[build] Adopt build without vendored Rust files

[fhanau]

Derived from #6125, but using plain rules_rust. Note that this can safely be merged by itself without needing a change to the internal rust configuration (as CI will show).

[ask-bonk]

Build migration review: switching from vendored crate BUILD files to crate.from_cargo. The bulk of the PR is deletion of generated files, which is great. A few items to address.

Critical: Missing derive feature for clap will break gen-compile-cache.
Minor: Dead code and stale comments from the migration.

[ask-bonk]

I'm Bonk, and I've done a quick review of your PR.

This PR migrates Rust dependency management from vendored crate BUILD files (crates_vendor mode) to crate.from_cargo with a Cargo.toml manifest, deleting ~18K lines of generated Bazel files and upgrading rules_rust from 0.69.0 to 0.70.0.

Issues found (ranked by severity):

1. Missing derive feature for clap (high): src/rust/gen-compile-cache/main.rs uses #[derive(Parser)] which requires clap's derive feature. The old cargo.bzl included it (features = ["derive", "std", "help"]), but the new Cargo.toml dropped it. The clap_derive crate was also removed from Cargo.lock. This will break the gen-compile-cache binary build.

2. Stale comments on RUST_TARGET_TRIPLES (low): The "macOS cross-compilation" and "macOS rosetta" comments were carried over from the old 2-entry list. Now that the list has all 5 supported triples, the "macOS rosetta" comment on aarch64-unknown-linux-gnu is incorrect.

3. Dead config setting groups in deps/rust/BUILD.bazel (low): The five platform config_setting_group definitions were only used by the removed CARGO_BAZEL select and are now unreferenced.

4. Bare # TODO in lol_html_c_api annotation (low): The TODO comment doesn't describe what needs to be done.

All issues posted as inline review comments with suggestions where applicable.

github run

[github-actions]

The generated output of @cloudflare/workers-types matches the snapshot in types/generated-snapshot 🎉

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 66.44%. Comparing base (b322726) to head (3bf1c00).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6643      +/-   ##
==========================================
- Coverage   66.49%   66.44%   -0.06%     
==========================================
  Files         405      405              
  Lines      117814   116097    -1717     
  Branches    19429    19429              
==========================================
- Hits        78342    77142    -1200     
+ Misses      27896    27374     -522     
- Partials    11576    11581       +5     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 72 untouched benchmarks
⏩ 129 skipped benchmarks¹

---

_{Comparing felix/030326-rules_rust-no-crate-files (3bf1c00) with main (b322726)}

Footnotes

1. 129 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩