harness: tidy build.rs comment + extract Fixture::fail helper

[Copilot]

Low-risk cleanup of seams left in crates/tests/fixtures/harness/ after the recent test-consolidation churn (#4339, #4342, #4344, #4347–#4354, #4365). No behaviour change.

• build.rs

  • Top-of-file comment claimed generated tests dispatched into harness::run_<group>(name) in the lib; in fact build.rs emits crate::run_fixture(group, name) and the dispatcher lives in tests/fixtures.rs. Updated.
  • Removed dead let _ = &mut groups; (and its incorrect "suppress unused warning" comment) — groups is iterated unconditionally on the line above.

• tests/fixtures.rs

  • Added Fixture::fail(stage, error) and routed 12 of the 14 unwrap_or_else(|e| panic!("[{}/{}] ...: {e}", f.group, f.name)) sites through it. stage: impl Display so format_args!("reader({path})") works without an extra allocation. The two unwrap_or_else(|| panic!(...)) ("expected to fail but succeeded") sites have a different shape and are left alone.

impl Fixture {
    fn fail(&self, stage: impl std::fmt::Display, error: impl std::fmt::Display) -> ! {
        panic!("[{}/{}] {stage}: {error}", self.group, self.name);
    }
}

// before
.unwrap_or_else(|e| panic!("[{}/{}] reader({}): {e}", f.group, f.name, rdl.display()));
// after
.unwrap_or_else(|e| f.fail(format_args!("reader({})", rdl.display()), e));

Verified that doc/CI sweep items from the plan (stale references to retired docs/{test,cross}-todo.md, linux.yml, tests/roundtrip/*, pre-rename Generic<T>/Ref) are already clean — no edits needed.

Out of scope, deferred to focused follow-ups: lifting the cfg(windows) gate on compile_fixtures.rs, replacing the hand-rolled fixture.toml parser with a real TOML dep, reshaping kind = "reader_no_input", and the msrv.yml / no_std.yml / slim_errors.yml workflow merges.

[Copilot]

Pull request overview

Cleans up the test-fixtures harness by correcting build.rs documentation and centralizing fixture failure formatting in a Fixture::fail helper, keeping panic output consistent across group runners.

Changes:

• Update build.rs top-level comment to reflect that generated tests call crate::run_fixture(group, name) and remove a dead unused-variable suppression.
• Add Fixture::fail(stage, error) and route most framed unwrap_or_else(... panic!(...)) sites through it for uniform error messages.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
crates/tests/fixtures/harness/tests/fixtures.rs	Adds Fixture::fail and refactors runner error handling to use it for consistent [group/name] stage: error panics.
crates/tests/fixtures/harness/build.rs	Fixes misleading comment about generated test dispatch and removes unnecessary groups mutability/unused suppression.

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