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diff --git a/toolkit/components/uniffi-js/ScaffoldingConverter.h b/toolkit/components/uniffi-js/ScaffoldingConverter.h
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+++ b/toolkit/components/uniffi-js/ScaffoldingConverter.h
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+/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* vim:set ts=2 sw=2 sts=2 et cindent: */
+/* 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/. */
+
+#ifndef mozilla_ScaffoldingConverter_h
+#define mozilla_ScaffoldingConverter_h
+
+#include <limits>
+#include <type_traits>
+#include "nsString.h"
+#include "mozilla/ResultVariant.h"
+#include "mozilla/dom/OwnedRustBuffer.h"
+#include "mozilla/dom/PrimitiveConversions.h"
+#include "mozilla/dom/TypedArray.h"
+#include "mozilla/dom/UniFFIBinding.h"
+#include "mozilla/dom/UniFFIPointer.h"
+#include "mozilla/dom/UniFFIPointerType.h"
+#include "mozilla/dom/UniFFIRust.h"
+#include "mozilla/dom/UniFFIScaffolding.h"
+
+namespace mozilla::uniffi {
+
+class ScaffoldingConverterTagDefault {};
+
+// Handle converting types between JS and Rust
+//
+// Scaffolding conversions are done using a 2 step process:
+//   - Call FromJs/FromRust to convert to an intermediate type
+//   - Call IntoJs/IntoRust to convert from that type to the target type
+//
+// The main reason for this is handling RustBuffers when other arguments fail
+// to convert.  By using OwnedRustBuffer as the intermediate type, we can
+// ensure those buffers get freed in that case.  Note that we can't use
+// OwnedRustBuffer as the Rust type.  Passing the buffer into Rust transfers
+// ownership so we shouldn't free the buffer in this case.
+//
+// For most other types, we just use the Rust type as the intermediate type.
+template <typename T, typename Tag = ScaffoldingConverterTagDefault>
+class ScaffoldingConverter {
+ public:
+  using RustType = T;
+  using IntermediateType = T;
+
+  // Convert a JS value to an intermedate type
+  //
+  // This inputs a const ref, because that's what the WebIDL bindings send to
+  // us.
+  //
+  // If this succeeds then IntoRust is also guaranteed to succeed
+  static mozilla::Result<IntermediateType, nsCString> FromJs(
+      const dom::ScaffoldingType& aValue) {
+    if (!aValue.IsDouble()) {
+      return Err("Bad argument type"_ns);
+    }
+    double value = aValue.GetAsDouble();
+
+    if (std::isnan(value)) {
+      return Err("NaN not allowed"_ns);
+    }
+
+    if constexpr (std::is_integral<RustType>::value) {
+      // Use PrimitiveConversionTraits_Limits rather than std::numeric_limits,
+      // since it handles JS-specific bounds like the 64-bit integer limits.
+      // (see Number.MAX_SAFE_INTEGER and Number.MIN_SAFE_INTEGER)
+      if (value < dom::PrimitiveConversionTraits_Limits<RustType>::min() ||
+          value > dom::PrimitiveConversionTraits_Limits<RustType>::max()) {
+        return Err("Out of bounds"_ns);
+      }
+    }
+
+    // Don't check float bounds for a few reasons.
+    //   - It's difficult because
+    //     PrimitiveConversionTraits_Limits<float>::min() is the smallest
+    //     positive value, rather than the most negative.
+    //   - A float value unlikely to overflow
+    //   - It's also likely that we can't do an exact conversion because the
+    //     float doesn't have enough precision, but it doesn't seem correct
+    //     to error out in that case.
+
+    RustType rv = static_cast<RustType>(value);
+    if constexpr (std::is_integral<RustType>::value) {
+      if (rv != value) {
+        return Err("Not an integer"_ns);
+      }
+    }
+
+    return rv;
+  }
+
+  // Convert an intermediate type to a Rust type
+  //
+  // IntoRust doesn't touch the JS data, so it's safe to call in a worker thread
+  static RustType IntoRust(IntermediateType aValue) { return aValue; }
+
+  // Convert an Rust type to an intermediate type
+  //
+  // This inputs a value since RustTypes are POD types
+  //
+  // If this succeeds then IntoJs is also guaranteed to succeed
+  static mozilla::Result<IntermediateType, nsCString> FromRust(
+      RustType aValue) {
+    if constexpr (std::is_same<RustType, int64_t>::value ||
+                  std::is_same<RustType, uint64_t>::value) {
+      // Check that the value can fit in a double (only needed for 64 bit types)
+      if (aValue < dom::PrimitiveConversionTraits_Limits<RustType>::min() ||
+          aValue > dom::PrimitiveConversionTraits_Limits<RustType>::max()) {
+        return Err("Out of bounds"_ns);
+      }
+    }
+    if constexpr (std::is_floating_point<RustType>::value) {
+      if (std::isnan(aValue)) {
+        return Err("NaN not allowed"_ns);
+      }
+    }
+    return aValue;
+  }
+
+  // Convert an intermedate type to a JS type
+  //
+  // This inputs an r-value reference since we may want to move data out of
+  // this type.
+  static void IntoJs(JSContext* aContext, IntermediateType&& aValue,
+                     dom::ScaffoldingType& aDest) {
+    aDest.SetAsDouble() = aValue;
+  }
+};
+
+template <>
+class ScaffoldingConverter<RustBuffer> {
+ public:
+  using RustType = RustBuffer;
+  using IntermediateType = OwnedRustBuffer;
+
+  static mozilla::Result<OwnedRustBuffer, nsCString> FromJs(
+      const dom::ScaffoldingType& aValue) {
+    if (!aValue.IsArrayBuffer()) {
+      return Err("Bad argument type"_ns);
+    }
+
+    return OwnedRustBuffer::FromArrayBuffer(aValue.GetAsArrayBuffer());
+  }
+
+  static RustBuffer IntoRust(OwnedRustBuffer&& aValue) {
+    return aValue.IntoRustBuffer();
+  }
+
+  static mozilla::Result<OwnedRustBuffer, nsCString> FromRust(
+      RustBuffer aValue) {
+    return OwnedRustBuffer(aValue);
+  }
+
+  static void IntoJs(JSContext* aContext, OwnedRustBuffer&& aValue,
+                     dom::ScaffoldingType& aDest) {
+    aDest.SetAsArrayBuffer().Init(aValue.IntoArrayBuffer(aContext));
+  }
+};
+
+// ScaffoldingConverter for object pointers
+template <const UniFFIPointerType* PointerType>
+class ScaffoldingObjectConverter {
+ public:
+  using RustType = void*;
+  using IntermediateType = void*;
+
+  static mozilla::Result<void*, nsCString> FromJs(
+      const dom::ScaffoldingType& aValue) {
+    if (!aValue.IsUniFFIPointer()) {
+      return Err("Bad argument type"_ns);
+    }
+    dom::UniFFIPointer& value = aValue.GetAsUniFFIPointer();
+    if (!value.IsSamePtrType(PointerType)) {
+      return Err("Bad pointer type"_ns);
+    }
+    return value.GetPtr();
+  }
+
+  static void* IntoRust(void* aValue) { return aValue; }
+
+  static mozilla::Result<void*, nsCString> FromRust(void* aValue) {
+    return aValue;
+  }
+
+  static void IntoJs(JSContext* aContext, void* aValue,
+                     dom::ScaffoldingType& aDest) {
+    aDest.SetAsUniFFIPointer() =
+        dom::UniFFIPointer::Create(aValue, PointerType);
+  }
+};
+
+// ScaffoldingConverter for void returns
+//
+// This doesn't implement the normal interface, it's only use is a the
+// ReturnConverter parameter of ScaffoldingCallHandler.
+template <>
+class ScaffoldingConverter<void> {
+ public:
+  using RustType = void;
+};
+
+}  // namespace mozilla::uniffi
+
+#endif  // mozilla_ScaffoldingConverter_h