Unstable Rust usage in Chromium

Policy

Unstable features are unsupported by default in Chromium. Any use of an unstable language or library feature should be agreed upon by the Rust toolchain team before enabling it.

Since Chromium imports the Rust toolchain at its HEAD and builds it in a nightly-like configuration, it is technically possible to depend on unstable features. However, unstable features often change in a backwards-incompatible way without a warning. If such incompatible changes are introduced, importing a new version of toolchain now requires the owner to fix forward, instead of being an automated process. This makes toolchain upgrades prohibitively difficult.

When an exception is required, consider:

• Whether the unstable feature brings significant value that is unattainable in stable alternatives
• The risk of breaking changes to the feature
• Ways to fallback in case a backward-incompatible toolchain change is introduced

The following directories have a stricter policy toward unstable features:

• //build accepts patches for community-maintained support for working with a stable version of the Rust compiler. See also rustc_nightly_capability and https://crbug.com/410596442
• //v8 tentatively disallows unstable features to provide community-maintained support for building V8 with a stable version of the Rust compiler. See also https://crbug.com/450577984

Exceptions

This section maintains a list of exceptions from the policy above:

• #![feature(portable_simd)] needed by the ETC1 encoder (see the discussion in the doc here
  • //ui/android/texture_compressor
  • //third_party/rust/bytemuck
• #![feature(linkage)] and #![feature(rustc_attrs)] needed to use PartitionAlloc from Rust (removal is tracked by https://crbug.com/410596442)
  • //build/rust/allocator
• maybe_uninit_write_slice, assert_matches, maybe_uninit_slice: Grandfathered-in exception in prototype code (i.e. not used and not shipping)
  • mojo/public/rust/...

How to allow unstable features in BUILD.gn

Example BUILD.gn for first-party code:

```
# BUILD.gn:
rust_static_library("some_target_name") {
  configs -= [ "//build/config/compiler:disallow_unstable_features" ]
  rustflags = [ "-Zallow-features=portable_simd" ]
}
```

Example gnrt_config.toml entry for a third-party crate (run tools/crates/run_gnrt.py gen to regenerate the crate's BUILD.gn file):

```
# gnrt_config.toml:
[crate.bytemuck.extra_kv]
allow_unstable_features = ["portable_simd"]
```