Adding upstream version 115.8.0esr.upstream/115.8.0esr

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

65 files changed, 12314 insertions, 0 deletions

diff --git a/toolkit/components/uniffi-bindgen-gecko-js/Cargo.toml b/toolkit/components/uniffi-bindgen-gecko-js/Cargo.toml
new file mode 100644
index 0000000000..b28f455e3b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/Cargo.toml
@@ -0,0 +1,21 @@
+[package]
+name = "uniffi-bindgen-gecko-js"
+version = "0.1.0"
+edition = "2018"
+license = "MPL-2.0"
+
+[[bin]]
+name = "uniffi-bindgen-gecko-js"
+path = "src/main.rs"
+
+[dependencies]
+anyhow = "1"
+askama = { version = "0.11", default-features = false, features = ["config"] }
+clap = { version = "3.1", default-features = false, features = ["std", "derive", "cargo"] }
+extend = "1.1"
+heck = "0.4"
+uniffi = { workspace = true }
+uniffi_bindgen = "0.23"
+serde = "1"
+toml = "0.5"
+camino = "1.0.8"
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/askama.toml b/toolkit/components/uniffi-bindgen-gecko-js/askama.toml
new file mode 100644
index 0000000000..066e3c468c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/askama.toml
@@ -0,0 +1,2 @@
+[general]
+dirs = ["src/templates"]
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/components/generated/README.md b/toolkit/components/uniffi-bindgen-gecko-js/components/generated/README.md
new file mode 100644
index 0000000000..4e4267acd4
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/components/generated/README.md
@@ -0,0 +1,6 @@
+This directory is where modules generated by UniFFI will be created.
+
+All files in this directory, other than this one, are generated and should
+not be hand-edited.
+
+To update these files, execute `./mach uniffi generate`
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/components/generated/RustTabs.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/components/generated/RustTabs.sys.mjs
new file mode 100644
index 0000000000..b4b14cea52
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/components/generated/RustTabs.sys.mjs
@@ -0,0 +1,1504 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+
+    // Reads a TabsStore pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerTabsStore() {
+        const pointerId = 0; // tabs:TabsStore
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a TabsStore pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerTabsStore(value) {
+        const pointerId = 0; // tabs:TabsStore
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+
+    // Reads a TabsBridgedEngine pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerTabsBridgedEngine() {
+        const pointerId = 1; // tabs:TabsBridgedEngine
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a TabsBridgedEngine pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerTabsBridgedEngine(value) {
+        const pointerId = 1; // tabs:TabsBridgedEngine
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI64 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class TabsBridgedEngine {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+
+    lastSync() {
+        const liftResult = (result) => FfiConverterI64.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                7, // tabs:tabs_dffd_TabsBridgedEngine_last_sync
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    setLastSync(lastSync) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(lastSync)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("lastSync");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                8, // tabs:tabs_dffd_TabsBridgedEngine_set_last_sync
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+                FfiConverterI64.lower(lastSync),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    syncId() {
+        const liftResult = (result) => FfiConverterOptionalstring.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                9, // tabs:tabs_dffd_TabsBridgedEngine_sync_id
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    resetSyncId() {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                10, // tabs:tabs_dffd_TabsBridgedEngine_reset_sync_id
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    ensureCurrentSyncId(newSyncId) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(newSyncId)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("newSyncId");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                11, // tabs:tabs_dffd_TabsBridgedEngine_ensure_current_sync_id
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+                FfiConverterString.lower(newSyncId),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    prepareForSync(clientData) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(clientData)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("clientData");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                12, // tabs:tabs_dffd_TabsBridgedEngine_prepare_for_sync
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+                FfiConverterString.lower(clientData),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    syncStarted() {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                13, // tabs:tabs_dffd_TabsBridgedEngine_sync_started
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    storeIncoming(incomingEnvelopesAsJson) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterSequencestring.checkType(incomingEnvelopesAsJson)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("incomingEnvelopesAsJson");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                14, // tabs:tabs_dffd_TabsBridgedEngine_store_incoming
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+                FfiConverterSequencestring.lower(incomingEnvelopesAsJson),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    apply() {
+        const liftResult = (result) => FfiConverterSequencestring.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                15, // tabs:tabs_dffd_TabsBridgedEngine_apply
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    setUploaded(newTimestamp,uploadedIds) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(newTimestamp)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("newTimestamp");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterSequenceTypeTabsGuid.checkType(uploadedIds)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("uploadedIds");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                16, // tabs:tabs_dffd_TabsBridgedEngine_set_uploaded
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+                FfiConverterI64.lower(newTimestamp),
+                FfiConverterSequenceTypeTabsGuid.lower(uploadedIds),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    syncFinished() {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                17, // tabs:tabs_dffd_TabsBridgedEngine_sync_finished
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    reset() {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                18, // tabs:tabs_dffd_TabsBridgedEngine_reset
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    wipe() {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                19, // tabs:tabs_dffd_TabsBridgedEngine_wipe
+                FfiConverterTypeTabsBridgedEngine.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTabsBridgedEngine extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new TabsBridgedEngine(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerTabsBridgedEngine());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerTabsBridgedEngine(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class TabsStore {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for TabsStore.
+     * 
+     * @returns {Promise<TabsStore>}: A promise that resolves
+     *      to a newly constructed TabsStore
+     */
+    static init(path) {
+        const liftResult = (result) => FfiConverterTypeTabsStore.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(path)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("path");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                0, // tabs:tabs_dffd_TabsStore_new
+                FfiConverterString.lower(path),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    getAll() {
+        const liftResult = (result) => FfiConverterSequenceTypeClientRemoteTabs.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                1, // tabs:tabs_dffd_TabsStore_get_all
+                FfiConverterTypeTabsStore.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    setLocalTabs(remoteTabs) {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterSequenceTypeRemoteTabRecord.checkType(remoteTabs)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("remoteTabs");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                2, // tabs:tabs_dffd_TabsStore_set_local_tabs
+                FfiConverterTypeTabsStore.lower(this),
+                FfiConverterSequenceTypeRemoteTabRecord.lower(remoteTabs),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    registerWithSyncManager() {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                3, // tabs:tabs_dffd_TabsStore_register_with_sync_manager
+                FfiConverterTypeTabsStore.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    reset() {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                4, // tabs:tabs_dffd_TabsStore_reset
+                FfiConverterTypeTabsStore.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sync(keyId,accessToken,syncKey,tokenserverUrl,localId) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = (data) => FfiConverterTypeTabsApiError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(keyId)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("keyId");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterString.checkType(accessToken)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("accessToken");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterString.checkType(syncKey)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("syncKey");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterString.checkType(tokenserverUrl)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("tokenserverUrl");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterString.checkType(localId)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("localId");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                5, // tabs:tabs_dffd_TabsStore_sync
+                FfiConverterTypeTabsStore.lower(this),
+                FfiConverterString.lower(keyId),
+                FfiConverterString.lower(accessToken),
+                FfiConverterString.lower(syncKey),
+                FfiConverterString.lower(tokenserverUrl),
+                FfiConverterString.lower(localId),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    bridgedEngine() {
+        const liftResult = (result) => FfiConverterTypeTabsBridgedEngine.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                6, // tabs:tabs_dffd_TabsStore_bridged_engine
+                FfiConverterTypeTabsStore.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTabsStore extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new TabsStore(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerTabsStore());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerTabsStore(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class ClientRemoteTabs {
+    constructor(clientId,clientName,deviceType,lastModified,remoteTabs) {
+        try {
+            FfiConverterString.checkType(clientId)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("clientId");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterString.checkType(clientName)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("clientName");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeTabsDeviceType.checkType(deviceType)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("deviceType");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(lastModified)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("lastModified");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequenceTypeRemoteTabRecord.checkType(remoteTabs)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("remoteTabs");
+            }
+            throw e;
+        }
+        this.clientId = clientId;
+        this.clientName = clientName;
+        this.deviceType = deviceType;
+        this.lastModified = lastModified;
+        this.remoteTabs = remoteTabs;
+    }
+    equals(other) {
+        return (
+            this.clientId == other.clientId &&
+            this.clientName == other.clientName &&
+            this.deviceType == other.deviceType &&
+            this.lastModified == other.lastModified &&
+            this.remoteTabs == other.remoteTabs
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeClientRemoteTabs extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new ClientRemoteTabs(
+            FfiConverterString.read(dataStream), 
+            FfiConverterString.read(dataStream), 
+            FfiConverterTypeTabsDeviceType.read(dataStream), 
+            FfiConverterI64.read(dataStream), 
+            FfiConverterSequenceTypeRemoteTabRecord.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterString.write(dataStream, value.clientId);
+        FfiConverterString.write(dataStream, value.clientName);
+        FfiConverterTypeTabsDeviceType.write(dataStream, value.deviceType);
+        FfiConverterI64.write(dataStream, value.lastModified);
+        FfiConverterSequenceTypeRemoteTabRecord.write(dataStream, value.remoteTabs);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterString.computeSize(value.clientId);
+        totalSize += FfiConverterString.computeSize(value.clientName);
+        totalSize += FfiConverterTypeTabsDeviceType.computeSize(value.deviceType);
+        totalSize += FfiConverterI64.computeSize(value.lastModified);
+        totalSize += FfiConverterSequenceTypeRemoteTabRecord.computeSize(value.remoteTabs);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterString.checkType(value.clientId);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".clientId");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterString.checkType(value.clientName);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".clientName");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeTabsDeviceType.checkType(value.deviceType);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".deviceType");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(value.lastModified);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".lastModified");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequenceTypeRemoteTabRecord.checkType(value.remoteTabs);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".remoteTabs");
+            }
+            throw e;
+        }
+    }
+}
+
+export class RemoteTabRecord {
+    constructor(title,urlHistory,icon,lastUsed) {
+        try {
+            FfiConverterString.checkType(title)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("title");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequencestring.checkType(urlHistory)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("urlHistory");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterOptionalstring.checkType(icon)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("icon");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(lastUsed)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("lastUsed");
+            }
+            throw e;
+        }
+        this.title = title;
+        this.urlHistory = urlHistory;
+        this.icon = icon;
+        this.lastUsed = lastUsed;
+    }
+    equals(other) {
+        return (
+            this.title == other.title &&
+            this.urlHistory == other.urlHistory &&
+            this.icon == other.icon &&
+            this.lastUsed == other.lastUsed
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeRemoteTabRecord extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new RemoteTabRecord(
+            FfiConverterString.read(dataStream), 
+            FfiConverterSequencestring.read(dataStream), 
+            FfiConverterOptionalstring.read(dataStream), 
+            FfiConverterI64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterString.write(dataStream, value.title);
+        FfiConverterSequencestring.write(dataStream, value.urlHistory);
+        FfiConverterOptionalstring.write(dataStream, value.icon);
+        FfiConverterI64.write(dataStream, value.lastUsed);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterString.computeSize(value.title);
+        totalSize += FfiConverterSequencestring.computeSize(value.urlHistory);
+        totalSize += FfiConverterOptionalstring.computeSize(value.icon);
+        totalSize += FfiConverterI64.computeSize(value.lastUsed);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterString.checkType(value.title);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".title");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequencestring.checkType(value.urlHistory);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".urlHistory");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterOptionalstring.checkType(value.icon);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".icon");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(value.lastUsed);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".lastUsed");
+            }
+            throw e;
+        }
+    }
+}
+
+export const TabsDeviceType = {
+    DESKTOP: 1,
+    MOBILE: 2,
+    TABLET: 3,
+    VR: 4,
+    TV: 5,
+    UNKNOWN: 6,
+};
+
+Object.freeze(TabsDeviceType);
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTabsDeviceType extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return TabsDeviceType.DESKTOP
+            case 2:
+                return TabsDeviceType.MOBILE
+            case 3:
+                return TabsDeviceType.TABLET
+            case 4:
+                return TabsDeviceType.VR
+            case 5:
+                return TabsDeviceType.TV
+            case 6:
+                return TabsDeviceType.UNKNOWN
+            default:
+                return new Error("Unknown TabsDeviceType variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === TabsDeviceType.DESKTOP) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        if (value === TabsDeviceType.MOBILE) {
+            dataStream.writeInt32(2);
+            return;
+        }
+        if (value === TabsDeviceType.TABLET) {
+            dataStream.writeInt32(3);
+            return;
+        }
+        if (value === TabsDeviceType.VR) {
+            dataStream.writeInt32(4);
+            return;
+        }
+        if (value === TabsDeviceType.TV) {
+            dataStream.writeInt32(5);
+            return;
+        }
+        if (value === TabsDeviceType.UNKNOWN) {
+            dataStream.writeInt32(6);
+            return;
+        }
+        return new Error("Unknown TabsDeviceType variant");
+    }
+
+    static computeSize(value) {
+        return 4;
+    }
+
+    static checkType(value) {
+      if (!Number.isInteger(value) || value < 1 || value > 6) {
+          throw new UniFFITypeError(`${value} is not a valid value for TabsDeviceType`);
+      }
+    }
+}
+
+
+
+export class TabsApiError extends Error {}
+
+
+export class SyncError extends TabsApiError {
+
+    constructor(
+        reason,
+        ...params
+    ) {
+        super(...params);
+        this.reason = reason;
+    }
+    toString() {
+        return `SyncError: ${super.toString()}`
+    }
+}
+
+export class SqlError extends TabsApiError {
+
+    constructor(
+        reason,
+        ...params
+    ) {
+        super(...params);
+        this.reason = reason;
+    }
+    toString() {
+        return `SqlError: ${super.toString()}`
+    }
+}
+
+export class UnexpectedTabsError extends TabsApiError {
+
+    constructor(
+        reason,
+        ...params
+    ) {
+        super(...params);
+        this.reason = reason;
+    }
+    toString() {
+        return `UnexpectedTabsError: ${super.toString()}`
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTabsApiError extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return new SyncError(
+                    FfiConverterString.read(dataStream)
+                    );
+            case 2:
+                return new SqlError(
+                    FfiConverterString.read(dataStream)
+                    );
+            case 3:
+                return new UnexpectedTabsError(
+                    FfiConverterString.read(dataStream)
+                    );
+            default:
+                throw new Error("Unknown TabsApiError variant");
+        }
+    }
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        if (value instanceof SyncError) {
+            totalSize += FfiConverterString.computeSize(value.reason);
+            return totalSize;
+        }
+        if (value instanceof SqlError) {
+            totalSize += FfiConverterString.computeSize(value.reason);
+            return totalSize;
+        }
+        if (value instanceof UnexpectedTabsError) {
+            totalSize += FfiConverterString.computeSize(value.reason);
+            return totalSize;
+        }
+        throw new Error("Unknown TabsApiError variant");
+    }
+    static write(dataStream, value) {
+        if (value instanceof SyncError) {
+            dataStream.writeInt32(1);
+            FfiConverterString.write(dataStream, value.reason);
+            return;
+        }
+        if (value instanceof SqlError) {
+            dataStream.writeInt32(2);
+            FfiConverterString.write(dataStream, value.reason);
+            return;
+        }
+        if (value instanceof UnexpectedTabsError) {
+            dataStream.writeInt32(3);
+            FfiConverterString.write(dataStream, value.reason);
+            return;
+        }
+        throw new Error("Unknown TabsApiError variant");
+    }
+
+    static errorClass = TabsApiError;
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalstring extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterString.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterString.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterString.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterString.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequencestring extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterString.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterString.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterString.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterString.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequenceTypeClientRemoteTabs extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterTypeClientRemoteTabs.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterTypeClientRemoteTabs.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterTypeClientRemoteTabs.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterTypeClientRemoteTabs.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequenceTypeRemoteTabRecord extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterTypeRemoteTabRecord.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterTypeRemoteTabRecord.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterTypeRemoteTabRecord.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterTypeRemoteTabRecord.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequenceTypeTabsGuid extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterTypeTabsGuid.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterTypeTabsGuid.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterTypeTabsGuid.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterTypeTabsGuid.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTabsGuid extends FfiConverter {
+    static lift(buf) {
+        return FfiConverterString.lift(buf);    
+    }
+    
+    static lower(buf) {
+        return FfiConverterString.lower(buf);
+    }
+    
+    static write(dataStream, value) {
+        FfiConverterString.write(dataStream, value);
+    } 
+    
+    static read(buf) {
+        return FfiConverterString.read(buf);
+    }
+    
+    static computeSize(value) {
+        return FfiConverterString.computeSize(value);
+    }
+}
+// TODO: We should also allow JS to customize the type eventually.
+
+
+
+
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/components/moz.build b/toolkit/components/uniffi-bindgen-gecko-js/components/moz.build
new file mode 100644
index 0000000000..0aca753d89
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/components/moz.build
@@ -0,0 +1,12 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+EXTRA_JS_MODULES += [
+    "generated/RustTabs.sys.mjs",
+]
+
+with Files("**"):
+    BUG_COMPONENT = ("Toolkit", "UniFFI Bindings")
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/config.toml b/toolkit/components/uniffi-bindgen-gecko-js/config.toml
new file mode 100644
index 0000000000..da1cd21c25
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/config.toml
@@ -0,0 +1,5 @@
+[fixture_callbacks.receiver_thread]
+default = "worker"
+main = [
+  "log_even_numbers_main_thread",
+]
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/README.md b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/README.md
new file mode 100644
index 0000000000..632ba88e00
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/README.md
@@ -0,0 +1,36 @@
+This directory contains generated code for the UniFFI examples/fixtures and JS
+unit tests for it.
+
+This is only built if the `--enable-uniffi-fixtures` flag is present in
+`mozconfig`.  There's no benefit to including this in a release build.
+
+To add additional examples/fixtures:
+  - For most of these steps, find the code for existing fixtures and use it as a template for the new code.
+  - Edit `toolkit/components/uniffi-bindgen-gecko-js/mach_commands.py`
+    - Add an entry to `FIXTURE_UDL_FILES`
+  - Edit `toolkit/library/rust/shared/Cargo.toml`
+    - Add an optional dependency for the fixture.
+    - Add the feature to the list of features enabled by `uniffi_fixtures`.
+  - Edit `toolkit/library/rust/shared/lib.rs`:
+     - Add an `extern crate [name]` to the `uniffi_fixtures` mod
+       - Note: [name] is the name from the `[lib]` section in the Cargo.toml
+         for the example/fixture crate.  This does not always match the package
+         name for the crate.
+     - Add `[name]::reexport_uniffi_scaffolding` to the `uniffi_fixtures` mod
+  - Edit `toolkit/components/uniffi-bindgen-gecko-js/fixtures/moz.build` and add the fixture name to the `components`
+    list.
+  - Add a test module to the `toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/` directory and an entry for it
+    in `toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell.ini`
+  - Run `mach vendor rust` to vendor in the Rust code.
+  - Run `mach uniffi generate` to generate the scaffolding code.
+  - Check in any new files
+
+To run the tests:
+  - Make sure you have a `mozconfig` file containing the line `ac_add_options --enable-uniffi-fixtures`
+  - Run `mach uniffi generate` if:
+    - You've added or updated a fixture
+    - You've made changes to `uniffi-bindgen-gecko-js`
+  - Run `mach build`
+  - Run `mach xpcshell-test toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/`
+    - You can also use a path to specific test file
+    - For subsequent runs, if you only modify the test files, then you can re-run this step directly
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/README.md b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/README.md
new file mode 100644
index 0000000000..91372fd31c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/README.md
@@ -0,0 +1 @@
+This directory is where files generated by Uniffi will be created.
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustArithmetic.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustArithmetic.sys.mjs
new file mode 100644
index 0000000000..ca50baec4a
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustArithmetic.sys.mjs
@@ -0,0 +1,491 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterU64 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+        if (value < 0) {
+            throw new UniFFITypeError(`${value} exceeds the U64 bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterBool extends FfiConverter {
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value == 1;
+    }
+    static lower(value) {
+        if (value) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint8(this.lower(value))
+    }
+    static read(dataStream) {
+        return this.lift(dataStream.readUint8())
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+
+
+export class ArithmeticError extends Error {}
+
+
+export class IntegerOverflow extends ArithmeticError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `IntegerOverflow: ${super.toString()}`
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeArithmeticError extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return new IntegerOverflow(FfiConverterString.read(dataStream));
+            default:
+                throw new Error("Unknown ArithmeticError variant");
+        }
+    }
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        if (value instanceof IntegerOverflow) {
+            return totalSize;
+        }
+        throw new Error("Unknown ArithmeticError variant");
+    }
+    static write(dataStream, value) {
+        if (value instanceof IntegerOverflow) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        throw new Error("Unknown ArithmeticError variant");
+    }
+
+    static errorClass = ArithmeticError;
+}
+
+
+
+
+
+export function add(a,b) {
+
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = (data) => FfiConverterTypeArithmeticError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(a)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("a");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterU64.checkType(b)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("b");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                22, // arithmetic:arithmetic_fd12_add
+                FfiConverterU64.lower(a),
+                FfiConverterU64.lower(b),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function sub(a,b) {
+
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = (data) => FfiConverterTypeArithmeticError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(a)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("a");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterU64.checkType(b)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("b");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                23, // arithmetic:arithmetic_fd12_sub
+                FfiConverterU64.lower(a),
+                FfiConverterU64.lower(b),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function div(dividend,divisor) {
+
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(dividend)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("dividend");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterU64.checkType(divisor)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("divisor");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                24, // arithmetic:arithmetic_fd12_div
+                FfiConverterU64.lower(dividend),
+                FfiConverterU64.lower(divisor),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function equal(a,b) {
+
+        const liftResult = (result) => FfiConverterBool.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(a)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("a");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterU64.checkType(b)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("b");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                25, // arithmetic:arithmetic_fd12_equal
+                FfiConverterU64.lower(a),
+                FfiConverterU64.lower(b),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustCustomTypes.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustCustomTypes.sys.mjs
new file mode 100644
index 0000000000..c43ccbf490
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustCustomTypes.sys.mjs
@@ -0,0 +1,464 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI64 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class CustomTypesDemo {
+    constructor(url,handle) {
+        try {
+            FfiConverterTypeUrl.checkType(url)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("url");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeHandle.checkType(handle)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("handle");
+            }
+            throw e;
+        }
+        this.url = url;
+        this.handle = handle;
+    }
+    equals(other) {
+        return (
+            this.url == other.url &&
+            this.handle == other.handle
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeCustomTypesDemo extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new CustomTypesDemo(
+            FfiConverterTypeUrl.read(dataStream), 
+            FfiConverterTypeHandle.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterTypeUrl.write(dataStream, value.url);
+        FfiConverterTypeHandle.write(dataStream, value.handle);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterTypeUrl.computeSize(value.url);
+        totalSize += FfiConverterTypeHandle.computeSize(value.handle);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterTypeUrl.checkType(value.url);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".url");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeHandle.checkType(value.handle);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".handle");
+            }
+            throw e;
+        }
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypeCustomTypesDemo extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypeCustomTypesDemo.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypeCustomTypesDemo.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypeCustomTypesDemo.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypeCustomTypesDemo.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeHandle extends FfiConverter {
+    static lift(buf) {
+        return FfiConverterI64.lift(buf);    
+    }
+    
+    static lower(buf) {
+        return FfiConverterI64.lower(buf);
+    }
+    
+    static write(dataStream, value) {
+        FfiConverterI64.write(dataStream, value);
+    } 
+    
+    static read(buf) {
+        return FfiConverterI64.read(buf);
+    }
+    
+    static computeSize(value) {
+        return FfiConverterI64.computeSize(value);
+    }
+}
+// TODO: We should also allow JS to customize the type eventually.
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeUrl extends FfiConverter {
+    static lift(buf) {
+        return FfiConverterString.lift(buf);    
+    }
+    
+    static lower(buf) {
+        return FfiConverterString.lower(buf);
+    }
+    
+    static write(dataStream, value) {
+        FfiConverterString.write(dataStream, value);
+    } 
+    
+    static read(buf) {
+        return FfiConverterString.read(buf);
+    }
+    
+    static computeSize(value) {
+        return FfiConverterString.computeSize(value);
+    }
+}
+// TODO: We should also allow JS to customize the type eventually.
+
+
+
+
+
+export function getCustomTypesDemo(demo) {
+
+        const liftResult = (result) => FfiConverterTypeCustomTypesDemo.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterOptionalTypeCustomTypesDemo.checkType(demo)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("demo");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                109, // custom_types:custom_types_881f_get_custom_types_demo
+                FfiConverterOptionalTypeCustomTypesDemo.lower(demo),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustExternalTypes.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustExternalTypes.sys.mjs
new file mode 100644
index 0000000000..90889d1cd0
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustExternalTypes.sys.mjs
@@ -0,0 +1,356 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterF64 extends FfiConverter {
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypeLine extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypeLine.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypeLine.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypeLine.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypeLine.computeSize(value)
+    }
+}
+
+import {
+  FfiConverterTypeLine,
+  Line,
+} from "resource://gre/modules/RustGeometry.sys.mjs";
+
+// Export the FFIConverter object to make external types work.
+export { FfiConverterTypeLine, Line };
+
+import {
+  FfiConverterTypePoint,
+  Point,
+} from "resource://gre/modules/RustGeometry.sys.mjs";
+
+// Export the FFIConverter object to make external types work.
+export { FfiConverterTypePoint, Point };
+
+
+
+
+
+export function gradient(value) {
+
+        const liftResult = (result) => FfiConverterF64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterOptionalTypeLine.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                110, // external_types:external_types_c083_gradient
+                FfiConverterOptionalTypeLine.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustFixtureCallbacks.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustFixtureCallbacks.sys.mjs
new file mode 100644
index 0000000000..221669920a
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustFixtureCallbacks.sys.mjs
@@ -0,0 +1,639 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+
+/**
+ * Handler for a single UniFFI CallbackInterface
+ *
+ * This class stores objects that implement a callback interface in a handle
+ * map, allowing them to be referenced by the Rust code using an integer
+ * handle.
+ *
+ * While the callback object is stored in the map, it allows the Rust code to
+ * call methods on the object using the callback object handle, a method id,
+ * and an ArrayBuffer packed with the method arguments.
+ *
+ * When the Rust code drops its reference, it sends a call with the methodId=0,
+ * which causes callback object to be removed from the map.
+ */
+class UniFFICallbackHandler {
+    #name;
+    #interfaceId;
+    #handleCounter;
+    #handleMap;
+    #methodHandlers;
+    #allowNewCallbacks
+
+    /**
+     * Create a UniFFICallbackHandler
+     * @param {string} name - Human-friendly name for this callback interface
+     * @param {int} interfaceId - Interface ID for this CallbackInterface.
+     * @param {UniFFICallbackMethodHandler[]} methodHandlers -- UniFFICallbackHandler for each method, in the same order as the UDL file
+     */
+     constructor(name, interfaceId, methodHandlers) {
+        this.#name = name;
+        this.#interfaceId = interfaceId;
+        this.#handleCounter = 0;
+        this.#handleMap = new Map();
+        this.#methodHandlers = methodHandlers;
+        this.#allowNewCallbacks = true;
+
+        UniFFIScaffolding.registerCallbackHandler(this.#interfaceId, this.invokeCallback.bind(this));
+        Services.obs.addObserver(this, "xpcom-shutdown");
+     }
+
+    /**
+     * Store a callback object in the handle map and return the handle
+     *
+     * @param {obj} callbackObj - Object that implements the callback interface
+     * @returns {int} - Handle for this callback object, this is what gets passed back to Rust.
+     */
+    storeCallbackObj(callbackObj) {
+        if (!this.#allowNewCallbacks) {
+            throw new UniFFIError(`No new callbacks allowed for ${this.#name}`);
+        }
+        const handle = this.#handleCounter;
+        this.#handleCounter += 1;
+        this.#handleMap.set(handle, new UniFFICallbackHandleMapEntry(callbackObj, Components.stack.caller.formattedStack.trim()));
+        return handle;
+    }
+
+    /**
+     * Get a previously stored callback object
+     *
+     * @param {int} handle - Callback object handle, returned from `storeCallbackObj()`
+     * @returns {obj} - Callback object
+     */
+    getCallbackObj(handle) {
+        return this.#handleMap.get(handle).callbackObj;
+    }
+
+    /**
+     * Set if new callbacks are allowed for this handler
+     *
+     * This is called with false during shutdown to ensure the callback maps don't
+     * prevent JS objects from being GCed.
+     */
+    setAllowNewCallbacks(allow) {
+        this.#allowNewCallbacks = allow
+    }
+
+    /**
+     * Check that no callbacks are currently registered
+     *
+     * If there are callbacks registered a UniFFIError will be thrown.  This is
+     * called during shutdown to generate an alert if there are leaked callback
+     * interfaces.
+     */
+    assertNoRegisteredCallbacks() {
+        if (this.#handleMap.size > 0) {
+            const entry = this.#handleMap.values().next().value;
+            throw new UniFFIError(`UniFFI interface ${this.#name} has ${this.#handleMap.size} registered callbacks at xpcom-shutdown. This likely indicates a UniFFI callback leak.\nStack trace for the first leaked callback:\n${entry.stackTrace}.`);
+        }
+    }
+
+    /**
+     * Invoke a method on a stored callback object
+     * @param {int} handle - Object handle
+     * @param {int} methodId - Method identifier.  This the 1-based index of
+     *   the method from the UDL file.  0 is the special drop method, which
+     *   removes the callback object from the handle map.
+     * @param {ArrayBuffer} argsArrayBuffer - Arguments to pass to the method, packed in an ArrayBuffer
+     */
+    invokeCallback(handle, methodId, argsArrayBuffer) {
+        try {
+            this.#invokeCallbackInner(handle, methodId, argsArrayBuffer);
+        } catch (e) {
+            console.error(`internal error invoking callback: ${e}`)
+        }
+    }
+
+    #invokeCallbackInner(handle, methodId, argsArrayBuffer) {
+        const callbackObj = this.getCallbackObj(handle);
+        if (callbackObj === undefined) {
+            throw new UniFFIError(`${this.#name}: invalid callback handle id: ${handle}`);
+        }
+
+        // Special-cased drop method, remove the object from the handle map and
+        // return an empty array buffer
+        if (methodId == 0) {
+            this.#handleMap.delete(handle);
+            return;
+        }
+
+        // Get the method data, converting from 1-based indexing
+        const methodHandler = this.#methodHandlers[methodId - 1];
+        if (methodHandler === undefined) {
+            throw new UniFFIError(`${this.#name}: invalid method id: ${methodId}`)
+        }
+
+        methodHandler.call(callbackObj, argsArrayBuffer);
+    }
+
+    /**
+     * xpcom-shutdown observer method
+     *
+     * This handles:
+     *  - Deregistering ourselves as the UniFFI callback handler
+     *  - Checks for any leftover stored callbacks which indicate memory leaks
+     */
+    observe(aSubject, aTopic, aData) {
+        if (aTopic == "xpcom-shutdown") {
+            try {
+                this.setAllowNewCallbacks(false);
+                this.assertNoRegisteredCallbacks();
+                UniFFIScaffolding.deregisterCallbackHandler(this.#interfaceId);
+            } catch (ex) {
+                console.error(`UniFFI Callback interface error during xpcom-shutdown: ${ex}`);
+                Cc["@mozilla.org/xpcom/debug;1"]
+                    .getService(Ci.nsIDebug2)
+                    .abort(ex.filename, ex.lineNumber);
+            }
+         }
+    }
+}
+
+/**
+ * Handles calling a single method for a callback interface
+ */
+class UniFFICallbackMethodHandler {
+    #name;
+    #argsConverters;
+
+    /**
+     * Create a UniFFICallbackMethodHandler
+
+     * @param {string} name -- Name of the method to call on the callback object
+     * @param {FfiConverter[]} argsConverters - FfiConverter for each argument type
+     */
+    constructor(name, argsConverters) {
+        this.#name = name;
+        this.#argsConverters = argsConverters;
+    }
+
+    /**
+     * Invoke the method
+     *
+     * @param {obj} callbackObj -- Object implementing the callback interface for this method
+     * @param {ArrayBuffer} argsArrayBuffer -- Arguments for the method, packed in an ArrayBuffer
+     */
+     call(callbackObj, argsArrayBuffer) {
+        const argsStream = new ArrayBufferDataStream(argsArrayBuffer);
+        const args = this.#argsConverters.map(converter => converter.read(argsStream));
+        callbackObj[this.#name](...args);
+    }
+}
+
+/**
+ * UniFFICallbackHandler.handleMap entry
+ *
+ * @property callbackObj - Callback object, this must implement the callback interface.
+ * @property {string} stackTrace - Stack trace from when the callback object was registered.  This is used to proved extra context when debugging leaked callback objects.
+ */
+class UniFFICallbackHandleMapEntry {
+    constructor(callbackObj, stackTrace) {
+        this.callbackObj = callbackObj;
+        this.stackTrace = stackTrace
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI32 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -2147483648 || value > 2147483647) {
+            throw new UniFFITypeError(`${value} exceeds the I32 bounds`);
+        }
+    }
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt32()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterCallbackInterfaceLogger extends FfiConverter {
+    static lower(callbackObj) {
+        return callbackHandlerLogger.storeCallbackObj(callbackObj)
+    }
+
+    static lift(handleId) {
+        return callbackHandlerLogger.getCallbackObj(handleId)
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readInt64())
+    }
+
+    static write(dataStream, callbackObj) {
+        dataStream.writeInt64(this.lower(callbackObj))
+    }
+
+    static computeSize(callbackObj) {
+        return 8;
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequencei32 extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterI32.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterI32.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterI32.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterI32.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+
+// Define callback interface handlers, this must come after the type loop since they reference the FfiConverters defined above.
+
+const callbackHandlerLogger = new UniFFICallbackHandler(
+    "fixture_callbacks:Logger",
+    0,
+    [
+        new UniFFICallbackMethodHandler(
+            "log",
+            [
+                FfiConverterString,
+            ],
+        ),
+        new UniFFICallbackMethodHandler(
+            "finished",
+            [
+            ],
+        ),
+    ]
+);
+
+// Allow the shutdown-related functionality to be tested in the unit tests
+UnitTestObjs.callbackHandlerLogger = callbackHandlerLogger;
+
+
+
+
+
+export function logEvenNumbers(logger,items) {
+
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterCallbackInterfaceLogger.checkType(logger)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("logger");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterSequencei32.checkType(items)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("items");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                107, // fixture_callbacks:fixture_callbacks_fdf_log_even_numbers
+                FfiConverterCallbackInterfaceLogger.lower(logger),
+                FfiConverterSequencei32.lower(items),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function logEvenNumbersMainThread(logger,items) {
+
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterCallbackInterfaceLogger.checkType(logger)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("logger");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterSequencei32.checkType(items)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("items");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callSync(
+                108, // fixture_callbacks:fixture_callbacks_fdf_log_even_numbers_main_thread
+                FfiConverterCallbackInterfaceLogger.lower(logger),
+                FfiConverterSequencei32.lower(items),
+            )
+        }
+        return handleRustResult(functionCall(), liftResult, liftError);
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustGeometry.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustGeometry.sys.mjs
new file mode 100644
index 0000000000..15a9c55c0c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustGeometry.sys.mjs
@@ -0,0 +1,514 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterF64 extends FfiConverter {
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class Line {
+    constructor(start,end) {
+        try {
+            FfiConverterTypePoint.checkType(start)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("start");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypePoint.checkType(end)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("end");
+            }
+            throw e;
+        }
+        this.start = start;
+        this.end = end;
+    }
+    equals(other) {
+        return (
+            this.start.equals(other.start) &&
+            this.end.equals(other.end)
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeLine extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new Line(
+            FfiConverterTypePoint.read(dataStream), 
+            FfiConverterTypePoint.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterTypePoint.write(dataStream, value.start);
+        FfiConverterTypePoint.write(dataStream, value.end);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterTypePoint.computeSize(value.start);
+        totalSize += FfiConverterTypePoint.computeSize(value.end);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterTypePoint.checkType(value.start);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".start");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypePoint.checkType(value.end);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".end");
+            }
+            throw e;
+        }
+    }
+}
+
+export class Point {
+    constructor(coordX,coordY) {
+        try {
+            FfiConverterF64.checkType(coordX)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("coordX");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(coordY)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("coordY");
+            }
+            throw e;
+        }
+        this.coordX = coordX;
+        this.coordY = coordY;
+    }
+    equals(other) {
+        return (
+            this.coordX == other.coordX &&
+            this.coordY == other.coordY
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypePoint extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new Point(
+            FfiConverterF64.read(dataStream), 
+            FfiConverterF64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterF64.write(dataStream, value.coordX);
+        FfiConverterF64.write(dataStream, value.coordY);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterF64.computeSize(value.coordX);
+        totalSize += FfiConverterF64.computeSize(value.coordY);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterF64.checkType(value.coordX);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".coordX");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(value.coordY);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".coordY");
+            }
+            throw e;
+        }
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypePoint extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypePoint.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypePoint.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypePoint.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypePoint.computeSize(value)
+    }
+}
+
+
+
+
+
+export function gradient(ln) {
+
+        const liftResult = (result) => FfiConverterF64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeLine.checkType(ln)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("ln");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                20, // geometry:geometry_f26e_gradient
+                FfiConverterTypeLine.lower(ln),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function intersection(ln1,ln2) {
+
+        const liftResult = (result) => FfiConverterOptionalTypePoint.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeLine.checkType(ln1)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("ln1");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterTypeLine.checkType(ln2)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("ln2");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                21, // geometry:geometry_f26e_intersection
+                FfiConverterTypeLine.lower(ln1),
+                FfiConverterTypeLine.lower(ln2),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustRondpoint.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustRondpoint.sys.mjs
new file mode 100644
index 0000000000..97990fc054
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustRondpoint.sys.mjs
@@ -0,0 +1,3363 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+
+    // Reads a Retourneur pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerRetourneur() {
+        const pointerId = 2; // rondpoint:Retourneur
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a Retourneur pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerRetourneur(value) {
+        const pointerId = 2; // rondpoint:Retourneur
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+
+    // Reads a Stringifier pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerStringifier() {
+        const pointerId = 3; // rondpoint:Stringifier
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a Stringifier pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerStringifier(value) {
+        const pointerId = 3; // rondpoint:Stringifier
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+
+    // Reads a Optionneur pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerOptionneur() {
+        const pointerId = 4; // rondpoint:Optionneur
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a Optionneur pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerOptionneur(value) {
+        const pointerId = 4; // rondpoint:Optionneur
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterU8 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 256) {
+            throw new UniFFITypeError(`${value} exceeds the U8 bounds`);
+        }
+    }
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint8(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint8()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI8 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -128 || value > 127) {
+            throw new UniFFITypeError(`${value} exceeds the I8 bounds`);
+        }
+    }
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt8(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt8()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterU16 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 65535) {
+            throw new UniFFITypeError(`${value} exceeds the U16 bounds`);
+        }
+    }
+    static computeSize() {
+        return 2;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint16(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint16()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI16 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -32768 || value > 32767) {
+            throw new UniFFITypeError(`${value} exceeds the I16 bounds`);
+        }
+    }
+    static computeSize() {
+        return 2;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt16(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt16()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterU32 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 4294967295) {
+            throw new UniFFITypeError(`${value} exceeds the U32 bounds`);
+        }
+    }
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint32()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI32 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -2147483648 || value > 2147483647) {
+            throw new UniFFITypeError(`${value} exceeds the I32 bounds`);
+        }
+    }
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt32()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterU64 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+        if (value < 0) {
+            throw new UniFFITypeError(`${value} exceeds the U64 bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterI64 extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterF32 extends FfiConverter {
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat32()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterF64 extends FfiConverter {
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterBool extends FfiConverter {
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value == 1;
+    }
+    static lower(value) {
+        if (value) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint8(this.lower(value))
+    }
+    static read(dataStream) {
+        return this.lift(dataStream.readUint8())
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class Optionneur {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for Optionneur.
+     * 
+     * @returns {Promise<Optionneur>}: A promise that resolves
+     *      to a newly constructed Optionneur
+     */
+    static init() {
+        const liftResult = (result) => FfiConverterTypeOptionneur.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                55, // rondpoint:rondpoint_7b7b_Optionneur_new
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    sinonBoolean(value = false) {
+        const liftResult = (result) => FfiConverterBool.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterBool.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                56, // rondpoint:rondpoint_7b7b_Optionneur_sinon_boolean
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterBool.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonString(value = "default") {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                57, // rondpoint:rondpoint_7b7b_Optionneur_sinon_string
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterString.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonSequence(value = []) {
+        const liftResult = (result) => FfiConverterSequencestring.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterSequencestring.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                58, // rondpoint:rondpoint_7b7b_Optionneur_sinon_sequence
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterSequencestring.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonNull(value = null) {
+        const liftResult = (result) => FfiConverterOptionalstring.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterOptionalstring.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                59, // rondpoint:rondpoint_7b7b_Optionneur_sinon_null
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterOptionalstring.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonZero(value = 0) {
+        const liftResult = (result) => FfiConverterOptionali32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterOptionali32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                60, // rondpoint:rondpoint_7b7b_Optionneur_sinon_zero
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterOptionali32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU8Dec(value = 42) {
+        const liftResult = (result) => FfiConverterU8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                61, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u8_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI8Dec(value = -42) {
+        const liftResult = (result) => FfiConverterI8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                62, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i8_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU16Dec(value = 42) {
+        const liftResult = (result) => FfiConverterU16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                63, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u16_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI16Dec(value = 42) {
+        const liftResult = (result) => FfiConverterI16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                64, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i16_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU32Dec(value = 42) {
+        const liftResult = (result) => FfiConverterU32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                65, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u32_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI32Dec(value = 42) {
+        const liftResult = (result) => FfiConverterI32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                66, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i32_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU64Dec(value = 42) {
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                67, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u64_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI64Dec(value = 42) {
+        const liftResult = (result) => FfiConverterI64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                68, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i64_dec
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU8Hex(value = 0xff) {
+        const liftResult = (result) => FfiConverterU8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                69, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u8_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI8Hex(value = -127) {
+        const liftResult = (result) => FfiConverterI8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                70, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i8_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU16Hex(value = 0xffff) {
+        const liftResult = (result) => FfiConverterU16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                71, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u16_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI16Hex(value = 0x7f) {
+        const liftResult = (result) => FfiConverterI16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                72, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i16_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU32Hex(value = 0xffffffff) {
+        const liftResult = (result) => FfiConverterU32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                73, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u32_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI32Hex(value = 0x7fffffff) {
+        const liftResult = (result) => FfiConverterI32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                74, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i32_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU64Hex(value = 0xffffffffffffffff) {
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                75, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u64_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonI64Hex(value = 0x7fffffffffffffff) {
+        const liftResult = (result) => FfiConverterI64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                76, // rondpoint:rondpoint_7b7b_Optionneur_sinon_i64_hex
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterI64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonU32Oct(value = 0o755) {
+        const liftResult = (result) => FfiConverterU32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                77, // rondpoint:rondpoint_7b7b_Optionneur_sinon_u32_oct
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterU32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonF32(value = 42.0) {
+        const liftResult = (result) => FfiConverterF32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                78, // rondpoint:rondpoint_7b7b_Optionneur_sinon_f32
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterF32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonF64(value = 42.1) {
+        const liftResult = (result) => FfiConverterF64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                79, // rondpoint:rondpoint_7b7b_Optionneur_sinon_f64
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterF64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    sinonEnum(value = Enumeration.TROIS) {
+        const liftResult = (result) => FfiConverterTypeEnumeration.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeEnumeration.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                80, // rondpoint:rondpoint_7b7b_Optionneur_sinon_enum
+                FfiConverterTypeOptionneur.lower(this),
+                FfiConverterTypeEnumeration.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeOptionneur extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new Optionneur(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerOptionneur());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerOptionneur(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class Retourneur {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for Retourneur.
+     * 
+     * @returns {Promise<Retourneur>}: A promise that resolves
+     *      to a newly constructed Retourneur
+     */
+    static init() {
+        const liftResult = (result) => FfiConverterTypeRetourneur.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                26, // rondpoint:rondpoint_7b7b_Retourneur_new
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    identiqueI8(value) {
+        const liftResult = (result) => FfiConverterI8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                27, // rondpoint:rondpoint_7b7b_Retourneur_identique_i8
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterI8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueU8(value) {
+        const liftResult = (result) => FfiConverterU8.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                28, // rondpoint:rondpoint_7b7b_Retourneur_identique_u8
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterU8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueI16(value) {
+        const liftResult = (result) => FfiConverterI16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                29, // rondpoint:rondpoint_7b7b_Retourneur_identique_i16
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterI16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueU16(value) {
+        const liftResult = (result) => FfiConverterU16.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                30, // rondpoint:rondpoint_7b7b_Retourneur_identique_u16
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterU16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueI32(value) {
+        const liftResult = (result) => FfiConverterI32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                31, // rondpoint:rondpoint_7b7b_Retourneur_identique_i32
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterI32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueU32(value) {
+        const liftResult = (result) => FfiConverterU32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                32, // rondpoint:rondpoint_7b7b_Retourneur_identique_u32
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterU32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueI64(value) {
+        const liftResult = (result) => FfiConverterI64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                33, // rondpoint:rondpoint_7b7b_Retourneur_identique_i64
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterI64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueU64(value) {
+        const liftResult = (result) => FfiConverterU64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                34, // rondpoint:rondpoint_7b7b_Retourneur_identique_u64
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterU64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueFloat(value) {
+        const liftResult = (result) => FfiConverterF32.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                35, // rondpoint:rondpoint_7b7b_Retourneur_identique_float
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterF32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueDouble(value) {
+        const liftResult = (result) => FfiConverterF64.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                36, // rondpoint:rondpoint_7b7b_Retourneur_identique_double
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterF64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueBoolean(value) {
+        const liftResult = (result) => FfiConverterBool.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterBool.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                37, // rondpoint:rondpoint_7b7b_Retourneur_identique_boolean
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterBool.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueString(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                38, // rondpoint:rondpoint_7b7b_Retourneur_identique_string
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterString.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueNombresSignes(value) {
+        const liftResult = (result) => FfiConverterTypeDictionnaireNombresSignes.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeDictionnaireNombresSignes.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                39, // rondpoint:rondpoint_7b7b_Retourneur_identique_nombres_signes
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterTypeDictionnaireNombresSignes.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueNombres(value) {
+        const liftResult = (result) => FfiConverterTypeDictionnaireNombres.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeDictionnaireNombres.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                40, // rondpoint:rondpoint_7b7b_Retourneur_identique_nombres
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterTypeDictionnaireNombres.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    identiqueOptionneurDictionnaire(value) {
+        const liftResult = (result) => FfiConverterTypeOptionneurDictionnaire.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeOptionneurDictionnaire.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                41, // rondpoint:rondpoint_7b7b_Retourneur_identique_optionneur_dictionnaire
+                FfiConverterTypeRetourneur.lower(this),
+                FfiConverterTypeOptionneurDictionnaire.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeRetourneur extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new Retourneur(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerRetourneur());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerRetourneur(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class Stringifier {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for Stringifier.
+     * 
+     * @returns {Promise<Stringifier>}: A promise that resolves
+     *      to a newly constructed Stringifier
+     */
+    static init() {
+        const liftResult = (result) => FfiConverterTypeStringifier.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                42, // rondpoint:rondpoint_7b7b_Stringifier_new
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    wellKnownString(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                43, // rondpoint:rondpoint_7b7b_Stringifier_well_known_string
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterString.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringI8(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                44, // rondpoint:rondpoint_7b7b_Stringifier_to_string_i8
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterI8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringU8(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU8.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                45, // rondpoint:rondpoint_7b7b_Stringifier_to_string_u8
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterU8.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringI16(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                46, // rondpoint:rondpoint_7b7b_Stringifier_to_string_i16
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterI16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringU16(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU16.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                47, // rondpoint:rondpoint_7b7b_Stringifier_to_string_u16
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterU16.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringI32(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                48, // rondpoint:rondpoint_7b7b_Stringifier_to_string_i32
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterI32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringU32(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                49, // rondpoint:rondpoint_7b7b_Stringifier_to_string_u32
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterU32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringI64(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterI64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                50, // rondpoint:rondpoint_7b7b_Stringifier_to_string_i64
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterI64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringU64(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterU64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                51, // rondpoint:rondpoint_7b7b_Stringifier_to_string_u64
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterU64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringFloat(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF32.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                52, // rondpoint:rondpoint_7b7b_Stringifier_to_string_float
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterF32.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringDouble(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterF64.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                53, // rondpoint:rondpoint_7b7b_Stringifier_to_string_double
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterF64.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    toStringBoolean(value) {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterBool.checkType(value)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("value");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                54, // rondpoint:rondpoint_7b7b_Stringifier_to_string_boolean
+                FfiConverterTypeStringifier.lower(this),
+                FfiConverterBool.lower(value),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeStringifier extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new Stringifier(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerStringifier());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerStringifier(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class Dictionnaire {
+    constructor(un,deux,petitNombre,grosNombre) {
+        try {
+            FfiConverterTypeEnumeration.checkType(un)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("un");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterBool.checkType(deux)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("deux");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU8.checkType(petitNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(grosNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("grosNombre");
+            }
+            throw e;
+        }
+        this.un = un;
+        this.deux = deux;
+        this.petitNombre = petitNombre;
+        this.grosNombre = grosNombre;
+    }
+    equals(other) {
+        return (
+            this.un == other.un &&
+            this.deux == other.deux &&
+            this.petitNombre == other.petitNombre &&
+            this.grosNombre == other.grosNombre
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeDictionnaire extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new Dictionnaire(
+            FfiConverterTypeEnumeration.read(dataStream), 
+            FfiConverterBool.read(dataStream), 
+            FfiConverterU8.read(dataStream), 
+            FfiConverterU64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterTypeEnumeration.write(dataStream, value.un);
+        FfiConverterBool.write(dataStream, value.deux);
+        FfiConverterU8.write(dataStream, value.petitNombre);
+        FfiConverterU64.write(dataStream, value.grosNombre);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterTypeEnumeration.computeSize(value.un);
+        totalSize += FfiConverterBool.computeSize(value.deux);
+        totalSize += FfiConverterU8.computeSize(value.petitNombre);
+        totalSize += FfiConverterU64.computeSize(value.grosNombre);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterTypeEnumeration.checkType(value.un);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".un");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterBool.checkType(value.deux);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".deux");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU8.checkType(value.petitNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(value.grosNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".grosNombre");
+            }
+            throw e;
+        }
+    }
+}
+
+export class DictionnaireNombres {
+    constructor(petitNombre,courtNombre,nombreSimple,grosNombre) {
+        try {
+            FfiConverterU8.checkType(petitNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU16.checkType(courtNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("courtNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU32.checkType(nombreSimple)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("nombreSimple");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(grosNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("grosNombre");
+            }
+            throw e;
+        }
+        this.petitNombre = petitNombre;
+        this.courtNombre = courtNombre;
+        this.nombreSimple = nombreSimple;
+        this.grosNombre = grosNombre;
+    }
+    equals(other) {
+        return (
+            this.petitNombre == other.petitNombre &&
+            this.courtNombre == other.courtNombre &&
+            this.nombreSimple == other.nombreSimple &&
+            this.grosNombre == other.grosNombre
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeDictionnaireNombres extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new DictionnaireNombres(
+            FfiConverterU8.read(dataStream), 
+            FfiConverterU16.read(dataStream), 
+            FfiConverterU32.read(dataStream), 
+            FfiConverterU64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterU8.write(dataStream, value.petitNombre);
+        FfiConverterU16.write(dataStream, value.courtNombre);
+        FfiConverterU32.write(dataStream, value.nombreSimple);
+        FfiConverterU64.write(dataStream, value.grosNombre);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterU8.computeSize(value.petitNombre);
+        totalSize += FfiConverterU16.computeSize(value.courtNombre);
+        totalSize += FfiConverterU32.computeSize(value.nombreSimple);
+        totalSize += FfiConverterU64.computeSize(value.grosNombre);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterU8.checkType(value.petitNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU16.checkType(value.courtNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".courtNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU32.checkType(value.nombreSimple);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".nombreSimple");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(value.grosNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".grosNombre");
+            }
+            throw e;
+        }
+    }
+}
+
+export class DictionnaireNombresSignes {
+    constructor(petitNombre,courtNombre,nombreSimple,grosNombre) {
+        try {
+            FfiConverterI8.checkType(petitNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI16.checkType(courtNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("courtNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI32.checkType(nombreSimple)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("nombreSimple");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(grosNombre)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("grosNombre");
+            }
+            throw e;
+        }
+        this.petitNombre = petitNombre;
+        this.courtNombre = courtNombre;
+        this.nombreSimple = nombreSimple;
+        this.grosNombre = grosNombre;
+    }
+    equals(other) {
+        return (
+            this.petitNombre == other.petitNombre &&
+            this.courtNombre == other.courtNombre &&
+            this.nombreSimple == other.nombreSimple &&
+            this.grosNombre == other.grosNombre
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeDictionnaireNombresSignes extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new DictionnaireNombresSignes(
+            FfiConverterI8.read(dataStream), 
+            FfiConverterI16.read(dataStream), 
+            FfiConverterI32.read(dataStream), 
+            FfiConverterI64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterI8.write(dataStream, value.petitNombre);
+        FfiConverterI16.write(dataStream, value.courtNombre);
+        FfiConverterI32.write(dataStream, value.nombreSimple);
+        FfiConverterI64.write(dataStream, value.grosNombre);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterI8.computeSize(value.petitNombre);
+        totalSize += FfiConverterI16.computeSize(value.courtNombre);
+        totalSize += FfiConverterI32.computeSize(value.nombreSimple);
+        totalSize += FfiConverterI64.computeSize(value.grosNombre);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterI8.checkType(value.petitNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".petitNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI16.checkType(value.courtNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".courtNombre");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI32.checkType(value.nombreSimple);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".nombreSimple");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(value.grosNombre);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".grosNombre");
+            }
+            throw e;
+        }
+    }
+}
+
+export class OptionneurDictionnaire {
+    constructor(i8Var = -8,u8Var = 8,i16Var = -16,u16Var = 0x10,i32Var = -32,u32Var = 32,i64Var = -64,u64Var = 64,floatVar = 4.0,doubleVar = 8.0,booleanVar = true,stringVar = "default",listVar = [],enumerationVar = Enumeration.DEUX,dictionnaireVar = null) {
+        try {
+            FfiConverterI8.checkType(i8Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("i8Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU8.checkType(u8Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("u8Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI16.checkType(i16Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("i16Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU16.checkType(u16Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("u16Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI32.checkType(i32Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("i32Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU32.checkType(u32Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("u32Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(i64Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("i64Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(u64Var)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("u64Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF32.checkType(floatVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("floatVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(doubleVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("doubleVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterBool.checkType(booleanVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("booleanVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterString.checkType(stringVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("stringVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequencestring.checkType(listVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("listVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeEnumeration.checkType(enumerationVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("enumerationVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterOptionalTypeminusculeMajusculeEnum.checkType(dictionnaireVar)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("dictionnaireVar");
+            }
+            throw e;
+        }
+        this.i8Var = i8Var;
+        this.u8Var = u8Var;
+        this.i16Var = i16Var;
+        this.u16Var = u16Var;
+        this.i32Var = i32Var;
+        this.u32Var = u32Var;
+        this.i64Var = i64Var;
+        this.u64Var = u64Var;
+        this.floatVar = floatVar;
+        this.doubleVar = doubleVar;
+        this.booleanVar = booleanVar;
+        this.stringVar = stringVar;
+        this.listVar = listVar;
+        this.enumerationVar = enumerationVar;
+        this.dictionnaireVar = dictionnaireVar;
+    }
+    equals(other) {
+        return (
+            this.i8Var == other.i8Var &&
+            this.u8Var == other.u8Var &&
+            this.i16Var == other.i16Var &&
+            this.u16Var == other.u16Var &&
+            this.i32Var == other.i32Var &&
+            this.u32Var == other.u32Var &&
+            this.i64Var == other.i64Var &&
+            this.u64Var == other.u64Var &&
+            this.floatVar == other.floatVar &&
+            this.doubleVar == other.doubleVar &&
+            this.booleanVar == other.booleanVar &&
+            this.stringVar == other.stringVar &&
+            this.listVar == other.listVar &&
+            this.enumerationVar == other.enumerationVar &&
+            this.dictionnaireVar == other.dictionnaireVar
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeOptionneurDictionnaire extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new OptionneurDictionnaire(
+            FfiConverterI8.read(dataStream), 
+            FfiConverterU8.read(dataStream), 
+            FfiConverterI16.read(dataStream), 
+            FfiConverterU16.read(dataStream), 
+            FfiConverterI32.read(dataStream), 
+            FfiConverterU32.read(dataStream), 
+            FfiConverterI64.read(dataStream), 
+            FfiConverterU64.read(dataStream), 
+            FfiConverterF32.read(dataStream), 
+            FfiConverterF64.read(dataStream), 
+            FfiConverterBool.read(dataStream), 
+            FfiConverterString.read(dataStream), 
+            FfiConverterSequencestring.read(dataStream), 
+            FfiConverterTypeEnumeration.read(dataStream), 
+            FfiConverterOptionalTypeminusculeMajusculeEnum.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterI8.write(dataStream, value.i8Var);
+        FfiConverterU8.write(dataStream, value.u8Var);
+        FfiConverterI16.write(dataStream, value.i16Var);
+        FfiConverterU16.write(dataStream, value.u16Var);
+        FfiConverterI32.write(dataStream, value.i32Var);
+        FfiConverterU32.write(dataStream, value.u32Var);
+        FfiConverterI64.write(dataStream, value.i64Var);
+        FfiConverterU64.write(dataStream, value.u64Var);
+        FfiConverterF32.write(dataStream, value.floatVar);
+        FfiConverterF64.write(dataStream, value.doubleVar);
+        FfiConverterBool.write(dataStream, value.booleanVar);
+        FfiConverterString.write(dataStream, value.stringVar);
+        FfiConverterSequencestring.write(dataStream, value.listVar);
+        FfiConverterTypeEnumeration.write(dataStream, value.enumerationVar);
+        FfiConverterOptionalTypeminusculeMajusculeEnum.write(dataStream, value.dictionnaireVar);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterI8.computeSize(value.i8Var);
+        totalSize += FfiConverterU8.computeSize(value.u8Var);
+        totalSize += FfiConverterI16.computeSize(value.i16Var);
+        totalSize += FfiConverterU16.computeSize(value.u16Var);
+        totalSize += FfiConverterI32.computeSize(value.i32Var);
+        totalSize += FfiConverterU32.computeSize(value.u32Var);
+        totalSize += FfiConverterI64.computeSize(value.i64Var);
+        totalSize += FfiConverterU64.computeSize(value.u64Var);
+        totalSize += FfiConverterF32.computeSize(value.floatVar);
+        totalSize += FfiConverterF64.computeSize(value.doubleVar);
+        totalSize += FfiConverterBool.computeSize(value.booleanVar);
+        totalSize += FfiConverterString.computeSize(value.stringVar);
+        totalSize += FfiConverterSequencestring.computeSize(value.listVar);
+        totalSize += FfiConverterTypeEnumeration.computeSize(value.enumerationVar);
+        totalSize += FfiConverterOptionalTypeminusculeMajusculeEnum.computeSize(value.dictionnaireVar);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterI8.checkType(value.i8Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".i8Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU8.checkType(value.u8Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".u8Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI16.checkType(value.i16Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".i16Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU16.checkType(value.u16Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".u16Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI32.checkType(value.i32Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".i32Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU32.checkType(value.u32Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".u32Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterI64.checkType(value.i64Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".i64Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterU64.checkType(value.u64Var);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".u64Var");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF32.checkType(value.floatVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".floatVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(value.doubleVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".doubleVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterBool.checkType(value.booleanVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".booleanVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterString.checkType(value.stringVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".stringVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterSequencestring.checkType(value.listVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".listVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterTypeEnumeration.checkType(value.enumerationVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".enumerationVar");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterOptionalTypeminusculeMajusculeEnum.checkType(value.dictionnaireVar);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".dictionnaireVar");
+            }
+            throw e;
+        }
+    }
+}
+
+export class MinusculeMajusculeDict {
+    constructor(minusculeMajusculeField) {
+        try {
+            FfiConverterBool.checkType(minusculeMajusculeField)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("minusculeMajusculeField");
+            }
+            throw e;
+        }
+        this.minusculeMajusculeField = minusculeMajusculeField;
+    }
+    equals(other) {
+        return (
+            this.minusculeMajusculeField == other.minusculeMajusculeField
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeminusculeMajusculeDict extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new MinusculeMajusculeDict(
+            FfiConverterBool.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterBool.write(dataStream, value.minusculeMajusculeField);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterBool.computeSize(value.minusculeMajusculeField);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterBool.checkType(value.minusculeMajusculeField);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".minusculeMajusculeField");
+            }
+            throw e;
+        }
+    }
+}
+
+export const Enumeration = {
+    UN: 1,
+    DEUX: 2,
+    TROIS: 3,
+};
+
+Object.freeze(Enumeration);
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeEnumeration extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return Enumeration.UN
+            case 2:
+                return Enumeration.DEUX
+            case 3:
+                return Enumeration.TROIS
+            default:
+                return new Error("Unknown Enumeration variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === Enumeration.UN) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        if (value === Enumeration.DEUX) {
+            dataStream.writeInt32(2);
+            return;
+        }
+        if (value === Enumeration.TROIS) {
+            dataStream.writeInt32(3);
+            return;
+        }
+        return new Error("Unknown Enumeration variant");
+    }
+
+    static computeSize(value) {
+        return 4;
+    }
+
+    static checkType(value) {
+      if (!Number.isInteger(value) || value < 1 || value > 3) {
+          throw new UniFFITypeError(`${value} is not a valid value for Enumeration`);
+      }
+    }
+}
+
+export class EnumerationAvecDonnees {}
+EnumerationAvecDonnees.Zero = class extends EnumerationAvecDonnees{
+    constructor(
+        ) {
+            super();
+        }
+}
+EnumerationAvecDonnees.Un = class extends EnumerationAvecDonnees{
+    constructor(
+        premier
+        ) {
+            super();
+            this.premier = premier;
+        }
+}
+EnumerationAvecDonnees.Deux = class extends EnumerationAvecDonnees{
+    constructor(
+        premier,
+        second
+        ) {
+            super();
+            this.premier = premier;
+            this.second = second;
+        }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeEnumerationAvecDonnees extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return new EnumerationAvecDonnees.Zero(
+                    );
+            case 2:
+                return new EnumerationAvecDonnees.Un(
+                    FfiConverterU32.read(dataStream)
+                    );
+            case 3:
+                return new EnumerationAvecDonnees.Deux(
+                    FfiConverterU32.read(dataStream),
+                    FfiConverterString.read(dataStream)
+                    );
+            default:
+                return new Error("Unknown EnumerationAvecDonnees variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value instanceof EnumerationAvecDonnees.Zero) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        if (value instanceof EnumerationAvecDonnees.Un) {
+            dataStream.writeInt32(2);
+            FfiConverterU32.write(dataStream, value.premier);
+            return;
+        }
+        if (value instanceof EnumerationAvecDonnees.Deux) {
+            dataStream.writeInt32(3);
+            FfiConverterU32.write(dataStream, value.premier);
+            FfiConverterString.write(dataStream, value.second);
+            return;
+        }
+        return new Error("Unknown EnumerationAvecDonnees variant");
+    }
+
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        if (value instanceof EnumerationAvecDonnees.Zero) {
+            return totalSize;
+        }
+        if (value instanceof EnumerationAvecDonnees.Un) {
+            totalSize += FfiConverterU32.computeSize(value.premier);
+            return totalSize;
+        }
+        if (value instanceof EnumerationAvecDonnees.Deux) {
+            totalSize += FfiConverterU32.computeSize(value.premier);
+            totalSize += FfiConverterString.computeSize(value.second);
+            return totalSize;
+        }
+        return new Error("Unknown EnumerationAvecDonnees variant");
+    }
+
+    static checkType(value) {
+      if (!(value instanceof EnumerationAvecDonnees)) {
+        throw new UniFFITypeError(`${value} is not a subclass instance of EnumerationAvecDonnees`);
+      }
+    }
+}
+
+export const MinusculeMajusculeEnum = {
+    MINUSCULE_MAJUSCULE_VARIANT: 1,
+};
+
+Object.freeze(MinusculeMajusculeEnum);
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeminusculeMajusculeEnum extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return MinusculeMajusculeEnum.MINUSCULE_MAJUSCULE_VARIANT
+            default:
+                return new Error("Unknown MinusculeMajusculeEnum variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === MinusculeMajusculeEnum.MINUSCULE_MAJUSCULE_VARIANT) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        return new Error("Unknown MinusculeMajusculeEnum variant");
+    }
+
+    static computeSize(value) {
+        return 4;
+    }
+
+    static checkType(value) {
+      if (!Number.isInteger(value) || value < 1 || value > 1) {
+          throw new UniFFITypeError(`${value} is not a valid value for MinusculeMajusculeEnum`);
+      }
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionali32 extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterI32.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterI32.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterI32.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterI32.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalstring extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterString.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterString.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterString.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterString.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypeminusculeMajusculeEnum extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypeminusculeMajusculeEnum.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypeminusculeMajusculeEnum.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypeminusculeMajusculeEnum.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypeminusculeMajusculeEnum.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequencestring extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterString.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterString.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterString.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterString.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequenceTypeEnumeration extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterTypeEnumeration.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterTypeEnumeration.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterTypeEnumeration.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterTypeEnumeration.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterMapStringTypeEnumerationAvecDonnees extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const map = {};
+        for (let i = 0; i < len; i++) {
+            const key = FfiConverterString.read(dataStream);
+            const value = FfiConverterTypeEnumerationAvecDonnees.read(dataStream);
+            map[key] = value;
+        }
+
+        return map;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(Object.keys(value).length);
+        for (const key in value) {
+            FfiConverterString.write(dataStream, key);
+            FfiConverterTypeEnumerationAvecDonnees.write(dataStream, value[key]);
+        }
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const key in value) {
+            size += FfiConverterString.computeSize(key);
+            size += FfiConverterTypeEnumerationAvecDonnees.computeSize(value[key]);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        for (const key in value) {
+            try {
+                FfiConverterString.checkType(key);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("(key)");
+                }
+                throw e;
+            }
+
+            try {
+                FfiConverterTypeEnumerationAvecDonnees.checkType(value[key]);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${key}]`);
+                }
+                throw e;
+            }
+        }
+    }
+}
+
+
+
+
+
+export function copieDictionnaire(d) {
+
+        const liftResult = (result) => FfiConverterTypeDictionnaire.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeDictionnaire.checkType(d)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("d");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                81, // rondpoint:rondpoint_7b7b_copie_dictionnaire
+                FfiConverterTypeDictionnaire.lower(d),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function copieEnumeration(e) {
+
+        const liftResult = (result) => FfiConverterTypeEnumeration.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeEnumeration.checkType(e)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("e");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                82, // rondpoint:rondpoint_7b7b_copie_enumeration
+                FfiConverterTypeEnumeration.lower(e),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function copieEnumerations(e) {
+
+        const liftResult = (result) => FfiConverterSequenceTypeEnumeration.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterSequenceTypeEnumeration.checkType(e)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("e");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                83, // rondpoint:rondpoint_7b7b_copie_enumerations
+                FfiConverterSequenceTypeEnumeration.lower(e),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function copieCarte(c) {
+
+        const liftResult = (result) => FfiConverterMapStringTypeEnumerationAvecDonnees.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterMapStringTypeEnumerationAvecDonnees.checkType(c)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("c");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                84, // rondpoint:rondpoint_7b7b_copie_carte
+                FfiConverterMapStringTypeEnumerationAvecDonnees.lower(c),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function switcheroo(b) {
+
+        const liftResult = (result) => FfiConverterBool.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterBool.checkType(b)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("b");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                85, // rondpoint:rondpoint_7b7b_switcheroo
+                FfiConverterBool.lower(b),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustSprites.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustSprites.sys.mjs
new file mode 100644
index 0000000000..edb80ea246
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustSprites.sys.mjs
@@ -0,0 +1,681 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+
+    // Reads a Sprite pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerSprite() {
+        const pointerId = 5; // sprites:Sprite
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a Sprite pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerSprite(value) {
+        const pointerId = 5; // sprites:Sprite
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterF64 extends FfiConverter {
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat64()
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class Sprite {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for Sprite.
+     * 
+     * @returns {Promise<Sprite>}: A promise that resolves
+     *      to a newly constructed Sprite
+     */
+    static init(initialPosition) {
+        const liftResult = (result) => FfiConverterTypeSprite.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterOptionalTypePoint.checkType(initialPosition)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("initialPosition");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                86, // sprites:sprites_3c8d_Sprite_new
+                FfiConverterOptionalTypePoint.lower(initialPosition),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+    /**
+     * An async constructor for Sprite.
+     * 
+     * @returns {Promise<Sprite>}: A promise that resolves
+     *      to a newly constructed Sprite
+     */
+    static newRelativeTo(reference,direction) {
+        const liftResult = (result) => FfiConverterTypeSprite.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypePoint.checkType(reference)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("reference");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterTypeVector.checkType(direction)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("direction");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                87, // sprites:sprites_3c8d_Sprite_new_relative_to
+                FfiConverterTypePoint.lower(reference),
+                FfiConverterTypeVector.lower(direction),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    getPosition() {
+        const liftResult = (result) => FfiConverterTypePoint.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                88, // sprites:sprites_3c8d_Sprite_get_position
+                FfiConverterTypeSprite.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    moveTo(position) {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypePoint.checkType(position)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("position");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                89, // sprites:sprites_3c8d_Sprite_move_to
+                FfiConverterTypeSprite.lower(this),
+                FfiConverterTypePoint.lower(position),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    moveBy(direction) {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeVector.checkType(direction)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("direction");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                90, // sprites:sprites_3c8d_Sprite_move_by
+                FfiConverterTypeSprite.lower(this),
+                FfiConverterTypeVector.lower(direction),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeSprite extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new Sprite(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerSprite());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerSprite(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class Point {
+    constructor(x,y) {
+        try {
+            FfiConverterF64.checkType(x)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("x");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(y)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("y");
+            }
+            throw e;
+        }
+        this.x = x;
+        this.y = y;
+    }
+    equals(other) {
+        return (
+            this.x == other.x &&
+            this.y == other.y
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypePoint extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new Point(
+            FfiConverterF64.read(dataStream), 
+            FfiConverterF64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterF64.write(dataStream, value.x);
+        FfiConverterF64.write(dataStream, value.y);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterF64.computeSize(value.x);
+        totalSize += FfiConverterF64.computeSize(value.y);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterF64.checkType(value.x);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".x");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(value.y);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".y");
+            }
+            throw e;
+        }
+    }
+}
+
+export class Vector {
+    constructor(dx,dy) {
+        try {
+            FfiConverterF64.checkType(dx)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("dx");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(dy)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("dy");
+            }
+            throw e;
+        }
+        this.dx = dx;
+        this.dy = dy;
+    }
+    equals(other) {
+        return (
+            this.dx == other.dx &&
+            this.dy == other.dy
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeVector extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new Vector(
+            FfiConverterF64.read(dataStream), 
+            FfiConverterF64.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterF64.write(dataStream, value.dx);
+        FfiConverterF64.write(dataStream, value.dy);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterF64.computeSize(value.dx);
+        totalSize += FfiConverterF64.computeSize(value.dy);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterF64.checkType(value.dx);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".dx");
+            }
+            throw e;
+        }
+        try {
+            FfiConverterF64.checkType(value.dy);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".dy");
+            }
+            throw e;
+        }
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypePoint extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypePoint.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypePoint.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypePoint.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypePoint.computeSize(value)
+    }
+}
+
+
+
+
+
+export function translate(position,direction) {
+
+        const liftResult = (result) => FfiConverterTypePoint.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypePoint.checkType(position)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("position");
+                }
+                throw e;
+            }
+            try {
+                FfiConverterTypeVector.checkType(direction)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("direction");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                91, // sprites:sprites_3c8d_translate
+                FfiConverterTypePoint.lower(position),
+                FfiConverterTypeVector.lower(direction),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustTodolist.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustTodolist.sys.mjs
new file mode 100644
index 0000000000..f191cc7f6b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated/RustTodolist.sys.mjs
@@ -0,0 +1,925 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+
+    // Reads a TodoList pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointerTodoList() {
+        const pointerId = 6; // todolist:TodoList
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a TodoList pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointerTodoList(value) {
+        const pointerId = 6; // todolist:TodoList
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterString extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
+
+export class TodoList {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+    /**
+     * An async constructor for TodoList.
+     * 
+     * @returns {Promise<TodoList>}: A promise that resolves
+     *      to a newly constructed TodoList
+     */
+    static init() {
+        const liftResult = (result) => FfiConverterTypeTodoList.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                92, // todolist:todolist_4b78_TodoList_new
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }}
+
+    addItem(todo) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(todo)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("todo");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                93, // todolist:todolist_4b78_TodoList_add_item
+                FfiConverterTypeTodoList.lower(this),
+                FfiConverterString.lower(todo),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    addEntry(entry) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterTypeTodoEntry.checkType(entry)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("entry");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                94, // todolist:todolist_4b78_TodoList_add_entry
+                FfiConverterTypeTodoList.lower(this),
+                FfiConverterTypeTodoEntry.lower(entry),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    getEntries() {
+        const liftResult = (result) => FfiConverterSequenceTypeTodoEntry.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                95, // todolist:todolist_4b78_TodoList_get_entries
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    getItems() {
+        const liftResult = (result) => FfiConverterSequencestring.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                96, // todolist:todolist_4b78_TodoList_get_items
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    addEntries(entries) {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterSequenceTypeTodoEntry.checkType(entries)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("entries");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                97, // todolist:todolist_4b78_TodoList_add_entries
+                FfiConverterTypeTodoList.lower(this),
+                FfiConverterSequenceTypeTodoEntry.lower(entries),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    addItems(items) {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterSequencestring.checkType(items)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("items");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                98, // todolist:todolist_4b78_TodoList_add_items
+                FfiConverterTypeTodoList.lower(this),
+                FfiConverterSequencestring.lower(items),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    getLastEntry() {
+        const liftResult = (result) => FfiConverterTypeTodoEntry.lift(result);
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                99, // todolist:todolist_4b78_TodoList_get_last_entry
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    getLast() {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                100, // todolist:todolist_4b78_TodoList_get_last
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    getFirst() {
+        const liftResult = (result) => FfiConverterString.lift(result);
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                101, // todolist:todolist_4b78_TodoList_get_first
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    clearItem(todo) {
+        const liftResult = (result) => undefined;
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(todo)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("todo");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                102, // todolist:todolist_4b78_TodoList_clear_item
+                FfiConverterTypeTodoList.lower(this),
+                FfiConverterString.lower(todo),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+    makeDefault() {
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                103, // todolist:todolist_4b78_TodoList_make_default
+                FfiConverterTypeTodoList.lower(this),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+    }
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTodoList extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new TodoList(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointerTodoList());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointerTodoList(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
+
+export class TodoEntry {
+    constructor(text) {
+        try {
+            FfiConverterString.checkType(text)
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("text");
+            }
+            throw e;
+        }
+        this.text = text;
+    }
+    equals(other) {
+        return (
+            this.text == other.text
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTodoEntry extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new TodoEntry(
+            FfiConverterString.read(dataStream)
+        );
+    }
+    static write(dataStream, value) {
+        FfiConverterString.write(dataStream, value.text);
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        totalSize += FfiConverterString.computeSize(value.text);
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        try {
+            FfiConverterString.checkType(value.text);
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".text");
+            }
+            throw e;
+        }
+    }
+}
+
+
+
+export class TodoError extends Error {}
+
+
+export class TodoDoesNotExist extends TodoError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `TodoDoesNotExist: ${super.toString()}`
+    }
+}
+
+export class EmptyTodoList extends TodoError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `EmptyTodoList: ${super.toString()}`
+    }
+}
+
+export class DuplicateTodo extends TodoError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `DuplicateTodo: ${super.toString()}`
+    }
+}
+
+export class EmptyString extends TodoError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `EmptyString: ${super.toString()}`
+    }
+}
+
+export class DeligatedError extends TodoError {
+
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+    toString() {
+        return `DeligatedError: ${super.toString()}`
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterTypeTodoError extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            case 1:
+                return new TodoDoesNotExist(FfiConverterString.read(dataStream));
+            case 2:
+                return new EmptyTodoList(FfiConverterString.read(dataStream));
+            case 3:
+                return new DuplicateTodo(FfiConverterString.read(dataStream));
+            case 4:
+                return new EmptyString(FfiConverterString.read(dataStream));
+            case 5:
+                return new DeligatedError(FfiConverterString.read(dataStream));
+            default:
+                throw new Error("Unknown TodoError variant");
+        }
+    }
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        if (value instanceof TodoDoesNotExist) {
+            return totalSize;
+        }
+        if (value instanceof EmptyTodoList) {
+            return totalSize;
+        }
+        if (value instanceof DuplicateTodo) {
+            return totalSize;
+        }
+        if (value instanceof EmptyString) {
+            return totalSize;
+        }
+        if (value instanceof DeligatedError) {
+            return totalSize;
+        }
+        throw new Error("Unknown TodoError variant");
+    }
+    static write(dataStream, value) {
+        if (value instanceof TodoDoesNotExist) {
+            dataStream.writeInt32(1);
+            return;
+        }
+        if (value instanceof EmptyTodoList) {
+            dataStream.writeInt32(2);
+            return;
+        }
+        if (value instanceof DuplicateTodo) {
+            dataStream.writeInt32(3);
+            return;
+        }
+        if (value instanceof EmptyString) {
+            dataStream.writeInt32(4);
+            return;
+        }
+        if (value instanceof DeligatedError) {
+            dataStream.writeInt32(5);
+            return;
+        }
+        throw new Error("Unknown TodoError variant");
+    }
+
+    static errorClass = TodoError;
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterOptionalTypeTodoList extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            FfiConverterTypeTodoList.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return FfiConverterTypeTodoList.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        FfiConverterTypeTodoList.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + FfiConverterTypeTodoList.computeSize(value)
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequencestring extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterString.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterString.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterString.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterString.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class FfiConverterSequenceTypeTodoEntry extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push(FfiConverterTypeTodoEntry.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            FfiConverterTypeTodoEntry.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += FfiConverterTypeTodoEntry.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                FfiConverterTypeTodoEntry.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
+
+
+
+
+
+export function getDefaultList() {
+
+        const liftResult = (result) => FfiConverterOptionalTypeTodoList.lift(result);
+        const liftError = null;
+        const functionCall = () => {
+            return UniFFIScaffolding.callAsync(
+                104, // todolist:todolist_4b78_get_default_list
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function setDefaultList(list) {
+
+        const liftResult = (result) => undefined;
+        const liftError = null;
+        const functionCall = () => {
+            try {
+                FfiConverterTypeTodoList.checkType(list)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("list");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                105, // todolist:todolist_4b78_set_default_list
+                FfiConverterTypeTodoList.lower(list),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
+
+export function createEntryWith(todo) {
+
+        const liftResult = (result) => FfiConverterTypeTodoEntry.lift(result);
+        const liftError = (data) => FfiConverterTypeTodoError.lift(data);
+        const functionCall = () => {
+            try {
+                FfiConverterString.checkType(todo)
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("todo");
+                }
+                throw e;
+            }
+            return UniFFIScaffolding.callAsync(
+                106, // todolist:todolist_4b78_create_entry_with
+                FfiConverterString.lower(todo),
+            )
+        }
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/moz.build b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/moz.build
new file mode 100644
index 0000000000..704efd8a9d
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/moz.build
@@ -0,0 +1,24 @@
+# -*- Mode: python; indent-tabs-mode: nil; tab-width: 40 -*-
+# vim: set filetype=python:
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+FINAL_LIBRARY = "xul"
+
+components = [
+    "Arithmetic",
+    "CustomTypes",
+    "ExternalTypes",
+    "FixtureCallbacks",
+    "Geometry",
+    "Rondpoint",
+    "Sprites",
+    "Todolist",
+]
+
+EXTRA_JS_MODULES += [
+    "generated/Rust{}.sys.mjs".format(component) for component in components
+]
+
+XPCSHELL_TESTS_MANIFESTS += ["tests/xpcshell/xpcshell.ini"]
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_arithmetic.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_arithmetic.js
new file mode 100644
index 0000000000..a221abc39f
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_arithmetic.js
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Arithmetic = ChromeUtils.importESModule(
+  "resource://gre/modules/RustArithmetic.sys.mjs"
+);
+
+add_task(async function () {
+  Assert.ok(Arithmetic.IntegerOverflow);
+  Assert.equal(await Arithmetic.add(2, 4), 6);
+  Assert.equal(await Arithmetic.add(4, 8), 12);
+  // For other backends we would have this test:
+  // await Assert.rejects(
+  //     Arithmetic.add(18446744073709551615, 1),
+  //     Arithmetic.IntegerOverflow,
+  //     "add() should throw IntegerOverflow")
+  //
+  // However, this doesn't work because JS number values are actually 64-bit
+  // floats, and that number is greater than the maximum "safe" integer.
+  //
+  // Instead, let's test that we reject numbers that are that big
+  await Assert.rejects(
+    Arithmetic.add(Number.MAX_SAFE_INTEGER + 1, 0),
+    /TypeError/,
+    "add() should throw TypeError when an input is > MAX_SAFE_INTEGER"
+  );
+
+  Assert.equal(await Arithmetic.sub(4, 2), 2);
+  Assert.equal(await Arithmetic.sub(8, 4), 4);
+  await Assert.rejects(
+    Arithmetic.sub(0, 1),
+    Arithmetic.IntegerOverflow,
+    "sub() should throw IntegerOverflow"
+  );
+
+  Assert.equal(await Arithmetic.div(8, 4), 2);
+  // Can't test this, because we don't allow Rust panics in FF
+  // Assert.rejects(
+  //     Arithmetic.div(8, 0),
+  //     (e) => Assert.equal(e, Arithmetic.UniFFIInternalError),
+  //     "Divide by 0 should throw UniFFIInternalError")
+  //
+  Assert.ok(await Arithmetic.equal(2, 2));
+  Assert.ok(await Arithmetic.equal(4, 4));
+
+  Assert.ok(!(await Arithmetic.equal(2, 4)));
+  Assert.ok(!(await Arithmetic.equal(4, 8)));
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_callbacks.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_callbacks.js
new file mode 100644
index 0000000000..67f0a167a8
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_callbacks.js
@@ -0,0 +1,47 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { logEvenNumbers, logEvenNumbersMainThread } = ChromeUtils.importESModule(
+  "resource://gre/modules/RustFixtureCallbacks.sys.mjs"
+);
+
+class Logger {
+  constructor() {
+    this.messages = [];
+    this.finishedPromise = new Promise((resolve, reject) => {
+      this.finishedResolve = resolve;
+      this.finishedReject = reject;
+    });
+  }
+
+  log(message) {
+    this.messages.push(message);
+  }
+
+  finished() {
+    this.finishedResolve(true);
+  }
+
+  async waitForFinish() {
+    // Set a timeout to avoid hanging the tests if the Rust code fails to call finished().
+    do_timeout(2000, () =>
+      this.finishedReject("Timeout waiting for finished()")
+    );
+    return this.finishedPromise;
+  }
+}
+
+add_task(async function testLogEvenNumbers() {
+  async function runTest(logEvenNumbersFunc) {
+    const logger = new Logger();
+    logEvenNumbersFunc(logger, [1, 1, 2, 3, 5, 8, 13]);
+    await logger.waitForFinish();
+    Assert.deepEqual(logger.messages, [
+      "Saw even number: 2",
+      "Saw even number: 8",
+    ]);
+  }
+
+  await runTest(logEvenNumbers);
+  await runTest(logEvenNumbersMainThread);
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_custom_types.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_custom_types.js
new file mode 100644
index 0000000000..cd26467494
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_custom_types.js
@@ -0,0 +1,13 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const CustomTypes = ChromeUtils.importESModule(
+  "resource://gre/modules/RustCustomTypes.sys.mjs"
+);
+
+add_task(async function () {
+  // JS right now doesn't treat custom types as anything but it's native counterparts
+  let demo = await CustomTypes.getCustomTypesDemo();
+  Assert.equal(demo.url, "http://example.com/");
+  Assert.equal(demo.handle, 123);
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_external_types.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_external_types.js
new file mode 100644
index 0000000000..e2f985cde7
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_external_types.js
@@ -0,0 +1,16 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const ExternalTypes = ChromeUtils.importESModule(
+  "resource://gre/modules/RustExternalTypes.sys.mjs"
+);
+
+add_task(async function () {
+  const line = new ExternalTypes.Line(
+    new ExternalTypes.Point(0, 0, "p1"),
+    new ExternalTypes.Point(2, 1, "p2")
+  );
+  Assert.equal(await ExternalTypes.gradient(line), 0.5);
+
+  Assert.equal(await ExternalTypes.gradient(null), 0.0);
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_geometry.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_geometry.js
new file mode 100644
index 0000000000..f06ecd46aa
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_geometry.js
@@ -0,0 +1,21 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Geometry = ChromeUtils.importESModule(
+  "resource://gre/modules/RustGeometry.sys.mjs"
+);
+
+add_task(async function () {
+  const ln1 = new Geometry.Line(
+    new Geometry.Point(0, 0, "p1"),
+    new Geometry.Point(1, 2, "p2")
+  );
+  const ln2 = new Geometry.Line(
+    new Geometry.Point(1, 1, "p3"),
+    new Geometry.Point(2, 2, "p4")
+  );
+  const origin = new Geometry.Point(0, 0);
+  Assert.ok((await Geometry.intersection(ln1, ln2)).equals(origin));
+  Assert.deepEqual(await Geometry.intersection(ln1, ln2), origin);
+  Assert.strictEqual(await Geometry.intersection(ln1, ln1), null);
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_rondpoint.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_rondpoint.js
new file mode 100644
index 0000000000..8c673b2e92
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_rondpoint.js
@@ -0,0 +1,311 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Rondpoint = ChromeUtils.importESModule(
+  "resource://gre/modules/RustRondpoint.sys.mjs"
+);
+
+const {
+  Dictionnaire,
+  Enumeration,
+  copieDictionnaire,
+  copieEnumeration,
+  copieEnumerations,
+  copieCarte,
+  EnumerationAvecDonnees,
+  switcheroo,
+  Retourneur,
+  DictionnaireNombresSignes,
+  DictionnaireNombres,
+  Stringifier,
+  Optionneur,
+  OptionneurDictionnaire,
+} = Rondpoint;
+add_task(async function () {
+  const dico = new Dictionnaire(Enumeration.DEUX, true, 0, 1235);
+  const copyDico = await copieDictionnaire(dico);
+  Assert.deepEqual(dico, copyDico);
+
+  Assert.equal(await copieEnumeration(Enumeration.DEUX), Enumeration.DEUX);
+  Assert.deepEqual(
+    await copieEnumerations([Enumeration.UN, Enumeration.DEUX]),
+    [Enumeration.UN, Enumeration.DEUX]
+  );
+  const obj = {
+    0: new EnumerationAvecDonnees.Zero(),
+    1: new EnumerationAvecDonnees.Un(1),
+    2: new EnumerationAvecDonnees.Deux(2, "deux"),
+  };
+
+  Assert.deepEqual(await copieCarte(obj), obj);
+
+  const zero = new EnumerationAvecDonnees.Zero();
+  const one = new EnumerationAvecDonnees.Un(1);
+  const two = new EnumerationAvecDonnees.Deux(2);
+  Assert.notEqual(zero, one);
+  Assert.notEqual(one, two);
+
+  Assert.deepEqual(zero, new EnumerationAvecDonnees.Zero());
+  Assert.deepEqual(one, new EnumerationAvecDonnees.Un(1));
+  Assert.notDeepEqual(one, new EnumerationAvecDonnees.Un(4));
+
+  Assert.ok(await switcheroo(false));
+  // Test the roundtrip across the FFI.
+  // This shows that the values we send come back in exactly the same state as we sent them.
+  // i.e. it shows that lowering from JS and lifting into rust is symmetrical with
+  //      lowering from rust and lifting into JS.
+
+  const rt = await Retourneur.init();
+
+  const affirmAllerRetour = async (arr, fn, equalFn) => {
+    for (const member of arr) {
+      if (equalFn) {
+        equalFn(await fn(member), member);
+      } else {
+        Assert.equal(await fn(member), member);
+      }
+    }
+  };
+
+  // Booleans
+  await affirmAllerRetour([true, false], rt.identiqueBoolean.bind(rt));
+
+  // Bytes
+  await affirmAllerRetour([-128, 127], rt.identiqueI8.bind(rt));
+  await affirmAllerRetour([0, 0xff], rt.identiqueU8.bind(rt));
+
+  // Shorts
+  await affirmAllerRetour([-32768, 32767], rt.identiqueI16.bind(rt));
+  await affirmAllerRetour([0, 0xffff], rt.identiqueU16.bind(rt));
+
+  // Ints
+  await affirmAllerRetour(
+    [0, 1, -1, -2147483648, 2147483647],
+    rt.identiqueI32.bind(rt)
+  );
+  await affirmAllerRetour([0, 0xffffffff], rt.identiqueU32.bind(rt));
+
+  // Longs
+  // NOTE: we cannot represent greater than `Number.MAX_SAFE_INTEGER`
+  await affirmAllerRetour(
+    [0, 1, -1, Number.MAX_SAFE_INTEGER, Number.MIN_SAFE_INTEGER],
+    rt.identiqueI64.bind(rt)
+  );
+  await affirmAllerRetour(
+    [0, Number.MAX_SAFE_INTEGER],
+    rt.identiqueU64.bind(rt)
+  );
+
+  // Floats
+  const equalFloats = (a, b) => Assert.ok(Math.abs(a - b) <= Number.EPSILON);
+  await affirmAllerRetour(
+    [0.0, 0.5, 0.25, 1.5],
+    rt.identiqueFloat.bind(rt),
+    equalFloats
+  );
+  //   Some float value's precision gets messed up, an example is 3.22, 100.223, etc
+  //   await affirmAllerRetour([0.0, 0.5, 0.25, 1.5, 100.223], rt.identiqueFloat.bind(rt), equalFloats);
+
+  // Double (although on the JS side doubles are limited since they are also represented by Number)
+  await affirmAllerRetour(
+    [0.0, 0.5, 0.25, 1.5],
+    rt.identiqueDouble.bind(rt),
+    equalFloats
+  );
+
+  // Strings
+  await affirmAllerRetour(
+    [
+      "",
+      "abc",
+      "null\u0000byte",
+      "été",
+      "ښي لاس ته لوستلو لوستل",
+      "😻emoji 👨‍👧‍👦multi-emoji, 🇨🇭a flag, a canal, panama",
+    ],
+    rt.identiqueString.bind(rt)
+  );
+
+  await affirmAllerRetour(
+    [-1, 0, 1].map(n => new DictionnaireNombresSignes(n, n, n, n)),
+    rt.identiqueNombresSignes.bind(rt),
+    (a, b) => Assert.deepEqual(a, b)
+  );
+
+  await affirmAllerRetour(
+    [0, 1].map(n => new DictionnaireNombres(n, n, n, n)),
+    rt.identiqueNombres.bind(rt),
+    (a, b) => Assert.deepEqual(a, b)
+  );
+
+  // Test one way across the FFI.
+  //
+  // We send one representation of a value to lib.rs, and it transforms it into another, a string.
+  // lib.rs sends the string back, and then we compare here in js.
+  //
+  // This shows that the values are transformed into strings the same way in both js and rust.
+  // i.e. if we assume that the string return works (we test this assumption elsewhere)
+  //      we show that lowering from js and lifting into rust has values that both js and rust
+  //      both stringify in the same way. i.e. the same values.
+  //
+  // If we roundtripping proves the symmetry of our lowering/lifting from here to rust, and lowering/lifting from rust to here,
+  // and this convinces us that lowering/lifting from here to rust is correct, then
+  // together, we've shown the correctness of the return leg.
+  const st = await Stringifier.init();
+
+  const affirmEnchaine = async (arr, fn) => {
+    for (const member of arr) {
+      Assert.equal(await fn(member), String(member));
+    }
+  };
+
+  // Booleans
+  await affirmEnchaine([true, false], st.toStringBoolean.bind(st));
+
+  // Bytes
+  await affirmEnchaine([-128, 127], st.toStringI8.bind(st));
+  await affirmEnchaine([0, 0xff], st.toStringU8.bind(st));
+
+  // Shorts
+  await affirmEnchaine([-32768, 32767], st.toStringI16.bind(st));
+  await affirmEnchaine([0, 0xffff], st.toStringU16.bind(st));
+
+  // Ints
+  await affirmEnchaine(
+    [0, 1, -1, -2147483648, 2147483647],
+    st.toStringI32.bind(st)
+  );
+  await affirmEnchaine([0, 0xffffffff], st.toStringU32.bind(st));
+
+  // Longs
+  // NOTE: we cannot represent greater than `Number.MAX_SAFE_INTEGER`
+  await affirmEnchaine(
+    [0, 1, -1, Number.MAX_SAFE_INTEGER, Number.MIN_SAFE_INTEGER],
+    st.toStringI64.bind(st)
+  );
+  await affirmEnchaine([0, Number.MAX_SAFE_INTEGER], st.toStringU64.bind(st));
+
+  // Floats
+  await affirmEnchaine([0.0, 0.5, 0.25, 1.5], st.toStringFloat.bind(st));
+
+  // Doubles
+  await affirmEnchaine([0.0, 0.5, 0.25, 1.5], st.toStringDouble.bind(st));
+
+  // Prove to ourselves that default arguments are being used.
+  // Step 1: call the methods without arguments, and check against the UDL.
+  const op = await Optionneur.init();
+
+  Assert.equal(await op.sinonString(), "default");
+
+  Assert.ok(!(await op.sinonBoolean()));
+
+  Assert.deepEqual(await op.sinonSequence(), []);
+
+  Assert.equal(await op.sinonNull(), null);
+  Assert.equal(await op.sinonZero(), 0);
+
+  // decimal integers
+  Assert.equal(await op.sinonI8Dec(), -42);
+  Assert.equal(await op.sinonU8Dec(), 42);
+  Assert.equal(await op.sinonI16Dec(), 42);
+  Assert.equal(await op.sinonU16Dec(), 42);
+  Assert.equal(await op.sinonI32Dec(), 42);
+  Assert.equal(await op.sinonU32Dec(), 42);
+  Assert.equal(await op.sinonI64Dec(), 42);
+  Assert.equal(await op.sinonU64Dec(), 42);
+
+  // hexadecimal integers
+  Assert.equal(await op.sinonI8Hex(), -0x7f);
+  Assert.equal(await op.sinonU8Hex(), 0xff);
+  Assert.equal(await op.sinonI16Hex(), 0x7f);
+  Assert.equal(await op.sinonU16Hex(), 0xffff);
+  Assert.equal(await op.sinonI32Hex(), 0x7fffffff);
+  Assert.equal(await op.sinonU32Hex(), 0xffffffff);
+  // The following are too big to be represented by js `Number`
+  // Assert.equal(await op.sinonI64Hex(), 0x7fffffffffffffff);
+  // Assert.equal(await op.sinonU64Hex(), 0xffffffffffffffff);
+
+  // octal integers
+  Assert.equal(await op.sinonU32Oct(), 0o755);
+
+  // floats
+  Assert.equal(await op.sinonF32(), 42.0);
+  Assert.equal(await op.sinonF64(), 42.1);
+
+  // enums
+  Assert.equal(await op.sinonEnum(), Enumeration.TROIS);
+
+  // Step 2. Convince ourselves that if we pass something else, then that changes the output.
+  //         We have shown something coming out of the sinon methods, but without eyeballing the Rust
+  //         we can't be sure that the arguments will change the return value.
+
+  await affirmAllerRetour(["foo", "bar"], op.sinonString.bind(op));
+  await affirmAllerRetour([true, false], op.sinonBoolean.bind(op));
+  await affirmAllerRetour([["a", "b"], []], op.sinonSequence.bind(op), (a, b) =>
+    Assert.deepEqual(a, b)
+  );
+
+  // Optionals
+  await affirmAllerRetour(["0", "1"], op.sinonNull.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonZero.bind(op));
+
+  // integers
+  await affirmAllerRetour([0, 1], op.sinonU8Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI8Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU16Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI16Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU32Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI32Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU64Dec.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI64Dec.bind(op));
+
+  await affirmAllerRetour([0, 1], op.sinonU8Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI8Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU16Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI16Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU32Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI32Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU64Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonI64Hex.bind(op));
+  await affirmAllerRetour([0, 1], op.sinonU32Oct.bind(op));
+
+  // Floats
+  await affirmAllerRetour([0.0, 1.0], op.sinonF32.bind(op));
+  await affirmAllerRetour([0.0, 1.0], op.sinonF64.bind(op));
+
+  // enums
+  await affirmAllerRetour(
+    [Enumeration.UN, Enumeration.DEUX, Enumeration.TROIS],
+    op.sinonEnum.bind(op)
+  );
+
+  // Testing defaulting properties in record types.
+  const defaultes = new OptionneurDictionnaire();
+  const explicite = new OptionneurDictionnaire(
+    -8,
+    8,
+    -16,
+    0x10,
+    -32,
+    32,
+    -64,
+    64,
+    4.0,
+    8.0,
+    true,
+    "default",
+    [],
+    Enumeration.DEUX,
+    null
+  );
+
+  Assert.deepEqual(defaultes, explicite);
+
+  // …and makes sure they travel across and back the FFI.
+
+  await affirmAllerRetour(
+    [defaultes, explicite],
+    rt.identiqueOptionneurDictionnaire.bind(rt),
+    (a, b) => Assert.deepEqual(a, b)
+  );
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_sprites.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_sprites.js
new file mode 100644
index 0000000000..3feb2fd34d
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_sprites.js
@@ -0,0 +1,28 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Sprites = ChromeUtils.importESModule(
+  "resource://gre/modules/RustSprites.sys.mjs"
+);
+
+add_task(async function () {
+  Assert.ok(Sprites.Sprite);
+
+  const sempty = await Sprites.Sprite.init(null);
+  Assert.deepEqual(await sempty.getPosition(), new Sprites.Point(0, 0));
+
+  const s = await Sprites.Sprite.init(new Sprites.Point(0, 1));
+  Assert.deepEqual(await s.getPosition(), new Sprites.Point(0, 1));
+
+  s.moveTo(new Sprites.Point(1, 2));
+  Assert.deepEqual(await s.getPosition(), new Sprites.Point(1, 2));
+
+  s.moveBy(new Sprites.Vector(-4, 2));
+  Assert.deepEqual(await s.getPosition(), new Sprites.Point(-3, 4));
+
+  const srel = await Sprites.Sprite.newRelativeTo(
+    new Sprites.Point(0, 1),
+    new Sprites.Vector(1, 1.5)
+  );
+  Assert.deepEqual(await srel.getPosition(), new Sprites.Point(1, 2.5));
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_todolist.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_todolist.js
new file mode 100644
index 0000000000..dac26d2be1
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_todolist.js
@@ -0,0 +1,71 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const { TodoList, TodoEntry, getDefaultList, setDefaultList } =
+  ChromeUtils.importESModule("resource://gre/modules/RustTodolist.sys.mjs");
+
+add_task(async function () {
+  const todo = await TodoList.init();
+  const entry = new TodoEntry("Write bindings for strings in records");
+
+  await todo.addItem("Write JS bindings");
+  Assert.equal(await todo.getLast(), "Write JS bindings");
+
+  await todo.addItem("Write tests for bindings");
+  Assert.equal(await todo.getLast(), "Write tests for bindings");
+
+  await todo.addEntry(entry);
+  Assert.equal(await todo.getLast(), "Write bindings for strings in records");
+  Assert.equal(
+    (await todo.getLastEntry()).text,
+    "Write bindings for strings in records"
+  );
+  Assert.ok((await todo.getLastEntry()).equals(entry));
+
+  await todo.addItem(
+    "Test Ünicode hàndling without an entry can't believe I didn't test this at first 🤣"
+  );
+  Assert.equal(
+    await todo.getLast(),
+    "Test Ünicode hàndling without an entry can't believe I didn't test this at first 🤣"
+  );
+
+  const entry2 = new TodoEntry(
+    "Test Ünicode hàndling in an entry can't believe I didn't test this at first 🤣"
+  );
+  await todo.addEntry(entry2);
+  Assert.equal(
+    (await todo.getLastEntry()).text,
+    "Test Ünicode hàndling in an entry can't believe I didn't test this at first 🤣"
+  );
+
+  const todo2 = await TodoList.init();
+  Assert.notEqual(todo, todo2);
+  Assert.notStrictEqual(todo, todo2);
+
+  Assert.strictEqual(await getDefaultList(), null);
+
+  await setDefaultList(todo);
+  Assert.deepEqual(
+    await todo.getItems(),
+    await (await getDefaultList()).getItems()
+  );
+
+  todo2.makeDefault();
+  Assert.deepEqual(
+    await todo2.getItems(),
+    await (await getDefaultList()).getItems()
+  );
+
+  await todo.addItem("Test liveness after being demoted from default");
+  Assert.equal(
+    await todo.getLast(),
+    "Test liveness after being demoted from default"
+  );
+
+  todo2.addItem("Test shared state through local vs default reference");
+  Assert.equal(
+    await (await getDefaultList()).getLast(),
+    "Test shared state through local vs default reference"
+  );
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_type_checking.js b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_type_checking.js
new file mode 100644
index 0000000000..bfeb07c82b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/test_type_checking.js
@@ -0,0 +1,123 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+const Arithmetic = ChromeUtils.importESModule(
+  "resource://gre/modules/RustArithmetic.sys.mjs"
+);
+const Geometry = ChromeUtils.importESModule(
+  "resource://gre/modules/RustGeometry.sys.mjs"
+);
+const TodoList = ChromeUtils.importESModule(
+  "resource://gre/modules/RustTodolist.sys.mjs"
+);
+const Rondpoint = ChromeUtils.importESModule(
+  "resource://gre/modules/RustRondpoint.sys.mjs"
+);
+const { UniFFITypeError } = ChromeUtils.importESModule(
+  "resource://gre/modules/UniFFI.sys.mjs"
+);
+
+add_task(async function testFunctionArguments() {
+  await Assert.rejects(
+    Arithmetic.add(2),
+    UniFFITypeError,
+    "add() call missing argument"
+  );
+  Assert.throws(
+    () => new Geometry.Point(0.0),
+    UniFFITypeError,
+    "Point constructor missing argument"
+  );
+});
+
+add_task(async function testObjectPointers() {
+  const todo = await TodoList.TodoList.init();
+  const stringifier = await Rondpoint.Stringifier.init();
+  await todo.getEntries(); // OK
+  todo[TodoList.UnitTestObjs.uniffiObjectPtr] =
+    stringifier[Rondpoint.UnitTestObjs.uniffiObjectPtr];
+
+  await Assert.rejects(
+    todo.getEntries(), // the pointer is incorrect, should throw
+    /Bad pointer type/,
+    "getEntries() with wrong pointer type"
+  );
+});
+
+add_task(async function testEnumTypeCheck() {
+  await Assert.rejects(
+    Rondpoint.copieEnumeration("invalid"), // Not an integer value
+    /e:/, // Ensure exception message includes the argument name
+    "copieEnumeration() with non-Enumeration value should throw"
+  );
+  await Assert.rejects(
+    Rondpoint.copieEnumeration(0), // Integer, but doesn't map to a variant
+    /e:/, // Ensure exception message includes the argument name
+    "copieEnumeration() with non-Enumeration value should throw"
+  );
+  await Assert.rejects(
+    Rondpoint.copieEnumeration(4), // Integer, but doesn't map to a variant
+    /e:/, // Ensure exception message includes the argument name
+    "copieEnumeration() with non-Enumeration value should throw"
+  );
+});
+
+add_task(async function testRecordTypeCheck() {
+  await Assert.rejects(
+    Geometry.gradient(123), // Not a Line object
+    UniFFITypeError,
+    "gradient with non-Line object should throw"
+  );
+
+  await Assert.rejects(
+    Geometry.gradient({
+      start: {
+        coordX: 0.0,
+        coordY: 0.0,
+      },
+      // missing the end field
+    }),
+    /ln.end/, // Ensure exception message includes the argument name
+    "gradient with Line object with missing end field should throw"
+  );
+});
+
+add_task(async function testOptionTypeCheck() {
+  const optionneur = await Rondpoint.Optionneur.init();
+  await Assert.rejects(
+    optionneur.sinonNull(0),
+    UniFFITypeError,
+    "sinonNull with non-string should throw"
+  );
+});
+
+add_task(async function testSequenceTypeCheck() {
+  const todo = await TodoList.TodoList.init();
+  await Assert.rejects(
+    todo.addEntries("not a list"),
+    UniFFITypeError,
+    "addEntries with non-list should throw"
+  );
+
+  await Assert.rejects(
+    todo.addEntries(["not TodoEntry"]),
+    /entries\[0]/,
+    "addEntries with non TodoEntry item should throw"
+  );
+});
+
+add_task(async function testMapTypeCheck() {
+  await Assert.rejects(
+    Rondpoint.copieCarte("not a map"),
+    UniFFITypeError,
+    "copieCarte with a non-map should throw"
+  );
+
+  await Assert.rejects(
+    Rondpoint.copieCarte({ x: 1 }),
+    /c\[x]/,
+    "copieCarte with a wrong value type should throw"
+  );
+
+  // TODO: test key types once we implement https://bugzilla.mozilla.org/show_bug.cgi?id=1809459
+});
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/xpcshell.ini b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/xpcshell.ini
new file mode 100644
index 0000000000..87d2b43489
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/fixtures/tests/xpcshell/xpcshell.ini
@@ -0,0 +1,9 @@
+[test_arithmetic.js]
+[test_callbacks.js]
+[test_geometry.js]
+[test_rondpoint.js]
+[test_sprites.js]
+[test_todolist.js]
+[test_type_checking.js]
+[test_custom_types.js]
+[test_external_types.js]
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/mach_commands.py b/toolkit/components/uniffi-bindgen-gecko-js/mach_commands.py
new file mode 100644
index 0000000000..68c2c3d58b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/mach_commands.py
@@ -0,0 +1,92 @@
+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+import os
+import subprocess
+
+from mach.decorators import Command, SubCommand
+
+# IMPORTANT: Please Request review from a DOM peer before
+# committing to using UniFFI. There are other ways to consume Rust from
+# JavaScript that might fit your use case better.
+UDL_FILES = [
+    "third_party/rust/tabs/src/tabs.udl",
+]
+
+FIXTURE_UDL_FILES = [
+    "third_party/rust/uniffi-example-geometry/src/geometry.udl",
+    "third_party/rust/uniffi-example-arithmetic/src/arithmetic.udl",
+    "third_party/rust/uniffi-example-rondpoint/src/rondpoint.udl",
+    "third_party/rust/uniffi-example-sprites/src/sprites.udl",
+    "third_party/rust/uniffi-example-todolist/src/todolist.udl",
+    "toolkit/components/uniffi-fixture-callbacks/src/callbacks.udl",
+    "toolkit/components/uniffi-example-custom-types/src/custom-types.udl",
+    "toolkit/components/uniffi-fixture-external-types/src/external-types.udl",
+]
+CPP_PATH = "toolkit/components/uniffi-js/UniFFIGeneratedScaffolding.cpp"
+JS_DIR = "toolkit/components/uniffi-bindgen-gecko-js/components/generated"
+FIXTURE_CPP_PATH = "toolkit/components/uniffi-js/UniFFIFixtureScaffolding.cpp"
+FIXTURE_JS_DIR = "toolkit/components/uniffi-bindgen-gecko-js/fixtures/generated"
+
+
+def build_uniffi_bindgen_gecko_js(command_context):
+    uniffi_root = crate_root(command_context)
+    print("Building uniffi-bindgen-gecko-js")
+    cmdline = [
+        "cargo",
+        "build",
+        "--release",
+        "--manifest-path",
+        os.path.join(command_context.topsrcdir, "Cargo.toml"),
+        "--package",
+        "uniffi-bindgen-gecko-js",
+    ]
+    subprocess.check_call(cmdline, cwd=uniffi_root)
+    print()
+    return os.path.join(
+        command_context.topsrcdir, "target", "release", "uniffi-bindgen-gecko-js"
+    )
+
+
+@Command(
+    "uniffi",
+    category="devenv",
+    description="Generate JS bindings using uniffi-bindgen-gecko-js",
+)
+def uniffi(command_context, *runargs, **lintargs):
+    """Run uniffi."""
+    command_context._sub_mach(["help", "uniffi"])
+    return 1
+
+
+@SubCommand(
+    "uniffi",
+    "generate",
+    description="Generate/regenerate bindings",
+)
+def generate_command(command_context):
+    binary_path = build_uniffi_bindgen_gecko_js(command_context)
+    cmdline = [
+        binary_path,
+        "--js-dir",
+        JS_DIR,
+        "--fixture-js-dir",
+        FIXTURE_JS_DIR,
+        "--cpp-path",
+        CPP_PATH,
+        "--fixture-cpp-path",
+        FIXTURE_CPP_PATH,
+    ]
+    if UDL_FILES:
+        cmdline += ["--udl-files"] + UDL_FILES
+    if FIXTURE_UDL_FILES:
+        cmdline += ["--fixture-udl-files"] + FIXTURE_UDL_FILES
+    subprocess.check_call(cmdline, cwd=command_context.topsrcdir)
+    return 0
+
+
+def crate_root(command_context):
+    return os.path.join(
+        command_context.topsrcdir, "toolkit", "components", "uniffi-bindgen-gecko-js"
+    )
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/ci_list.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/ci_list.rs
new file mode 100644
index 0000000000..8a6603aa7d
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/ci_list.rs
@@ -0,0 +1,193 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+//! Manage the universe of ComponentInterfaces / Configs
+//!
+//! uniffi-bindgen-gecko-js is unique because it generates bindings over a set of UDL files rather
+//! than just one.  This is because we want to generate the WebIDL statically rather than generate
+//! it.  To accomplish that, each WebIDL function inputs an opaque integer id that identifies which
+//! version of it should run, for example `CallSync` inputs a function id.  Operating on all UDL
+//! files at once simplifies the task of ensuring those ids are to be unique and consistent between
+//! the JS and c++ code.
+//!
+//! This module manages the list of ComponentInterface and the object ids.
+
+use crate::render::cpp::ComponentInterfaceCppExt;
+use crate::{Config, ConfigMap};
+use anyhow::{bail, Context, Result};
+use camino::Utf8PathBuf;
+use std::collections::{BTreeSet, HashMap, HashSet};
+use uniffi_bindgen::interface::{CallbackInterface, ComponentInterface, FfiFunction, Object};
+
+pub struct ComponentUniverse {
+    pub components: Vec<(ComponentInterface, Config)>,
+    pub fixture_components: Vec<(ComponentInterface, Config)>,
+}
+
+impl ComponentUniverse {
+    pub fn new(
+        udl_files: Vec<Utf8PathBuf>,
+        fixture_udl_files: Vec<Utf8PathBuf>,
+        config_map: ConfigMap,
+    ) -> Result<Self> {
+        let components = udl_files
+            .into_iter()
+            .map(|udl_file| parse_udl_file(udl_file, &config_map))
+            .collect::<Result<Vec<_>>>()?;
+        let fixture_components = fixture_udl_files
+            .into_iter()
+            .map(|udl_file| parse_udl_file(udl_file, &config_map))
+            .collect::<Result<Vec<_>>>()?;
+        let universe = Self {
+            components,
+            fixture_components,
+        };
+        universe.check_udl_namespaces_unique()?;
+        universe.check_callback_interfaces()?;
+        Ok(universe)
+    }
+
+    fn check_udl_namespaces_unique(&self) -> Result<()> {
+        let mut set = HashSet::new();
+        for ci in self.iter_cis() {
+            if !set.insert(ci.namespace()) {
+                bail!("UDL files have duplicate namespace: {}", ci.namespace());
+            }
+        }
+        Ok(())
+    }
+
+    fn check_callback_interfaces(&self) -> Result<()> {
+        // We don't currently support callback interfaces returning values or throwing errors.
+        for ci in self.iter_cis() {
+            for cbi in ci.callback_interface_definitions() {
+                for method in cbi.methods() {
+                    if method.return_type().is_some() {
+                        bail!("Callback interface method {}.{} throws an error, which is not yet supported", cbi.name(), method.name())
+                    }
+                    if method.throws_type().is_some() {
+                        bail!("Callback interface method {}.{} returns a value, which is not yet supported", cbi.name(), method.name())
+                    }
+                }
+            }
+        }
+        Ok(())
+    }
+
+    pub fn iter_cis(&self) -> impl Iterator<Item = &ComponentInterface> {
+        self.components
+            .iter()
+            .chain(self.fixture_components.iter())
+            .map(|(ci, _)| ci)
+    }
+}
+
+fn parse_udl_file(
+    udl_file: Utf8PathBuf,
+    config_map: &ConfigMap,
+) -> Result<(ComponentInterface, Config)> {
+    let udl = std::fs::read_to_string(&udl_file).context("Error reading UDL file")?;
+    let ci = ComponentInterface::from_webidl(&udl).context("Failed to parse UDL")?;
+    let config = config_map.get(ci.namespace()).cloned().unwrap_or_default();
+    Ok((ci, config))
+}
+
+pub struct FunctionIds<'a> {
+    // Map (CI namespace, func name) -> Ids
+    map: HashMap<(&'a str, &'a str), usize>,
+}
+
+impl<'a> FunctionIds<'a> {
+    pub fn new(cis: &'a ComponentUniverse) -> Self {
+        Self {
+            map: cis
+                .iter_cis()
+                .flat_map(|ci| {
+                    ci.exposed_functions()
+                        .into_iter()
+                        .map(move |f| (ci.namespace(), f.name()))
+                })
+                .enumerate()
+                .map(|(i, (namespace, name))| ((namespace, name), i))
+                // Sort using BTreeSet to guarantee the IDs remain stable across runs
+                .collect::<BTreeSet<_>>()
+                .into_iter()
+                .collect(),
+        }
+    }
+
+    pub fn get(&self, ci: &ComponentInterface, func: &FfiFunction) -> usize {
+        return *self.map.get(&(ci.namespace(), func.name())).unwrap();
+    }
+
+    pub fn name(&self, ci: &ComponentInterface, func: &FfiFunction) -> String {
+        format!("{}:{}", ci.namespace(), func.name())
+    }
+}
+
+pub struct ObjectIds<'a> {
+    // Map (CI namespace, object name) -> Ids
+    map: HashMap<(&'a str, &'a str), usize>,
+}
+
+impl<'a> ObjectIds<'a> {
+    pub fn new(cis: &'a ComponentUniverse) -> Self {
+        Self {
+            map: cis
+                .iter_cis()
+                .flat_map(|ci| {
+                    ci.object_definitions()
+                        .iter()
+                        .map(move |o| (ci.namespace(), o.name()))
+                })
+                .enumerate()
+                .map(|(i, (namespace, name))| ((namespace, name), i))
+                // Sort using BTreeSet to guarantee the IDs remain stable across runs
+                .collect::<BTreeSet<_>>()
+                .into_iter()
+                .collect(),
+        }
+    }
+
+    pub fn get(&self, ci: &ComponentInterface, obj: &Object) -> usize {
+        return *self.map.get(&(ci.namespace(), obj.name())).unwrap();
+    }
+
+    pub fn name(&self, ci: &ComponentInterface, obj: &Object) -> String {
+        format!("{}:{}", ci.namespace(), obj.name())
+    }
+}
+
+pub struct CallbackIds<'a> {
+    // Map (CI namespace, callback name) -> Ids
+    map: HashMap<(&'a str, &'a str), usize>,
+}
+
+impl<'a> CallbackIds<'a> {
+    pub fn new(cis: &'a ComponentUniverse) -> Self {
+        Self {
+            map: cis
+                .iter_cis()
+                .flat_map(|ci| {
+                    ci.callback_interface_definitions()
+                        .iter()
+                        .map(move |cb| (ci.namespace(), cb.name()))
+                })
+                .enumerate()
+                .map(|(i, (namespace, name))| ((namespace, name), i))
+                // Sort using BTreeSet to guarantee the IDs remain stable across runs
+                .collect::<BTreeSet<_>>()
+                .into_iter()
+                .collect(),
+        }
+    }
+
+    pub fn get(&self, ci: &ComponentInterface, cb: &CallbackInterface) -> usize {
+        return *self.map.get(&(ci.namespace(), cb.name())).unwrap();
+    }
+
+    pub fn name(&self, ci: &ComponentInterface, cb: &CallbackInterface) -> String {
+        format!("{}:{}", ci.namespace(), cb.name())
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/lib.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/lib.rs
new file mode 100644
index 0000000000..965ead26db
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/lib.rs
@@ -0,0 +1,158 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use anyhow::{Context, Result};
+use askama::Template;
+use camino::Utf8PathBuf;
+use clap::Parser;
+use serde::{Deserialize, Serialize};
+use std::collections::{HashMap, HashSet};
+use std::fs::File;
+use std::io::Write;
+use uniffi_bindgen::ComponentInterface;
+
+mod ci_list;
+mod render;
+
+use ci_list::{CallbackIds, ComponentUniverse, FunctionIds, ObjectIds};
+use render::cpp::CPPScaffoldingTemplate;
+use render::js::JSBindingsTemplate;
+
+#[derive(Debug, Parser)]
+#[clap(name = "uniffi-bindgen-gecko-js")]
+#[clap(version = clap::crate_version!())]
+#[clap(about = "JS bindings generator for Rust")]
+#[clap(propagate_version = true)]
+struct CliArgs {
+    // This is a really convoluted set of arguments, but we're only expecting to be called by
+    // `mach_commands.py`
+    #[clap(long, value_name = "FILE")]
+    js_dir: Utf8PathBuf,
+
+    #[clap(long, value_name = "FILE")]
+    fixture_js_dir: Utf8PathBuf,
+
+    #[clap(long, value_name = "FILE")]
+    cpp_path: Utf8PathBuf,
+
+    #[clap(long, value_name = "FILE")]
+    fixture_cpp_path: Utf8PathBuf,
+
+    #[clap(long, multiple_values = true, value_name = "FILES")]
+    udl_files: Vec<Utf8PathBuf>,
+
+    #[clap(long, multiple_values = true, value_name = "FILES")]
+    fixture_udl_files: Vec<Utf8PathBuf>,
+}
+
+/// Configuration for all components, read from `uniffi.toml`
+type ConfigMap = HashMap<String, Config>;
+
+/// Configuration for a single Component
+#[derive(Clone, Debug, Default, Deserialize, Serialize)]
+pub struct Config {
+    receiver_thread: ReceiverThreadConfig,
+}
+
+#[derive(Clone, Debug, Default, Deserialize, Serialize)]
+struct ReceiverThreadConfig {
+    #[serde(default)]
+    default: Option<String>,
+    #[serde(default)]
+    main: HashSet<String>,
+    #[serde(default)]
+    worker: HashSet<String>,
+}
+
+fn render(out_path: Utf8PathBuf, template: impl Template) -> Result<()> {
+    println!("rendering {}", out_path);
+    let contents = template.render()?;
+    let mut f =
+        File::create(&out_path).context(format!("Failed to create {:?}", out_path.file_name()))?;
+    write!(f, "{}\n", contents).context(format!("Failed to write to {}", out_path))
+}
+
+fn render_cpp(
+    path: Utf8PathBuf,
+    prefix: &str,
+    components: &Vec<(ComponentInterface, Config)>,
+    function_ids: &FunctionIds,
+    object_ids: &ObjectIds,
+    callback_ids: &CallbackIds,
+) -> Result<()> {
+    render(
+        path,
+        CPPScaffoldingTemplate {
+            prefix,
+            components,
+            function_ids,
+            object_ids,
+            callback_ids,
+        },
+    )
+}
+
+fn render_js(
+    out_dir: Utf8PathBuf,
+    components: &Vec<(ComponentInterface, Config)>,
+    function_ids: &FunctionIds,
+    object_ids: &ObjectIds,
+    callback_ids: &CallbackIds,
+) -> Result<()> {
+    for (ci, config) in components {
+        let template = JSBindingsTemplate {
+            ci,
+            config,
+            function_ids,
+            object_ids,
+            callback_ids,
+        };
+        let path = out_dir.join(template.js_module_name());
+        render(path, template)?;
+    }
+    Ok(())
+}
+
+pub fn run_main() -> Result<()> {
+    let args = CliArgs::parse();
+    let config_map: ConfigMap =
+        toml::from_str(include_str!("../config.toml")).expect("Error parsing config.toml");
+    let components = ComponentUniverse::new(args.udl_files, args.fixture_udl_files, config_map)?;
+    let function_ids = FunctionIds::new(&components);
+    let object_ids = ObjectIds::new(&components);
+    let callback_ids = CallbackIds::new(&components);
+
+    render_cpp(
+        args.cpp_path,
+        "UniFFI",
+        &components.components,
+        &function_ids,
+        &object_ids,
+        &callback_ids,
+    )?;
+    render_cpp(
+        args.fixture_cpp_path,
+        "UniFFIFixtures",
+        &components.fixture_components,
+        &function_ids,
+        &object_ids,
+        &callback_ids,
+    )?;
+    render_js(
+        args.js_dir,
+        &components.components,
+        &function_ids,
+        &object_ids,
+        &callback_ids,
+    )?;
+    render_js(
+        args.fixture_js_dir,
+        &components.fixture_components,
+        &function_ids,
+        &object_ids,
+        &callback_ids,
+    )?;
+
+    Ok(())
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/main.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/main.rs
new file mode 100644
index 0000000000..eefe72ba66
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/main.rs
@@ -0,0 +1,9 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use anyhow::Result;
+
+fn main() -> Result<()> {
+    uniffi_bindgen_gecko_js::run_main()
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/render/cpp.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/render/cpp.rs
new file mode 100644
index 0000000000..edbe4c7660
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/render/cpp.rs
@@ -0,0 +1,190 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use crate::{CallbackIds, Config, FunctionIds, ObjectIds};
+use askama::Template;
+use extend::ext;
+use heck::{ToShoutySnakeCase, ToUpperCamelCase};
+use std::collections::HashSet;
+use std::iter;
+use uniffi_bindgen::interface::{
+    CallbackInterface, ComponentInterface, FfiArgument, FfiFunction, FfiType, Object,
+};
+
+#[derive(Template)]
+#[template(path = "UniFFIScaffolding.cpp", escape = "none")]
+pub struct CPPScaffoldingTemplate<'a> {
+    // Prefix for each function name in.  This is related to how we handle the test fixtures.  For
+    // each function defined in the UniFFI namespace in UniFFI.webidl we:
+    //   - Generate a function in to handle it using the real UDL files
+    //   - Generate a different function in for handle it using the fixture UDL files
+    //   - Have a hand-written stub function that always calls the first function and only calls
+    //     the second function in if MOZ_UNIFFI_FIXTURES is defined.
+    pub prefix: &'a str,
+    pub components: &'a Vec<(ComponentInterface, Config)>,
+    pub function_ids: &'a FunctionIds<'a>,
+    pub object_ids: &'a ObjectIds<'a>,
+    pub callback_ids: &'a CallbackIds<'a>,
+}
+
+impl<'a> CPPScaffoldingTemplate<'a> {
+    fn has_any_objects(&self) -> bool {
+        self.components
+            .iter()
+            .any(|(ci, _)| ci.object_definitions().len() > 0)
+    }
+}
+
+// Define extension traits with methods used in our template code
+
+#[ext(name=ComponentInterfaceCppExt)]
+pub impl ComponentInterface {
+    // C++ pointer type name.  This needs to be a valid C++ type name and unique across all UDL
+    // files.
+    fn pointer_type(&self, object: &Object) -> String {
+        self._pointer_type(object.name())
+    }
+
+    fn _pointer_type(&self, name: &str) -> String {
+        format!(
+            "k{}{}PointerType",
+            self.namespace().to_upper_camel_case(),
+            name.to_upper_camel_case()
+        )
+    }
+
+    // Iterate over all functions to expose via the UniFFIScaffolding class
+    //
+    // This is basically all the user functions, except we don't expose the free methods for
+    // objects.  Freeing is handled by the UniFFIPointer class.
+    //
+    // Note: this function should return `impl Iterator<&FfiFunction>`, but that's not currently
+    // allowed for traits.
+    fn exposed_functions(&self) -> Vec<&FfiFunction> {
+        let excluded: HashSet<_> = self
+            .object_definitions()
+            .iter()
+            .map(|o| o.ffi_object_free().name())
+            .chain(
+                self.callback_interface_definitions()
+                    .iter()
+                    .map(|cbi| cbi.ffi_init_callback().name()),
+            )
+            .collect();
+        self.iter_user_ffi_function_definitions()
+            .filter(move |f| !excluded.contains(f.name()))
+            .collect()
+    }
+
+    // ScaffoldingConverter class
+    //
+    // This is used to convert types between the JS code and Rust
+    fn scaffolding_converter(&self, ffi_type: &FfiType) -> String {
+        match ffi_type {
+            FfiType::RustArcPtr(name) => {
+                format!("ScaffoldingObjectConverter<&{}>", self._pointer_type(name),)
+            }
+            _ => format!("ScaffoldingConverter<{}>", ffi_type.rust_type()),
+        }
+    }
+
+    // ScaffoldingCallHandler class
+    fn scaffolding_call_handler(&self, func: &FfiFunction) -> String {
+        let return_param = match func.return_type() {
+            Some(return_type) => self.scaffolding_converter(return_type),
+            None => "ScaffoldingConverter<void>".to_string(),
+        };
+        let all_params = iter::once(return_param)
+            .chain(
+                func.arguments()
+                    .into_iter()
+                    .map(|a| self.scaffolding_converter(&a.type_())),
+            )
+            .collect::<Vec<_>>()
+            .join(", ");
+        return format!("ScaffoldingCallHandler<{}>", all_params);
+    }
+}
+
+#[ext(name=FFIFunctionCppExt)]
+pub impl FfiFunction {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+
+    fn rust_name(&self) -> String {
+        self.name().to_string()
+    }
+
+    fn rust_return_type(&self) -> String {
+        match self.return_type() {
+            Some(t) => t.rust_type(),
+            None => "void".to_owned(),
+        }
+    }
+
+    fn rust_arg_list(&self) -> String {
+        let mut parts: Vec<String> = self.arguments().iter().map(|a| a.rust_type()).collect();
+        parts.push("RustCallStatus*".to_owned());
+        parts.join(", ")
+    }
+}
+
+#[ext(name=FFITypeCppExt)]
+pub impl FfiType {
+    // Type for the Rust scaffolding code
+    fn rust_type(&self) -> String {
+        match self {
+            FfiType::UInt8 => "uint8_t",
+            FfiType::Int8 => "int8_t",
+            FfiType::UInt16 => "uint16_t",
+            FfiType::Int16 => "int16_t",
+            FfiType::UInt32 => "uint32_t",
+            FfiType::Int32 => "int32_t",
+            FfiType::UInt64 => "uint64_t",
+            FfiType::Int64 => "int64_t",
+            FfiType::Float32 => "float",
+            FfiType::Float64 => "double",
+            FfiType::RustBuffer(_) => "RustBuffer",
+            FfiType::RustArcPtr(_) => "void *",
+            FfiType::ForeignCallback => "ForeignCallback",
+            FfiType::ForeignBytes => unimplemented!("ForeignBytes not supported"),
+        }
+        .to_owned()
+    }
+}
+
+#[ext(name=FFIArgumentCppExt)]
+pub impl FfiArgument {
+    fn rust_type(&self) -> String {
+        self.type_().rust_type()
+    }
+}
+
+#[ext(name=ObjectCppExt)]
+pub impl Object {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+}
+
+#[ext(name=CallbackInterfaceCppExt)]
+pub impl CallbackInterface {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+
+    /// Name of the static pointer to the JS callback handler
+    fn js_handler(&self) -> String {
+        format!("JS_CALLBACK_HANDLER_{}", self.name().to_shouty_snake_case())
+    }
+
+    /// Name of the C function handler
+    fn c_handler(&self, prefix: &str) -> String {
+        format!(
+            "{prefix}CallbackHandler{}",
+            self.name().to_upper_camel_case()
+        )
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/render/js.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/render/js.rs
new file mode 100644
index 0000000000..728be7db96
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/render/js.rs
@@ -0,0 +1,302 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use super::shared::*;
+use crate::{CallbackIds, Config, FunctionIds, ObjectIds};
+use askama::Template;
+use extend::ext;
+use heck::{ToLowerCamelCase, ToShoutySnakeCase, ToUpperCamelCase};
+use uniffi_bindgen::interface::{
+    Argument, CallbackInterface, ComponentInterface, Constructor, Enum, Error, Field, Function,
+    Literal, Method, Object, Radix, Record, Type,
+};
+
+fn arg_names(args: &[&Argument]) -> String {
+    args.iter()
+        .map(|arg| {
+            if let Some(default_value) = arg.default_value() {
+                format!("{} = {}", arg.nm(), default_value.render())
+            } else {
+                arg.nm()
+            }
+        })
+        .collect::<Vec<String>>()
+        .join(",")
+}
+
+fn render_enum_literal(typ: &Type, variant_name: &str) -> String {
+    if let Type::Enum(enum_name) = typ {
+        // TODO: This does not support complex enum literals yet.
+        return format!(
+            "{}.{}",
+            enum_name.to_upper_camel_case(),
+            variant_name.to_shouty_snake_case()
+        );
+    } else {
+        panic!("Rendering an enum literal on a type that is not an enum")
+    }
+}
+
+#[derive(Template)]
+#[template(path = "js/wrapper.sys.mjs", escape = "none")]
+pub struct JSBindingsTemplate<'a> {
+    pub ci: &'a ComponentInterface,
+    pub config: &'a Config,
+    pub function_ids: &'a FunctionIds<'a>,
+    pub object_ids: &'a ObjectIds<'a>,
+    pub callback_ids: &'a CallbackIds<'a>,
+}
+
+impl<'a> JSBindingsTemplate<'a> {
+    pub fn js_module_name(&self) -> String {
+        self.js_module_name_for_ci_namespace(self.ci.namespace())
+    }
+
+    fn external_type_module(&self, crate_name: &str) -> String {
+        format!(
+            "resource://gre/modules/{}",
+            self.js_module_name_for_crate_name(crate_name),
+        )
+    }
+
+    // TODO: Once https://phabricator.services.mozilla.com/D156116 is merged maybe the next two
+    // functions should use a map from the config file
+
+    fn js_module_name_for_ci_namespace(&self, namespace: &str) -> String {
+        // The plain namespace name is a bit too generic as a module name for m-c, so we
+        // prefix it with "Rust". Later we'll probably allow this to be customized.
+        format!("Rust{}.sys.mjs", namespace.to_upper_camel_case())
+    }
+
+    fn js_module_name_for_crate_name(&self, crate_name: &str) -> String {
+        let namespace = match crate_name {
+            "uniffi_geometry" => "geometry",
+            s => s,
+        };
+        self.js_module_name_for_ci_namespace(namespace)
+    }
+}
+
+// Define extension traits with methods used in our template code
+
+#[ext(name=LiteralJSExt)]
+pub impl Literal {
+    fn render(&self) -> String {
+        match self {
+            Literal::Boolean(inner) => inner.to_string(),
+            Literal::String(inner) => format!("\"{}\"", inner),
+            Literal::UInt(num, radix, _) => format!("{}", radix.render_num(num)),
+            Literal::Int(num, radix, _) => format!("{}", radix.render_num(num)),
+            Literal::Float(num, _) => num.clone(),
+            Literal::Enum(name, typ) => render_enum_literal(typ, name),
+            Literal::EmptyMap => "{}".to_string(),
+            Literal::EmptySequence => "[]".to_string(),
+            Literal::Null => "null".to_string(),
+        }
+    }
+}
+
+#[ext(name=RadixJSExt)]
+pub impl Radix {
+    fn render_num(
+        &self,
+        num: impl std::fmt::Display + std::fmt::LowerHex + std::fmt::Octal,
+    ) -> String {
+        match self {
+            Radix::Decimal => format!("{}", num),
+            Radix::Hexadecimal => format!("{:#x}", num),
+            Radix::Octal => format!("{:#o}", num),
+        }
+    }
+}
+
+#[ext(name=RecordJSExt)]
+pub impl Record {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+
+    fn constructor_field_list(&self) -> String {
+        self.fields()
+            .iter()
+            .map(|field| {
+                if let Some(default_value) = field.default_value() {
+                    format!("{} = {}", field.nm(), default_value.render())
+                } else {
+                    field.nm()
+                }
+            })
+            .collect::<Vec<String>>()
+            .join(",")
+    }
+}
+
+#[ext(name=CallbackInterfaceJSExt)]
+pub impl CallbackInterface {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+
+    fn handler(&self) -> String {
+        format!("callbackHandler{}", self.nm())
+    }
+}
+
+#[ext(name=FieldJSExt)]
+pub impl Field {
+    fn nm(&self) -> String {
+        self.name().to_lower_camel_case()
+    }
+
+    fn lower_fn(&self) -> String {
+        self.type_().lower_fn()
+    }
+
+    fn lift_fn(&self) -> String {
+        self.type_().lift_fn()
+    }
+
+    fn write_datastream_fn(&self) -> String {
+        self.type_().write_datastream_fn()
+    }
+
+    fn read_datastream_fn(&self) -> String {
+        self.type_().read_datastream_fn()
+    }
+
+    fn compute_size_fn(&self) -> String {
+        self.type_().compute_size_fn()
+    }
+
+    fn ffi_converter(&self) -> String {
+        self.type_().ffi_converter()
+    }
+}
+
+#[ext(name=ArgumentJSExt)]
+pub impl Argument {
+    fn nm(&self) -> String {
+        self.name().to_lower_camel_case()
+    }
+
+    fn lower_fn(&self) -> String {
+        self.type_().lower_fn()
+    }
+
+    fn lift_fn(&self) -> String {
+        self.type_().lift_fn()
+    }
+
+    fn write_datastream_fn(&self) -> String {
+        self.type_().write_datastream_fn()
+    }
+
+    fn read_datastream_fn(&self) -> String {
+        self.type_().read_datastream_fn()
+    }
+
+    fn compute_size_fn(&self) -> String {
+        self.type_().compute_size_fn()
+    }
+
+    fn ffi_converter(&self) -> String {
+        self.type_().ffi_converter()
+    }
+}
+
+#[ext(name=TypeJSExt)]
+pub impl Type {
+    // Render an expression to check if two instances of this type are equal
+    fn equals(&self, first: &str, second: &str) -> String {
+        match self {
+            Type::Record(_) => format!("{}.equals({})", first, second),
+            _ => format!("{} == {}", first, second),
+        }
+    }
+
+    fn lower_fn(&self) -> String {
+        format!("{}.lower", self.ffi_converter())
+    }
+
+    fn lift_fn(&self) -> String {
+        format!("{}.lift", self.ffi_converter())
+    }
+
+    fn write_datastream_fn(&self) -> String {
+        format!("{}.write", self.ffi_converter())
+    }
+
+    fn read_datastream_fn(&self) -> String {
+        format!("{}.read", self.ffi_converter())
+    }
+
+    fn compute_size_fn(&self) -> String {
+        format!("{}.computeSize", self.ffi_converter())
+    }
+
+    fn ffi_converter(&self) -> String {
+        format!(
+            "FfiConverter{}",
+            self.canonical_name().to_upper_camel_case()
+        )
+    }
+}
+
+#[ext(name=EnumJSExt)]
+pub impl Enum {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+}
+
+#[ext(name=FunctionJSExt)]
+pub impl Function {
+    fn arg_names(&self) -> String {
+        arg_names(self.arguments().as_slice())
+    }
+
+    fn nm(&self) -> String {
+        self.name().to_lower_camel_case()
+    }
+}
+
+#[ext(name=ErrorJSExt)]
+pub impl Error {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+}
+
+#[ext(name=ObjectJSExt)]
+pub impl Object {
+    fn nm(&self) -> String {
+        self.name().to_upper_camel_case()
+    }
+}
+
+#[ext(name=ConstructorJSExt)]
+pub impl Constructor {
+    fn nm(&self) -> String {
+        if self.is_primary_constructor() {
+            "init".to_string()
+        } else {
+            self.name().to_lower_camel_case()
+        }
+    }
+
+    fn arg_names(&self) -> String {
+        arg_names(&self.arguments().as_slice())
+    }
+}
+
+#[ext(name=MethodJSExt)]
+pub impl Method {
+    fn arg_names(&self) -> String {
+        arg_names(self.arguments().as_slice())
+    }
+
+    fn nm(&self) -> String {
+        self.name().to_lower_camel_case()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/render/mod.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/render/mod.rs
new file mode 100644
index 0000000000..f9ceeb9872
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/render/mod.rs
@@ -0,0 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+pub mod cpp;
+pub mod js;
+pub mod shared;
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/render/shared.rs b/toolkit/components/uniffi-bindgen-gecko-js/src/render/shared.rs
new file mode 100644
index 0000000000..c356158715
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/render/shared.rs
@@ -0,0 +1,39 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+License, v. 2.0. If a copy of the MPL was not distributed with this
+* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/// Extension traits that are shared across multiple render targets
+use crate::Config;
+use extend::ext;
+use uniffi_bindgen::interface::{Function, Method, Object};
+
+fn is_async(config: &Config, spec: &str) -> bool {
+    if config.receiver_thread.main.contains(spec) {
+        false
+    } else if config.receiver_thread.worker.contains(spec) {
+        true
+    } else {
+        match &config.receiver_thread.default {
+            Some(t) => t != "main",
+            _ => true,
+        }
+    }
+}
+
+#[ext]
+pub impl Function {
+    fn is_async(&self, config: &Config) -> bool {
+        is_async(config, self.name())
+    }
+}
+
+#[ext]
+pub impl Object {
+    fn is_constructor_async(&self, config: &Config) -> bool {
+        is_async(config, self.name())
+    }
+
+    fn is_method_async(&self, method: &Method, config: &Config) -> bool {
+        is_async(config, &format!("{}.{}", self.name(), method.name()))
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/UniFFIScaffolding.cpp b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/UniFFIScaffolding.cpp
new file mode 100644
index 0000000000..9fe981f206
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/UniFFIScaffolding.cpp
@@ -0,0 +1,155 @@
+// Generated by uniffi-bindgen-gecko-js.  DO NOT EDIT.
+
+#include "nsString.h"
+#include "nsPrintfCString.h"
+#include "mozilla/Logging.h"
+#include "mozilla/Maybe.h"
+#include "mozilla/StaticPtr.h"
+#include "mozilla/dom/UniFFICallbacks.h"
+#include "mozilla/dom/UniFFIScaffolding.h"
+#include "mozilla/dom/ScaffoldingCall.h"
+
+namespace mozilla::uniffi {
+
+using dom::ArrayBuffer;
+using dom::AutoEntryScript;
+using dom::GlobalObject;
+using dom::RootedDictionary;
+using dom::Promise;
+using dom::ScaffoldingType;
+using dom::Sequence;
+using dom::UniFFICallbackHandler;
+using dom::UniFFIPointer;
+using dom::UniFFIScaffoldingCallResult;
+
+// Define scaffolding functions from UniFFI
+extern "C" {
+  {%- for (ci, config) in components %}
+  {%- for func in ci.iter_user_ffi_function_definitions() %}
+  {{ func.rust_return_type() }} {{ func.rust_name() }}({{ func.rust_arg_list() }});
+  {%- endfor %}
+  {%- endfor %}
+}
+
+// Define pointer types
+{%- for (ci, config) in components %}
+{%- for object in ci.object_definitions() %}
+{%- let pointer_type = ci.pointer_type(object) %}
+const static mozilla::uniffi::UniFFIPointerType {{ pointer_type }} {
+  "{{ "{}::{}"|format(ci.namespace(), object.name()) }}"_ns,
+  {{ object.ffi_object_free().rust_name() }}
+};
+{%- endfor %}
+{%- endfor %}
+
+// Define the data we need per-callback interface
+{%- for (ci, config) in components %}
+{%- for cbi in ci.callback_interface_definitions() %}
+MOZ_CAN_RUN_SCRIPT
+extern "C" int {{ cbi.c_handler(prefix) }}(uint64_t aHandle, uint32_t aMethod, RustBuffer aArgs, RustBuffer* aOutBuffer) {
+    // Currently, we only support "fire-and-forget" async callbacks.  These are
+    // callbacks that run asynchronously without returning anything.  The main
+    // use case for callbacks is logging, which fits very well with this model.
+    //
+    // So, here we simple queue the callback and return immediately.
+    mozilla::uniffi::QueueCallback({{ callback_ids.get(ci, cbi) }}, aHandle, aMethod, aArgs);
+    return CALLBACK_INTERFACE_SUCCESS;
+}
+static StaticRefPtr<dom::UniFFICallbackHandler> {{ cbi.js_handler() }};
+{%- endfor %}
+{%- endfor %}
+
+// Define a lookup function for our callback interface info
+Maybe<CallbackInterfaceInfo> {{ prefix }}GetCallbackInterfaceInfo(uint64_t aInterfaceId) {
+    switch(aInterfaceId) {
+        {%- for (ci, config) in components %}
+        {%- for cbi in ci.callback_interface_definitions() %}
+        case {{ callback_ids.get(ci, cbi) }}: { // {{ callback_ids.name(ci, cbi) }}
+            return Some(CallbackInterfaceInfo {
+                "{{ cbi.name() }}",
+                &{{ cbi.js_handler() }},
+                {{ cbi.c_handler(prefix) }},
+                {{ cbi.ffi_init_callback().name() }},
+            });
+        }
+        {%- endfor %}
+        {%- endfor %}
+
+        default:
+            return Nothing();
+    }
+}
+
+Maybe<already_AddRefed<Promise>> {{ prefix }}CallAsync(const GlobalObject& aGlobal, uint64_t aId, const Sequence<ScaffoldingType>& aArgs, ErrorResult& aError) {
+  switch (aId) {
+    {%- for (ci, config) in components %}
+    {%- for func in ci.exposed_functions() %}
+    case {{ function_ids.get(ci, func) }}: { // {{ function_ids.name(ci, func) }}
+      using CallHandler = {{ ci.scaffolding_call_handler(func) }};
+      return Some(CallHandler::CallAsync({{ func.rust_name() }}, aGlobal, aArgs, "{{ func.name() }}: "_ns, aError));
+    }
+    {%- endfor %}
+    {%- endfor %}
+  }
+  return Nothing();
+}
+
+bool {{ prefix }}CallSync(const GlobalObject& aGlobal, uint64_t aId, const Sequence<ScaffoldingType>& aArgs, RootedDictionary<UniFFIScaffoldingCallResult>& aReturnValue, ErrorResult& aError) {
+  switch (aId) {
+    {%- for (ci, config) in components %}
+    {%- for func in ci.exposed_functions() %}
+    case {{ function_ids.get(ci, func) }}: { // {{ function_ids.name(ci, func) }}
+      using CallHandler = {{ ci.scaffolding_call_handler(func) }};
+      CallHandler::CallSync({{ func.rust_name() }}, aGlobal, aArgs, aReturnValue, "{{ func.name() }}: "_ns, aError);
+      return true;
+    }
+    {%- endfor %}
+    {%- endfor %}
+  }
+  return false;
+}
+
+Maybe<already_AddRefed<UniFFIPointer>> {{ prefix }}ReadPointer(const GlobalObject& aGlobal, uint64_t aId, const ArrayBuffer& aArrayBuff, long aPosition, ErrorResult& aError) {
+  {%- if self.has_any_objects() %}
+  const UniFFIPointerType* type;
+  switch (aId) {
+    {%- for (ci, config) in components %}
+    {%- for object in ci.object_definitions() %}
+    case {{ object_ids.get(ci, object) }}: { // {{ object_ids.name(ci, object) }}
+      type = &{{ ci.pointer_type(object) }};
+      break;
+    }
+    {%- endfor %}
+    {%- endfor %}
+    default:
+      return Nothing();
+  }
+  return Some(UniFFIPointer::Read(aArrayBuff, aPosition, type, aError));
+  {%- else %}
+  return Nothing();
+  {%- endif %}
+}
+
+bool {{ prefix }}WritePointer(const GlobalObject& aGlobal, uint64_t aId, const UniFFIPointer& aPtr, const ArrayBuffer& aArrayBuff, long aPosition, ErrorResult& aError) {
+  {%- if self.has_any_objects() %}
+  const UniFFIPointerType* type;
+  switch (aId) {
+    {%- for (ci, config) in components %}
+    {%- for object in ci.object_definitions() %}
+    case {{ object_ids.get(ci, object) }}: { // {{ object_ids.name(ci, object) }}
+      type = &{{ ci.pointer_type(object) }};
+      break;
+    }
+    {%- endfor %}
+    {%- endfor %}
+    default:
+      return false;
+  }
+  aPtr.Write(aArrayBuff, aPosition, type, aError);
+  return true;
+  {%- else %}
+  return false;
+  {%- endif %}
+}
+
+}  // namespace mozilla::uniffi
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Boolean.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Boolean.sys.mjs
new file mode 100644
index 0000000000..a38b6bdd94
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Boolean.sys.mjs
@@ -0,0 +1,22 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value == 1;
+    }
+    static lower(value) {
+        if (value) {
+            return 1;
+        } else {
+            return 0;
+        }
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint8(this.lower(value))
+    }
+    static read(dataStream) {
+        return this.lift(dataStream.readUint8())
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterface.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterface.sys.mjs
new file mode 100644
index 0000000000..0b24cbbe0b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterface.sys.mjs
@@ -0,0 +1,24 @@
+{%- let cbi = ci.get_callback_interface_definition(name).unwrap() %}
+{#- See CallbackInterfaceRuntime.sys.mjs and CallbackInterfaceHandler.sys.mjs for the callback interface handler definition, referenced here as `{{ cbi.handler() }}` #}
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static lower(callbackObj) {
+        return {{ cbi.handler() }}.storeCallbackObj(callbackObj)
+    }
+
+    static lift(handleId) {
+        return {{ cbi.handler() }}.getCallbackObj(handleId)
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readInt64())
+    }
+
+    static write(dataStream, callbackObj) {
+        dataStream.writeInt64(this.lower(callbackObj))
+    }
+
+    static computeSize(callbackObj) {
+        return 8;
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceHandler.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceHandler.sys.mjs
new file mode 100644
index 0000000000..c062d64e0c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceHandler.sys.mjs
@@ -0,0 +1,19 @@
+const {{ cbi.handler() }} = new UniFFICallbackHandler(
+    "{{ callback_ids.name(ci, cbi) }}",
+    {{ callback_ids.get(ci, cbi) }},
+    [
+        {%- for method in cbi.methods() %}
+        new UniFFICallbackMethodHandler(
+            "{{ method.nm() }}",
+            [
+                {%- for arg in method.arguments() %}
+                {{ arg.ffi_converter() }},
+                {%- endfor %}
+            ],
+        ),
+        {%- endfor %}
+    ]
+);
+
+// Allow the shutdown-related functionality to be tested in the unit tests
+UnitTestObjs.{{ cbi.handler() }} = {{ cbi.handler() }};
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceRuntime.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceRuntime.sys.mjs
new file mode 100644
index 0000000000..a4d88136ab
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CallbackInterfaceRuntime.sys.mjs
@@ -0,0 +1,195 @@
+
+/**
+ * Handler for a single UniFFI CallbackInterface
+ *
+ * This class stores objects that implement a callback interface in a handle
+ * map, allowing them to be referenced by the Rust code using an integer
+ * handle.
+ *
+ * While the callback object is stored in the map, it allows the Rust code to
+ * call methods on the object using the callback object handle, a method id,
+ * and an ArrayBuffer packed with the method arguments.
+ *
+ * When the Rust code drops its reference, it sends a call with the methodId=0,
+ * which causes callback object to be removed from the map.
+ */
+class UniFFICallbackHandler {
+    #name;
+    #interfaceId;
+    #handleCounter;
+    #handleMap;
+    #methodHandlers;
+    #allowNewCallbacks
+
+    /**
+     * Create a UniFFICallbackHandler
+     * @param {string} name - Human-friendly name for this callback interface
+     * @param {int} interfaceId - Interface ID for this CallbackInterface.
+     * @param {UniFFICallbackMethodHandler[]} methodHandlers -- UniFFICallbackHandler for each method, in the same order as the UDL file
+     */
+     constructor(name, interfaceId, methodHandlers) {
+        this.#name = name;
+        this.#interfaceId = interfaceId;
+        this.#handleCounter = 0;
+        this.#handleMap = new Map();
+        this.#methodHandlers = methodHandlers;
+        this.#allowNewCallbacks = true;
+
+        UniFFIScaffolding.registerCallbackHandler(this.#interfaceId, this.invokeCallback.bind(this));
+        Services.obs.addObserver(this, "xpcom-shutdown");
+     }
+
+    /**
+     * Store a callback object in the handle map and return the handle
+     *
+     * @param {obj} callbackObj - Object that implements the callback interface
+     * @returns {int} - Handle for this callback object, this is what gets passed back to Rust.
+     */
+    storeCallbackObj(callbackObj) {
+        if (!this.#allowNewCallbacks) {
+            throw new UniFFIError(`No new callbacks allowed for ${this.#name}`);
+        }
+        const handle = this.#handleCounter;
+        this.#handleCounter += 1;
+        this.#handleMap.set(handle, new UniFFICallbackHandleMapEntry(callbackObj, Components.stack.caller.formattedStack.trim()));
+        return handle;
+    }
+
+    /**
+     * Get a previously stored callback object
+     *
+     * @param {int} handle - Callback object handle, returned from `storeCallbackObj()`
+     * @returns {obj} - Callback object
+     */
+    getCallbackObj(handle) {
+        return this.#handleMap.get(handle).callbackObj;
+    }
+
+    /**
+     * Set if new callbacks are allowed for this handler
+     *
+     * This is called with false during shutdown to ensure the callback maps don't
+     * prevent JS objects from being GCed.
+     */
+    setAllowNewCallbacks(allow) {
+        this.#allowNewCallbacks = allow
+    }
+
+    /**
+     * Check that no callbacks are currently registered
+     *
+     * If there are callbacks registered a UniFFIError will be thrown.  This is
+     * called during shutdown to generate an alert if there are leaked callback
+     * interfaces.
+     */
+    assertNoRegisteredCallbacks() {
+        if (this.#handleMap.size > 0) {
+            const entry = this.#handleMap.values().next().value;
+            throw new UniFFIError(`UniFFI interface ${this.#name} has ${this.#handleMap.size} registered callbacks at xpcom-shutdown. This likely indicates a UniFFI callback leak.\nStack trace for the first leaked callback:\n${entry.stackTrace}.`);
+        }
+    }
+
+    /**
+     * Invoke a method on a stored callback object
+     * @param {int} handle - Object handle
+     * @param {int} methodId - Method identifier.  This the 1-based index of
+     *   the method from the UDL file.  0 is the special drop method, which
+     *   removes the callback object from the handle map.
+     * @param {ArrayBuffer} argsArrayBuffer - Arguments to pass to the method, packed in an ArrayBuffer
+     */
+    invokeCallback(handle, methodId, argsArrayBuffer) {
+        try {
+            this.#invokeCallbackInner(handle, methodId, argsArrayBuffer);
+        } catch (e) {
+            console.error(`internal error invoking callback: ${e}`)
+        }
+    }
+
+    #invokeCallbackInner(handle, methodId, argsArrayBuffer) {
+        const callbackObj = this.getCallbackObj(handle);
+        if (callbackObj === undefined) {
+            throw new UniFFIError(`${this.#name}: invalid callback handle id: ${handle}`);
+        }
+
+        // Special-cased drop method, remove the object from the handle map and
+        // return an empty array buffer
+        if (methodId == 0) {
+            this.#handleMap.delete(handle);
+            return;
+        }
+
+        // Get the method data, converting from 1-based indexing
+        const methodHandler = this.#methodHandlers[methodId - 1];
+        if (methodHandler === undefined) {
+            throw new UniFFIError(`${this.#name}: invalid method id: ${methodId}`)
+        }
+
+        methodHandler.call(callbackObj, argsArrayBuffer);
+    }
+
+    /**
+     * xpcom-shutdown observer method
+     *
+     * This handles:
+     *  - Deregistering ourselves as the UniFFI callback handler
+     *  - Checks for any leftover stored callbacks which indicate memory leaks
+     */
+    observe(aSubject, aTopic, aData) {
+        if (aTopic == "xpcom-shutdown") {
+            try {
+                this.setAllowNewCallbacks(false);
+                this.assertNoRegisteredCallbacks();
+                UniFFIScaffolding.deregisterCallbackHandler(this.#interfaceId);
+            } catch (ex) {
+                console.error(`UniFFI Callback interface error during xpcom-shutdown: ${ex}`);
+                Cc["@mozilla.org/xpcom/debug;1"]
+                    .getService(Ci.nsIDebug2)
+                    .abort(ex.filename, ex.lineNumber);
+            }
+         }
+    }
+}
+
+/**
+ * Handles calling a single method for a callback interface
+ */
+class UniFFICallbackMethodHandler {
+    #name;
+    #argsConverters;
+
+    /**
+     * Create a UniFFICallbackMethodHandler
+
+     * @param {string} name -- Name of the method to call on the callback object
+     * @param {FfiConverter[]} argsConverters - FfiConverter for each argument type
+     */
+    constructor(name, argsConverters) {
+        this.#name = name;
+        this.#argsConverters = argsConverters;
+    }
+
+    /**
+     * Invoke the method
+     *
+     * @param {obj} callbackObj -- Object implementing the callback interface for this method
+     * @param {ArrayBuffer} argsArrayBuffer -- Arguments for the method, packed in an ArrayBuffer
+     */
+     call(callbackObj, argsArrayBuffer) {
+        const argsStream = new ArrayBufferDataStream(argsArrayBuffer);
+        const args = this.#argsConverters.map(converter => converter.read(argsStream));
+        callbackObj[this.#name](...args);
+    }
+}
+
+/**
+ * UniFFICallbackHandler.handleMap entry
+ *
+ * @property callbackObj - Callback object, this must implement the callback interface.
+ * @property {string} stackTrace - Stack trace from when the callback object was registered.  This is used to proved extra context when debugging leaked callback objects.
+ */
+class UniFFICallbackHandleMapEntry {
+    constructor(callbackObj, stackTrace) {
+        this.callbackObj = callbackObj;
+        this.stackTrace = stackTrace
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CustomType.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CustomType.sys.mjs
new file mode 100644
index 0000000000..4ce4dc31af
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/CustomType.sys.mjs
@@ -0,0 +1,23 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static lift(buf) {
+        return {{ builtin.ffi_converter() }}.lift(buf);    
+    }
+    
+    static lower(buf) {
+        return {{ builtin.ffi_converter() }}.lower(buf);
+    }
+    
+    static write(dataStream, value) {
+        {{ builtin.ffi_converter() }}.write(dataStream, value);
+    } 
+    
+    static read(buf) {
+        return {{ builtin.ffi_converter() }}.read(buf);
+    }
+    
+    static computeSize(value) {
+        return {{ builtin.ffi_converter() }}.computeSize(value);
+    }
+}
+// TODO: We should also allow JS to customize the type eventually.
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Enum.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Enum.sys.mjs
new file mode 100644
index 0000000000..941784ed63
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Enum.sys.mjs
@@ -0,0 +1,115 @@
+{%- let enum_ = ci.get_enum_definition(name).unwrap() -%}
+
+{%- if enum_.is_flat() -%}
+
+export const {{ enum_.nm() }} = {
+    {%- for variant in enum_.variants() %}
+    {{ variant.name().to_shouty_snake_case() }}: {{loop.index}},
+    {%- endfor %}
+};
+
+Object.freeze({{ enum_.nm() }});
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            {%- for variant in enum_.variants() %}
+            case {{ loop.index }}:
+                return {{ enum_.nm() }}.{{ variant.name().to_shouty_snake_case() }}
+            {%- endfor %}
+            default:
+                return new Error("Unknown {{ enum_.nm() }} variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        {%- for variant in enum_.variants() %}
+        if (value === {{ enum_.nm() }}.{{ variant.name().to_shouty_snake_case() }}) {
+            dataStream.writeInt32({{ loop.index }});
+            return;
+        }
+        {%- endfor %}
+        return new Error("Unknown {{ enum_.nm() }} variant");
+    }
+
+    static computeSize(value) {
+        return 4;
+    }
+
+    static checkType(value) {
+      if (!Number.isInteger(value) || value < 1 || value > {{ enum_.variants().len() }}) {
+          throw new UniFFITypeError(`${value} is not a valid value for {{ enum_.nm() }}`);
+      }
+    }
+}
+
+{%- else -%}
+
+export class {{ enum_.nm() }} {}
+{%- for variant in enum_.variants() %}
+{{enum_.nm()}}.{{variant.name().to_upper_camel_case() }} = class extends {{ enum_.nm() }}{
+    constructor(
+        {% for field in variant.fields() -%}
+        {{ field.nm() }}{%- if loop.last %}{%- else %}, {%- endif %}
+        {% endfor -%}
+        ) {
+            super();
+            {%- for field in variant.fields() %}
+            this.{{field.nm()}} = {{ field.nm() }};
+            {%- endfor %}
+        }
+}
+{%- endfor %}
+
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            {%- for variant in enum_.variants() %}
+            case {{ loop.index }}:
+                return new {{ enum_.nm() }}.{{ variant.name().to_upper_camel_case()  }}(
+                    {%- for field in variant.fields() %}
+                    {{ field.ffi_converter() }}.read(dataStream){%- if loop.last %}{% else %}, {%- endif %}
+                    {%- endfor %}
+                    );
+            {%- endfor %}
+            default:
+                return new Error("Unknown {{ enum_.nm() }} variant");
+        }
+    }
+
+    static write(dataStream, value) {
+        {%- for variant in enum_.variants() %}
+        if (value instanceof {{enum_.nm()}}.{{ variant.name().to_upper_camel_case() }}) {
+            dataStream.writeInt32({{ loop.index }});
+            {%- for field in variant.fields() %}
+            {{ field.ffi_converter() }}.write(dataStream, value.{{ field.nm() }});
+            {%- endfor %}
+            return;
+        }
+        {%- endfor %}
+        return new Error("Unknown {{ enum_.nm() }} variant");
+    }
+
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        {%- for variant in enum_.variants() %}
+        if (value instanceof {{enum_.nm()}}.{{ variant.name().to_upper_camel_case() }}) {
+            {%- for field in variant.fields() %}
+            totalSize += {{ field.ffi_converter() }}.computeSize(value.{{ field.nm() }});
+            {%- endfor %}
+            return totalSize;
+        }
+        {%- endfor %}
+        return new Error("Unknown {{ enum_.nm() }} variant");
+    }
+
+    static checkType(value) {
+      if (!(value instanceof {{ enum_.nm() }})) {
+        throw new UniFFITypeError(`${value} is not a subclass instance of {{ enum_.nm() }}`);
+      }
+    }
+}
+
+{%- endif %}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Error.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Error.sys.mjs
new file mode 100644
index 0000000000..97b1d064d9
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Error.sys.mjs
@@ -0,0 +1,80 @@
+{%- let error = ci.get_error_definition(name).unwrap() %}
+{%- let string_type = Type::String %}
+{%- let string_ffi_converter = string_type.ffi_converter() %}
+
+export class {{ error.nm() }} extends Error {}
+{% for variant in error.variants() %}
+
+export class {{ variant.name().to_upper_camel_case() }} extends {{ error.nm() }} {
+{% if error.is_flat() %}
+    constructor(message, ...params) {
+        super(...params);
+        this.message = message;
+    }
+{%- else %}
+    constructor(
+        {% for field in variant.fields() -%}
+        {{field.nm()}},
+        {% endfor -%}
+        ...params
+    ) {
+        super(...params);
+        {%- for field in variant.fields() %}
+        this.{{field.nm()}} = {{ field.nm() }};
+        {%- endfor %}
+    }
+{%- endif %}
+    toString() {
+        return `{{ variant.name().to_upper_camel_case() }}: ${super.toString()}`
+    }
+}
+{%- endfor %}
+
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        switch (dataStream.readInt32()) {
+            {%- for variant in error.variants() %}
+            case {{ loop.index }}:
+                {%- if error.is_flat() %}
+                return new {{ variant.name().to_upper_camel_case()  }}({{ string_ffi_converter }}.read(dataStream));
+                {%- else %}
+                return new {{ variant.name().to_upper_camel_case()  }}(
+                    {%- for field in variant.fields() %}
+                    {{ field.ffi_converter() }}.read(dataStream){%- if loop.last %}{% else %}, {%- endif %}
+                    {%- endfor %}
+                    );
+                {%- endif %}
+            {%- endfor %}
+            default:
+                throw new Error("Unknown {{ error.nm() }} variant");
+        }
+    }
+    static computeSize(value) {
+        // Size of the Int indicating the variant
+        let totalSize = 4;
+        {%- for variant in error.variants() %}
+        if (value instanceof {{ variant.name().to_upper_camel_case() }}) {
+            {%- for field in variant.fields() %}
+            totalSize += {{ field.ffi_converter() }}.computeSize(value.{{ field.nm() }});
+            {%- endfor %}
+            return totalSize;
+        }
+        {%- endfor %}
+        throw new Error("Unknown {{ error.nm() }} variant");
+    }
+    static write(dataStream, value) {
+        {%- for variant in error.variants() %}
+        if (value instanceof {{ variant.name().to_upper_camel_case() }}) {
+            dataStream.writeInt32({{ loop.index }});
+            {%- for field in variant.fields() %}
+            {{ field.ffi_converter() }}.write(dataStream, value.{{ field.nm() }});
+            {%- endfor %}
+            return;
+        }
+        {%- endfor %}
+        throw new Error("Unknown {{ error.nm() }} variant");
+    }
+
+    static errorClass = {{ error.nm() }};
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/ExternalType.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/ExternalType.sys.mjs
new file mode 100644
index 0000000000..ebc658834b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/ExternalType.sys.mjs
@@ -0,0 +1,7 @@
+import {
+  {{ ffi_converter }},
+  {{ name }},
+} from "{{ self.external_type_module(crate_name) }}";
+
+// Export the FFIConverter object to make external types work.
+export { {{ ffi_converter }}, {{ name }} };
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float32.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float32.sys.mjs
new file mode 100644
index 0000000000..1030efa226
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float32.sys.mjs
@@ -0,0 +1,18 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat32()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float64.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float64.sys.mjs
new file mode 100644
index 0000000000..fc49046691
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Float64.sys.mjs
@@ -0,0 +1,18 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeFloat64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readFloat64()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Helpers.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Helpers.sys.mjs
new file mode 100644
index 0000000000..0daaa983ac
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Helpers.sys.mjs
@@ -0,0 +1,234 @@
+// Write/Read data to/from an ArrayBuffer
+class ArrayBufferDataStream {
+    constructor(arrayBuffer) {
+        this.dataView = new DataView(arrayBuffer);
+        this.pos = 0;
+    }
+
+    readUint8() {
+        let rv = this.dataView.getUint8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeUint8(value) {
+        this.dataView.setUint8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readUint16() {
+        let rv = this.dataView.getUint16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeUint16(value) {
+        this.dataView.setUint16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readUint32() {
+        let rv = this.dataView.getUint32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeUint32(value) {
+        this.dataView.setUint32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readUint64() {
+        let rv = this.dataView.getBigUint64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeUint64(value) {
+        this.dataView.setBigUint64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+
+    readInt8() {
+        let rv = this.dataView.getInt8(this.pos);
+        this.pos += 1;
+        return rv;
+    }
+
+    writeInt8(value) {
+        this.dataView.setInt8(this.pos, value);
+        this.pos += 1;
+    }
+
+    readInt16() {
+        let rv = this.dataView.getInt16(this.pos);
+        this.pos += 2;
+        return rv;
+    }
+
+    writeInt16(value) {
+        this.dataView.setInt16(this.pos, value);
+        this.pos += 2;
+    }
+
+    readInt32() {
+        let rv = this.dataView.getInt32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeInt32(value) {
+        this.dataView.setInt32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readInt64() {
+        let rv = this.dataView.getBigInt64(this.pos);
+        this.pos += 8;
+        return Number(rv);
+    }
+
+    writeInt64(value) {
+        this.dataView.setBigInt64(this.pos, BigInt(value));
+        this.pos += 8;
+    }
+
+    readFloat32() {
+        let rv = this.dataView.getFloat32(this.pos);
+        this.pos += 4;
+        return rv;
+    }
+
+    writeFloat32(value) {
+        this.dataView.setFloat32(this.pos, value);
+        this.pos += 4;
+    }
+
+    readFloat64() {
+        let rv = this.dataView.getFloat64(this.pos);
+        this.pos += 8;
+        return rv;
+    }
+
+    writeFloat64(value) {
+        this.dataView.setFloat64(this.pos, value);
+        this.pos += 8;
+    }
+
+
+    writeString(value) {
+      const encoder = new TextEncoder();
+      // Note: in order to efficiently write this data, we first write the
+      // string data, reserving 4 bytes for the size.
+      const dest = new Uint8Array(this.dataView.buffer, this.pos + 4);
+      const encodeResult = encoder.encodeInto(value, dest);
+      if (encodeResult.read != value.length) {
+        throw new UniFFIError(
+            "writeString: out of space when writing to ArrayBuffer.  Did the computeSize() method returned the wrong result?"
+        );
+      }
+      const size = encodeResult.written;
+      // Next, go back and write the size before the string data
+      this.dataView.setUint32(this.pos, size);
+      // Finally, advance our position past both the size and string data
+      this.pos += size + 4;
+    }
+
+    readString() {
+      const decoder = new TextDecoder();
+      const size = this.readUint32();
+      const source = new Uint8Array(this.dataView.buffer, this.pos, size)
+      const value = decoder.decode(source);
+      this.pos += size;
+      return value;
+    }
+
+    {%- for object in ci.object_definitions() %}
+
+    // Reads a {{ object.nm() }} pointer from the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    readPointer{{ object.nm() }}() {
+        const pointerId = {{ object_ids.get(ci, object) }}; // {{ object_ids.name(ci, object) }}
+        const res = UniFFIScaffolding.readPointer(pointerId, this.dataView.buffer, this.pos);
+        this.pos += 8;
+        return res;
+    }
+
+    // Writes a {{ object.nm() }} pointer into the data stream
+    // UniFFI Pointers are **always** 8 bytes long. That is enforced
+    // by the C++ and Rust Scaffolding code.
+    writePointer{{ object.nm() }}(value) {
+        const pointerId = {{ object_ids.get(ci, object) }}; // {{ object_ids.name(ci, object) }}
+        UniFFIScaffolding.writePointer(pointerId, value, this.dataView.buffer, this.pos);
+        this.pos += 8;
+    }
+    {% endfor %}
+}
+
+function handleRustResult(result, liftCallback, liftErrCallback) {
+    switch (result.code) {
+        case "success":
+            return liftCallback(result.data);
+
+        case "error":
+            throw liftErrCallback(result.data);
+
+        case "internal-error":
+            let message = result.internalErrorMessage;
+            if (message) {
+                throw new UniFFIInternalError(message);
+            } else {
+                throw new UniFFIInternalError("Unknown error");
+            }
+
+        default:
+            throw new UniFFIError(`Unexpected status code: ${result.code}`);
+    }
+}
+
+class UniFFIError {
+    constructor(message) {
+        this.message = message;
+    }
+
+    toString() {
+        return `UniFFIError: ${this.message}`
+    }
+}
+
+class UniFFIInternalError extends UniFFIError {}
+
+// Base class for FFI converters
+class FfiConverter {
+    // throw `UniFFITypeError` if a value to be converted has an invalid type
+    static checkType(value) {
+        if (value === undefined ) {
+            throw new UniFFITypeError(`undefined`);
+        }
+        if (value === null ) {
+            throw new UniFFITypeError(`null`);
+        }
+    }
+}
+
+// Base class for FFI converters that lift/lower by reading/writing to an ArrayBuffer
+class FfiConverterArrayBuffer extends FfiConverter {
+    static lift(buf) {
+        return this.read(new ArrayBufferDataStream(buf));
+    }
+
+    static lower(value) {
+        const buf = new ArrayBuffer(this.computeSize(value));
+        const dataStream = new ArrayBufferDataStream(buf);
+        this.write(dataStream, value);
+        return buf;
+    }
+}
+
+// Symbols that are used to ensure that Object constructors
+// can only be used with a proper UniFFI pointer
+const uniffiObjectPtr = Symbol("uniffiObjectPtr");
+const constructUniffiObject = Symbol("constructUniffiObject");
+UnitTestObjs.uniffiObjectPtr = uniffiObjectPtr;
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int16.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int16.sys.mjs
new file mode 100644
index 0000000000..63c26bce8a
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int16.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -32768 || value > 32767) {
+            throw new UniFFITypeError(`${value} exceeds the I16 bounds`);
+        }
+    }
+    static computeSize() {
+        return 2;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt16(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt16()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int32.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int32.sys.mjs
new file mode 100644
index 0000000000..502092eb16
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int32.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -2147483648 || value > 2147483647) {
+            throw new UniFFITypeError(`${value} exceeds the I32 bounds`);
+        }
+    }
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt32()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int64.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int64.sys.mjs
new file mode 100644
index 0000000000..d56296712d
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int64.sys.mjs
@@ -0,0 +1,24 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt64()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int8.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int8.sys.mjs
new file mode 100644
index 0000000000..63e543b1fa
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Int8.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < -128 || value > 127) {
+            throw new UniFFITypeError(`${value} exceeds the I8 bounds`);
+        }
+    }
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeInt8(value)
+    }
+    static read(dataStream) {
+        return dataStream.readInt8()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Map.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Map.sys.mjs
new file mode 100644
index 0000000000..5b6e6dc172
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Map.sys.mjs
@@ -0,0 +1,54 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const map = {};
+        for (let i = 0; i < len; i++) {
+            const key = {{ key_type.ffi_converter() }}.read(dataStream);
+            const value = {{ value_type.ffi_converter() }}.read(dataStream);
+            map[key] = value;
+        }
+
+        return map;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(Object.keys(value).length);
+        for (const key in value) {
+            {{ key_type.ffi_converter() }}.write(dataStream, key);
+            {{ value_type.ffi_converter() }}.write(dataStream, value[key]);
+        }
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const key in value) {
+            size += {{ key_type.ffi_converter() }}.computeSize(key);
+            size += {{ value_type.ffi_converter() }}.computeSize(value[key]);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        for (const key in value) {
+            try {
+                {{ key_type.ffi_converter() }}.checkType(key);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("(key)");
+                }
+                throw e;
+            }
+
+            try {
+                {{ value_type.ffi_converter() }}.checkType(value[key]);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${key}]`);
+                }
+                throw e;
+            }
+        }
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Object.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Object.sys.mjs
new file mode 100644
index 0000000000..e03291089e
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Object.sys.mjs
@@ -0,0 +1,68 @@
+{%- let object = ci.get_object_definition(name).unwrap() -%}
+export class {{ object.nm() }} {
+    // Use `init` to instantiate this class.
+    // DO NOT USE THIS CONSTRUCTOR DIRECTLY
+    constructor(opts) {
+        if (!Object.prototype.hasOwnProperty.call(opts, constructUniffiObject)) {
+            throw new UniFFIError("Attempting to construct an object using the JavaScript constructor directly" +
+            "Please use a UDL defined constructor, or the init function for the primary constructor")
+        }
+        if (!opts[constructUniffiObject] instanceof UniFFIPointer) {
+            throw new UniFFIError("Attempting to create a UniFFI object with a pointer that is not an instance of UniFFIPointer")
+        }
+        this[uniffiObjectPtr] = opts[constructUniffiObject];
+    }
+
+    {%- for cons in object.constructors() %}
+    {%- if object.is_constructor_async(config) %}
+    /**
+     * An async constructor for {{ object.nm() }}.
+     * 
+     * @returns {Promise<{{ object.nm() }}>}: A promise that resolves
+     *      to a newly constructed {{ object.nm() }}
+     */
+    {%- else %}
+    /**
+     * A constructor for {{ object.nm() }}.
+     * 
+     * @returns { {{ object.nm() }} }
+     */
+    {%- endif %}
+    static {{ cons.nm() }}({{cons.arg_names()}}) {
+        {%- call js::call_constructor(cons, type_, object.is_constructor_async(config)) -%}
+    }
+    {%- endfor %}
+
+    {%- for meth in object.methods() %}
+
+    {{ meth.nm() }}({{ meth.arg_names() }}) {
+        {%- call js::call_method(meth, type_, object.is_method_async(meth, config)) %}
+    }
+    {%- endfor %}
+
+}
+
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static lift(value) {
+        const opts = {};
+        opts[constructUniffiObject] = value;
+        return new {{ object.nm() }}(opts);
+    }
+
+    static lower(value) {
+        return value[uniffiObjectPtr];
+    }
+
+    static read(dataStream) {
+        return this.lift(dataStream.readPointer{{ object.nm() }}());
+    }
+
+    static write(dataStream, value) {
+        dataStream.writePointer{{ object.nm() }}(value[uniffiObjectPtr]);
+    }
+
+    static computeSize(value) {
+        return 8;
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Optional.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Optional.sys.mjs
new file mode 100644
index 0000000000..a37e04abe5
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Optional.sys.mjs
@@ -0,0 +1,36 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static checkType(value) {
+        if (value !== undefined && value !== null) {
+            {{ inner.ffi_converter() }}.checkType(value)
+        }
+    }
+
+    static read(dataStream) {
+        const code = dataStream.readUint8(0);
+        switch (code) {
+            case 0:
+                return null
+            case 1:
+                return {{ inner.ffi_converter() }}.read(dataStream)
+            default:
+                throw UniFFIError(`Unexpected code: ${code}`);
+        }
+    }
+
+    static write(dataStream, value) {
+        if (value === null || value === undefined) {
+            dataStream.writeUint8(0);
+            return;
+        }
+        dataStream.writeUint8(1);
+        {{ inner.ffi_converter() }}.write(dataStream, value)
+    }
+
+    static computeSize(value) {
+        if (value === null || value === undefined) {
+            return 1;
+        }
+        return 1 + {{ inner.ffi_converter() }}.computeSize(value)
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Record.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Record.sys.mjs
new file mode 100644
index 0000000000..0dbb0e5800
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Record.sys.mjs
@@ -0,0 +1,65 @@
+{%- let record = ci.get_record_definition(name).unwrap() -%}
+export class {{ record.nm() }} {
+    constructor({{ record.constructor_field_list() }}) {
+        {%- for field in record.fields() %}
+        try {
+            {{ field.ffi_converter() }}.checkType({{ field.nm() }})
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart("{{ field.nm() }}");
+            }
+            throw e;
+        }
+        {%- endfor %}
+
+        {%- for field in record.fields() %}
+        this.{{field.nm()}} = {{ field.nm() }};
+        {%- endfor %}
+    }
+    equals(other) {
+        return (
+            {%- for field in record.fields() %}
+            {{ field.type_().equals("this.{}"|format(field.nm()), "other.{}"|format(field.nm())) }}{% if !loop.last %} &&{% endif %}
+            {%- endfor %}
+        )
+    }
+}
+
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        return new {{record.nm()}}(
+            {%- for field in record.fields() %}
+            {{ field.read_datastream_fn() }}(dataStream)
+            {%- if !loop.last %}, {% endif %}
+            {%- endfor %}
+        );
+    }
+    static write(dataStream, value) {
+        {%- for field in record.fields() %}
+        {{ field.write_datastream_fn() }}(dataStream, value.{{field.nm()}});
+        {%- endfor %}
+    }
+
+    static computeSize(value) {
+        let totalSize = 0;
+        {%- for field in record.fields() %}
+        totalSize += {{ field.ffi_converter() }}.computeSize(value.{{ field.nm() }});
+        {%- endfor %}
+        return totalSize
+    }
+
+    static checkType(value) {
+        super.checkType(value);
+        {%- for field in record.fields() %}
+        try {
+            {{ field.ffi_converter() }}.checkType(value.{{ field.nm() }});
+        } catch (e) {
+            if (e instanceof UniFFITypeError) {
+                e.addItemDescriptionPart(".{{ field.nm() }}");
+            }
+            throw e;
+        }
+        {%- endfor %}
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Sequence.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Sequence.sys.mjs
new file mode 100644
index 0000000000..4b4c078670
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Sequence.sys.mjs
@@ -0,0 +1,43 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverterArrayBuffer {
+    static read(dataStream) {
+        const len = dataStream.readInt32();
+        const arr = [];
+        for (let i = 0; i < len; i++) {
+            arr.push({{ inner.ffi_converter() }}.read(dataStream));
+        }
+        return arr;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeInt32(value.length);
+        value.forEach((innerValue) => {
+            {{ inner.ffi_converter() }}.write(dataStream, innerValue);
+        })
+    }
+
+    static computeSize(value) {
+        // The size of the length
+        let size = 4;
+        for (const innerValue of value) {
+            size += {{ inner.ffi_converter() }}.computeSize(innerValue);
+        }
+        return size;
+    }
+
+    static checkType(value) {
+        if (!Array.isArray(value)) {
+            throw new UniFFITypeError(`${value} is not an array`);
+        }
+        value.forEach((innerValue, idx) => {
+            try {
+                {{ inner.ffi_converter() }}.checkType(innerValue);
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart(`[${idx}]`);
+                }
+                throw e;
+            }
+        })
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/String.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/String.sys.mjs
new file mode 100644
index 0000000000..d016e3f21b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/String.sys.mjs
@@ -0,0 +1,32 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (typeof value !== "string") {
+            throw new UniFFITypeError(`${value} is not a string`);
+        }
+    }
+
+    static lift(buf) {
+        const decoder = new TextDecoder();
+        const utf8Arr = new Uint8Array(buf);
+        return decoder.decode(utf8Arr);
+    }
+    static lower(value) {
+        const encoder = new TextEncoder();
+        return encoder.encode(value).buffer;
+    }
+
+    static write(dataStream, value) {
+        dataStream.writeString(value);
+    }
+
+    static read(dataStream) {
+        return dataStream.readString();
+    }
+
+    static computeSize(value) {
+        const encoder = new TextEncoder();
+        return 4 + encoder.encode(value).length
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/TopLevelFunctions.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/TopLevelFunctions.sys.mjs
new file mode 100644
index 0000000000..601eb74d7c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/TopLevelFunctions.sys.mjs
@@ -0,0 +1,6 @@
+{%- for func in ci.function_definitions() %}
+
+export function {{ func.nm() }}({{ func.arg_names() }}) {
+{% call js::call_scaffolding_function(func) %}
+}
+{%- endfor %}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Types.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Types.sys.mjs
new file mode 100644
index 0000000000..d17748420c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/Types.sys.mjs
@@ -0,0 +1,89 @@
+{%- if !ci.callback_interface_definitions().is_empty() %}
+{%- include "CallbackInterfaceRuntime.sys.mjs" %}
+
+{% endif %}
+
+{%- for type_ in ci.iter_types() %}
+{%- let ffi_converter = type_.ffi_converter() %}
+{%- match type_ %}
+
+{%- when Type::Boolean %}
+{%- include "Boolean.sys.mjs" %}
+
+{%- when Type::UInt8 %}
+{%- include "UInt8.sys.mjs" %}
+
+{%- when Type::UInt16 %}
+{%- include "UInt16.sys.mjs" %}
+
+{%- when Type::UInt32 %}
+{%- include "UInt32.sys.mjs" %}
+
+{%- when Type::UInt64 %}
+{%- include "UInt64.sys.mjs" %}
+
+{%- when Type::Int8 %}
+{%- include "Int8.sys.mjs" %}
+
+{%- when Type::Int16 %}
+{%- include "Int16.sys.mjs" %}
+
+{%- when Type::Int32 %}
+{%- include "Int32.sys.mjs" %}
+
+{%- when Type::Int64 %}
+{%- include "Int64.sys.mjs" %}
+
+{%- when Type::Float32 %}
+{%- include "Float32.sys.mjs" %}
+
+{%- when Type::Float64 %}
+{%- include "Float64.sys.mjs" %}
+
+{%- when Type::Record with (name) %}
+{%- include "Record.sys.mjs" %}
+
+{%- when Type::Optional with (inner) %}
+{%- include "Optional.sys.mjs" %}
+
+{%- when Type::String %}
+{%- include "String.sys.mjs" %}
+
+{%- when Type::Sequence with (inner) %}
+{%- include "Sequence.sys.mjs" %}
+
+{%- when Type::Map with (key_type, value_type) %}
+{%- include "Map.sys.mjs" %}
+
+{%- when Type::Error with (name) %}
+{%- include "Error.sys.mjs" %}
+
+{%- when Type::Enum with (name) %}
+{%- include "Enum.sys.mjs" %}
+
+{%- when Type::Object with (name) %}
+{%- include "Object.sys.mjs" %}
+
+{%- when Type::Custom with { name, builtin } %}
+{%- include "CustomType.sys.mjs" %}
+
+{%- when Type::External with { name, crate_name } %}
+{%- include "ExternalType.sys.mjs" %}
+
+{%- when Type::CallbackInterface with (name) %}
+{%- include "CallbackInterface.sys.mjs" %}
+
+{%- else %}
+{#- TODO implement the other types #}
+
+{%- endmatch %}
+
+{% endfor %}
+
+{%- if !ci.callback_interface_definitions().is_empty() %}
+// Define callback interface handlers, this must come after the type loop since they reference the FfiConverters defined above.
+
+{% for cbi in ci.callback_interface_definitions() %}
+{%- include "CallbackInterfaceHandler.sys.mjs" %}
+{% endfor %}
+{% endif %}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt16.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt16.sys.mjs
new file mode 100644
index 0000000000..569d6d2ebd
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt16.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 65535) {
+            throw new UniFFITypeError(`${value} exceeds the U16 bounds`);
+        }
+    }
+    static computeSize() {
+        return 2;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint16(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint16()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt32.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt32.sys.mjs
new file mode 100644
index 0000000000..cfeffb1ecb
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt32.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 4294967295) {
+            throw new UniFFITypeError(`${value} exceeds the U32 bounds`);
+        }
+    }
+    static computeSize() {
+        return 4;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint32(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint32()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt64.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt64.sys.mjs
new file mode 100644
index 0000000000..a62a0b7e6c
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt64.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isSafeInteger(value)) {
+            throw new UniFFITypeError(`${value} exceeds the safe integer bounds`);
+        }
+        if (value < 0) {
+            throw new UniFFITypeError(`${value} exceeds the U64 bounds`);
+        }
+    }
+    static computeSize() {
+        return 8;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint64(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint64()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt8.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt8.sys.mjs
new file mode 100644
index 0000000000..2f08aeee1b
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/UInt8.sys.mjs
@@ -0,0 +1,27 @@
+// Export the FFIConverter object to make external types work.
+export class {{ ffi_converter }} extends FfiConverter {
+    static checkType(value) {
+        super.checkType(value);
+        if (!Number.isInteger(value)) {
+            throw new UniFFITypeError(`${value} is not an integer`);
+        }
+        if (value < 0 || value > 256) {
+            throw new UniFFITypeError(`${value} exceeds the U8 bounds`);
+        }
+    }
+    static computeSize() {
+        return 1;
+    }
+    static lift(value) {
+        return value;
+    }
+    static lower(value) {
+        return value;
+    }
+    static write(dataStream, value) {
+        dataStream.writeUint8(value)
+    }
+    static read(dataStream) {
+        return dataStream.readUint8()
+    }
+}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/macros.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/macros.sys.mjs
new file mode 100644
index 0000000000..efec33e4e6
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/macros.sys.mjs
@@ -0,0 +1,62 @@
+{%- macro call_scaffolding_function(func) %}
+{%- call _call_scaffolding_function(func, func.return_type(), "", func.is_async(config)) -%}
+{%- endmacro %}
+
+{%- macro call_constructor(cons, object_type, is_async) %}
+{%- call _call_scaffolding_function(cons, Some(object_type), "", is_async) -%}
+{%- endmacro %}
+
+{%- macro call_method(method, object_type, is_async) %}
+{%- call _call_scaffolding_function(method, method.return_type(), object_type.ffi_converter(), is_async) -%}
+{%- endmacro %}
+
+{%- macro _call_scaffolding_function(func, return_type, receiver_ffi_converter, is_async) %}
+        {%- match return_type %}
+        {%- when Some with (return_type) %}
+        const liftResult = (result) => {{ return_type.ffi_converter() }}.lift(result);
+        {%- else %}
+        const liftResult = (result) => undefined;
+        {%- endmatch %}
+        {%- match func.throws_type() %}
+        {%- when Some with (err_type) %}
+        const liftError = (data) => {{ err_type.ffi_converter() }}.lift(data);
+        {%- else %}
+        const liftError = null;
+        {%- endmatch %}
+        const functionCall = () => {
+            {%- for arg in func.arguments() %}
+            try {
+                {{ arg.ffi_converter() }}.checkType({{ arg.nm() }})
+            } catch (e) {
+                if (e instanceof UniFFITypeError) {
+                    e.addItemDescriptionPart("{{ arg.nm() }}");
+                }
+                throw e;
+            }
+            {%- endfor %}
+
+            {%- if is_async %}
+            return UniFFIScaffolding.callAsync(
+            {%- else %}
+            return UniFFIScaffolding.callSync(
+            {%- endif %}
+                {{ function_ids.get(ci, func.ffi_func()) }}, // {{ function_ids.name(ci, func.ffi_func()) }}
+                {%- if receiver_ffi_converter != "" %}
+                {{ receiver_ffi_converter }}.lower(this),
+                {%- endif %}
+                {%- for arg in func.arguments() %}
+                {{ arg.lower_fn() }}({{ arg.nm() }}),
+                {%- endfor %}
+            )
+        }
+
+        {%- if is_async %}
+        try {
+            return functionCall().then((result) => handleRustResult(result, liftResult, liftError));
+        }  catch (error) {
+            return Promise.reject(error)
+        }
+        {%- else %}
+        return handleRustResult(functionCall(), liftResult, liftError);
+        {%- endif %}
+{%- endmacro %}
diff --git a/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/wrapper.sys.mjs b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/wrapper.sys.mjs
new file mode 100644
index 0000000000..0c33c05e4f
--- /dev/null
+++ b/toolkit/components/uniffi-bindgen-gecko-js/src/templates/js/wrapper.sys.mjs
@@ -0,0 +1,15 @@
+// This file was autogenerated by the `uniffi-bindgen-gecko-js` crate.
+// Trust me, you don't want to mess with it!
+
+import { UniFFITypeError } from "resource://gre/modules/UniFFI.sys.mjs";
+
+{% import "macros.sys.mjs" as js %}
+
+// Objects intended to be used in the unit tests
+export var UnitTestObjs = {};
+
+{% include "Helpers.sys.mjs" %}
+
+{% include "Types.sys.mjs" %}
+
+{% include "TopLevelFunctions.sys.mjs" %}