Bug: RustBuffer uses size_t instead of uint64_t, causing crashes on 32-bit ARM

[sfourdrinier]

Bug Report

Description

The generated RustBuffer struct in cpp/includes/RustBuffer.h uses size_t for the capacity and len fields, but UniFFI's C ABI specification requires uint64_t on all platforms. This causes a critical ABI mismatch on 32-bit ARM architectures.

Platforms Affected

• ❌ 32-bit ARM (armeabi-v7a): Crashes immediately during argument marshalling
• ✅ 64-bit platforms: Works by accident (size_t == uint64_t == 8 bytes)

Reproduction

1. Build a React Native app using uniffi-bindgen-react-native on a 32-bit ARM device (e.g., Amazon Fire TV, armeabi-v7a)
2. Call any UniFFI function that accepts a RustBuffer parameter
3. App crashes with SIGSEGV before any Rust code executes

Crash Details

Process: <app>, PID: <pid>
Signal: SIGSEGV (Segmentation fault)

#00 pc <addr> <lib>.so (NativeModule::cpp_uniffi_<crate>_fn_constructor_<type>_new+102)

The crash occurs in the C++ JSI wrapper during constructor marshalling. The stack/register corruption happens because:

• C++ passes RustBuffer with 4-byte size_t fields on 32-bit ARM
• Rust expects RustBuffer with 8-byte u64 fields per UniFFI spec

Root Cause

The RustBuffer struct definition doesn't match UniFFI's ABI:

// Current (incorrect) - cpp/includes/RustBuffer.h
struct RustBuffer {
  size_t capacity;  // 4 bytes on 32-bit ARM, 8 bytes on 64-bit
  size_t len;       // 4 bytes on 32-bit ARM, 8 bytes on 64-bit
  uint8_t *data;
};

UniFFI's ABI specification mandates uint64_t (8 bytes) on ALL platforms:

• https://mozilla.github.io/uniffi-rs/0.27/internals/api/uniffi/struct.RustBuffer.html

Fix

Change size_t to uint64_t to match UniFFI's ABI across all platforms.

PR submitted: #313

Environment

• Device: Amazon Fire TV (armeabi-v7a)
• Framework: Expo SDK 54 / React Native 0.81
• uniffi-bindgen-react-native: latest from main branch

References

• UniFFI RustBuffer ABI: https://mozilla.github.io/uniffi-rs/0.27/internals/api/uniffi/struct.RustBuffer.html
• Related UniFFI issue: RustBufferByReference misreports its size mozilla/uniffi-rs#2681

[sfourdrinier]

Fix submitted in PR #313