[sorinirimies]

Summary

cargo gpu install finds the toolchain by text-matching channel = "..." in the checked-out rustc_codegen_spirv/build.rs; cargo xtask update-toolchain rewrites the same line with a regex. Nothing tested that contract, so drift (e.g. indenting the line, or build.rs and rust-toolchain.toml disagreeing) would only show up when users install.

The format has to stay readable for older rust-gpu checkouts, so this keeps it and only makes it more lenient.

 get_channel_from_rustc_codegen_spirv_build_script(package)
   contents = read(build.rs)
-  find line.strip_prefix(`channel = "`); slice to next `"`
+  parse_toolchain_channel(contents)          // pure, unit-tested
+    find line.trim_start().strip_prefix(`channel = "`)
+    split at closing `"`; error if missing or empty

Evidence

New tests in spirv_source::test:

parse_toolchain_channel_of_real_build_script                  ok  // guards the real file
build_script_channel_matches_rust_toolchain_toml              ok  // guards xtask drift
parse_toolchain_channel_takes_first_match_and_allows_indent   ok  // old impl: Err
parse_toolchain_channel_errors                                ok

cargo test -p cargo-gpu-install --features test spirv_source: 9 passed. Clippy -D warnings clean.

Merge Danger

Door: two-way

Blast Radius: small — every input the old parser accepted yields the same channel, except channel = "", which now errors instead of returning an empty string.