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+# Generating Javascript bindings with UniFFI
+
+Firefox supports auto-generating JS bindings for Rust components using [UniFFI](https://mozilla.github.io/uniffi-rs/).
+
+## How it works
+
+The Rust crate contains a
+[UniFFI Definition Language (UDL) file](https://mozilla.github.io/uniffi-rs/udl_file_spec.html), which describes the
+interface to generate bindings for.
+
+The UniFFI core generates the scaffolding: Rust code which acts as the FFI layer from the UDL file.  The functions of
+this layer all use the C calling convention and all structs use a C layout, this is the de facto standard for FFI
+interoperability.
+
+The [`uniffi-bindgen-gecko-js`](https://searchfox.org/mozilla-central/source/toolkit/components/uniffi-bindgen-gecko-js)
+tool, which lives in the Firefox source tree, generates 2 things:
+  - A JS interface for the scaffolding code, which uses [WebIDL](/dom/bindings/webidl/index.rst)
+  - A module that uses the scaffolding to provide the bindings API.
+
+Currently, this generated code gets checked in to source control.  We are working on a system to avoid this and
+auto-generate it at build time instead (see [bugzilla 1756214](https://bugzilla.mozilla.org/show_bug.cgi?id=1756214)).
+
+## Before creating new bindings with UniFFI
+
+Keep a few things in mind before you create a new set of bindings:
+
+ - **UniFFI was not written to maximize performance.**  It's code is efficient enough to handle many use cases, but at this
+   point should probably be avoided for performance critical components.
+ - **uniffi-bindgen-gecko-js bindings run with chrome privileges.**  Make sure this is acceptable for your project
+ - **Only a subset of Rust types can be exposed via the FFI.**  Check the [UniFFI Book](https://mozilla.github.io/uniffi-rs/) to see what
+   types are compatible with UniFFI.
+
+If any of these are blockers for your work, consider discussing it further with the UniFFI devs to see if we can support
+your project:
+
+  - Chat with us on `#uniffi` on Matrix/Element
+  - File an issue on [mozilla/uniffi](https://github.com/mozilla/uniffi-rs/)
+
+## Creating new bindings with UniFFI
+
+You can see an example of this feature in use: [when application-services swapped the tabs js sync engine with rust](https://bugzilla.mozilla.org/show_bug.cgi?id=1791851)
+
+Here's how you can create a new set of bindings using UniFFI:
+
+  1. UniFFI your crate (if it isn't already):
+      - [Create a UDL file to describe your interface](https://mozilla.github.io/uniffi-rs/udl_file_spec.html)
+      - [Set up your Rust crate to generate scaffolding](https://mozilla.github.io/uniffi-rs/tutorial/Rust_scaffolding.html)
+  2. Add your crate as a Firefox dependency (if it isn't already)
+      - **If the code will exist in the mozilla-central repo:**
+        - Create a new directory for the Rust crate
+        - Edit `toolkit/library/rust/shared/Cargo.toml` and add a dependency to your library path
+      - **If the code exists in an external repo:**
+        - Edit `toolkit/library/rust/shared/Cargo.toml` and add a dependency to your library URL
+        - Run `mach vendor rust` to vendor in your Rust code
+  3. Generate bindings code for your crate
+      - Add the path of your UDL (that you made in step 1) in `toolkit/components/uniffi-bindgen-gecko-js/mach_commands.py`
+      - Run `./mach uniffi generate`
+          - add your newly generated `Rust{udl-name}.sys.mjs` file to `toolkit/components/uniffi-bindgen-gecko-js/components/moz.build`
+      - Then simply import your module to the file you want to use it in and start using your APIs!
+
+        Example from tabs module:
+
+        ``` js
+        ChromeUtils.defineESModuleGetters(lazy, {
+          ...
+          TabsStore: "resource://gre/modules/RustTabs.sys.mjs",
+        });
+        ...
+        this._rustStore = await lazy.TabsStore.init(path);
+        ```