compiletest: Do the known-directives check only once, and improve its error message

[Zalathar]

This PR is a combination of three changes:

• Store the list of known directives in a set, so that checking a directive name doesn't require a linear scan
• Extract the known-directives check out of iter_directives and do it only once, instead of multiple times per file
• Improve the error message for unknown directives, as requested on Zulip

The change to error messages is fused with the extraction, since doing it independently would have been more awkward.

---

Before

Testing stage1 with compiletest suite=coverage mode=coverage-map (aarch64-apple-darwin)
errors encountered during EarlyProps parsing: /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs
2025-11-05T01:55:46.440012Z ERROR compiletest::directives: /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs:2: detected unknown compiletest test directive `add-core-stubs`

thread '<unnamed>' (36268582) panicked at src/tools/compiletest/src/directives.rs:72:13:
errors encountered during EarlyProps parsing
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

After

Testing stage1 with compiletest suite=coverage mode=coverage-map (aarch64-apple-darwin)

thread '<unnamed>' (36270772) panicked at src/tools/compiletest/src/lib.rs:876:9:
directives check failed:
ERROR: unknown compiletest directive `add-core-stubs` at /Users/stuart/Dev/rust/rust/tests/coverage/trivial.rs:2
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

[rustbot]

Some changes occurred in src/tools/compiletest

cc @jieyouxu

compiletest directives have been modified. Please add or update docs for the
new or modified directive in src/doc/rustc-dev-guide/.

[rustbot]

r? @Kobzol

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[Zalathar]

This was originally going to wait until after #147955, but then Ralf and I both got annoyed by the error message, and improving the error message would be a lot harder without extracting the check first.

[jieyouxu]

Thanks. Eventually, I would like us to move towards a late rejection, where if we fail to match the directive against any registered handle, then that constitutes an unknown directive. But in the meantime, making this "stop the bleeding" logic nicer is also a good improvement.

View changes since this review

[jieyouxu]

We can land this first.
@bors r+ rollup

[bors]

📌 Commit 795e906 has been approved by jieyouxu

It is now in the queue for this repository.