[JoshuaWink]

Phase 4 of docs/rust-plan.md: a linter for Rust code that uses codeupipe, in packages/codeupipe-lint-rs.

What it is

• A codeupipe pipeline of 11 Filters, in the order of the TypeScript linter. The steps carry LintRun, a payload type of its own (it holds syntax trees), so the linter is also a real use of custom payload types.
• It reads the source with syn and checks the rules the compiler does not: CUP-I000, A003, A005, I003, I005, I006, I007, and the warnings I008 and I009.
• The rule ids, the report, the exit codes (0 / 1 / 2), the cup: allow-... markers, --strict, --exclude, --disable and cup.config.json are those of the Python and TypeScript linters.
• A001, A002, A008, A009, I004 and I001 have no rule: the Rust types refuse that code at compile time.
• Rust details: file names in snake_case, and I008 reported on each impl block so its marker sits by the code. Text in a doc comment or a string is not a marker.
• It is a Cargo workspace of its own (it needs syn and serde_json), so the runtime still builds with no download.

The repository passes it

• Six step types declared inside test functions in codeupipe-rs/tests moved to the top of their files. The test of a plain nested pipeline's label has an allow-anonymous marker, on purpose.
• 45 cup: allow-plain markers, each with a reason: the runtime's own types, the conformance runner's JSON reader and entry point, and the linter's helpers.

CI

A new job, Rust linter: fmt, clippy, tests, the linter with --strict on all the Rust code of the repository (runtime, examples, tests, conformance runner, the linter), and Rust 1.75.

Checked locally

• Linter: 31 tests, clippy and fmt clean; --strict on the repository: No findings (56 suppressed).
• Core crate: 75 tests, clippy and fmt clean. Conformance rust:run: 30 / 30.
• Python: 576 tests, codeupipe_lint.invariants --strict clean.
• Not run locally: the Rust 1.75 build of the linter (only 1.89 is installed). Every dependency declares an MSRV of 1.71 or lower, and Cargo.lock is version 3.

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