uniffi_bindgen code generator and CLI dependencies compiled into liblivekit_uniffi.a static library (+11.5 MB object code)

[siddarth-byte]

Describe the bug

The uniffi dependency in livekit-uniffi/Cargo.toml (livekit/rust-sdks) unconditionally enables the "cli" feature under [dependencies]. This causes uniffi_bindgen — a build-time code generator — and all its transitive dependencies (reqwest, hyper, ring, clap, serde_json, askama) to be compiled into liblivekit_uniffi.a inside RustLiveKitUniFFI.xcframework.

~11.5 MB of build-tool object code ends up in the static library (26% of the 45 MB .a). These are tools for generating Swift/Kotlin/Python/Ruby bindings from Rust UDL — they have no runtime purpose.

Current Cargo.toml:

[dependencies]
uniffi = { version = "0.30.0", features = ["cli", "scaffolding-ffi-buffer-fns"] }

Because "cli" is unconditional, Cargo's feature unification compiles uniffi_bindgen and its full dependency tree into the static library — even when only the [lib] target is built. UniFFI's own docs recommend running the bindgen binary with --features=uniffi/cli (ref), implying cli should be opt-in — but the current Cargo.toml enables it unconditionally. UniFFI #1482 documents the same underlying issue.

A possible fix is to gate "cli" behind an optional feature with required-features on the [[bin]] target — this is exactly how UniFFI's own Cargo.toml is structured upstream:

[features]
bindgen = ["uniffi/cli"]

[dependencies]
uniffi = { version = "0.30.0", features = ["scaffolding-ffi-buffer-fns"] }

[[bin]]
name = "uniffi-bindgen"
path = "bindgen.rs"
required-features = ["bindgen"]

This way cargo build --lib skips the binary and excludes all CLI dependencies from the static library. The bindgen binary remains available via cargo run --features bindgen --bin uniffi-bindgen.

Introduced in v2.11.0 via PR #822. All versions from v2.11.0 onwards (including v2.12.1) are affected. Root cause is in livekit/rust-sdks — livekit-uniffi/Cargo.toml.

SDK Version

• livekit/client-sdk-swift v2.12.1 (latest)
• livekit/livekit-uniffi-xcframework v0.0.5
• Affected since v2.11.0

iOS/macOS Version

All — build-time packaging issue, not OS-specific.

Xcode Version

Xcode 26.2 / Swift 6.2.3. Issue is Xcode-independent (originates in the Rust Cargo build).

Steps to Reproduce

1. Download RustLiveKitUniFFI.xcframework from the v0.0.5 release:

   curl -L -O "https://github.com/livekit/livekit-uniffi-xcframework/releases/download/0.0.5/RustLiveKitUniFFI.xcframework.zip"
   unzip RustLiveKitUniFFI.xcframework.zip

2. Check the static library size:

   BINARY="RustLiveKitUniFFI.xcframework/ios-arm64/liblivekit_uniffi.a"
   ls -lh "$BINARY"   # → 45 MB

3. Count build-tool symbols that should not be present:

   nm "$BINARY" | grep -c "uniffi_bindgen"    # → 5,163
   nm "$BINARY" | grep -c "reqwest"           # → 566
   nm "$BINARY" | grep -c "clap"              # → 1,041

4. Measure build-tool object file bloat:

   mkdir objfiles && cd objfiles && ar x "../$BINARY"
   ls -l *.o | grep -iE "(uniffi_bindgen|uniffi_pipeline|reqwest|hyper|ring-|askama|serde_json|clap)" \
     | awk '{sum+=$5} END {printf "%.1f MB\n", sum/1048576}'
   # → 11.5 MB

Expected behavior

The static library should contain only runtime code (livekit_protocol, tokio, jsonwebtoken, prost). Build-time code generators and their dependencies should not be shipped. Gating the "cli" feature should reduce the .a by ~11.5 MB.

Screenshots

N/A — binary analysis issue.

Logs

Sample nm output — note Python and Ruby code generators inside the iOS binary:

__ZN...uniffi_bindgen..bindings..python..pipeline..nodes..Field...
__ZN...uniffi_bindgen..bindings..ruby..gen_ruby..Config...
__ZN...uniffi_bindgen..bindings..swift..gen_swift..Config...

Of the 35,701 executable text symbols, 8,948 (25.1%) come from build-tool dependencies.

Full bloat breakdown (click to expand)

Crate	Symbols	Object size	Purpose
uniffi_bindgen	5,163	7.9 MB	Multi-language code generator
uniffi_pipeline	3,474	0.1 MB	Binding AST pipeline
ring	2,740	0.5 MB	TLS crypto (bindgen dependency)
serde_json	1,278	0.2 MB	JSON parsing (bindgen dependency)
hyper	1,170	0.6 MB	HTTP implementation (bindgen dep)
clap	1,041	0.9 MB	CLI argument parser (bindgen dep)
reqwest	566	1.3 MB	HTTP client (bindgen dependency)
askama	146	~0 MB	Template engine (bindgen dependency)
Total	~15,578	11.5 MB	None of these are runtime deps

Legitimate runtime dependencies (expected, should remain):

Component	Symbols	Purpose
livekit_protocol	2,625	Signaling protocol
tokio	2,286	Async runtime
jsonwebtoken	283	Access token validation
prost	211	Protobuf serialization
livekit_api	175	API types

---

Discovered during binary analysis while integrating LiveKit v2.12.1 into an iOS monorepo. Happy to provide additional data or test a fixed build.

[pblazej]

@ladvoc wdyt?

the alternative would be to support dylibs "properly" in cargo-swift (which I'm also considering).

[pblazej]

antoniusnaumann/cargo-swift#96

[pblazej]

also cc @jhugman