`src/tools/rustdoc-gui-test` uses compiletest directives + `TestProp` handling internals in a questionable way

I was wondering why compiletest's Config had a default, it turns out it's used by

https://github.com/rust-lang/rust/blob/9535feebd5741a55fc24e84060e82d41a75dac6e/src/tools/rustdoc-gui-test/src/main.rs#L114-L137

to piggy-back off of compiletest's directive parsing and compile/run-flags extraction.

This happens to work because only //@ {compile,run}-flags directives are used, which do not otherwise happen to rely on other parts of the compiletest config or get validated in a way that requires other parts of the compiletest config (for now).

tests/rustdoc-gui/src/extend_css/lib.rs
1://@ compile-flags: --extend-css extra.css

tests/rustdoc-gui/src/theme_css/lib.rs
1://@ compile-flags: --theme custom-theme.css

tests/rustdoc-gui/src/scrape_examples/src/lib.rs
1://@ run-flags:-Zrustdoc-scrape-examples

tests/rustdoc-gui/src/link_to_definition/lib.rs
1://@ compile-flags: -Zunstable-options --generate-link-to-definition

IMO this is very questionable, because this bypasses any validation that compiletest might perform between parsing CLI args passed from bootstrap, and then constructing a validated Config. Furthermore, you'll appear to be able to use //@ only-xxx and other conditional test execution directives, where in reality they do not work at all (or are completely wrong) because the dummy config prevents compiletest conditional test directives from having any info to work with properly.

In other words, compiletest tries to be internally self-consistent / coherent, but it can't enforce that if its internals get exposed and "intercepted".

Possible mitigations / better approaches

• If //@ {run,compile}-flags is all rustdoc-gui-test needs, it might be better to just reimplement a naive per-line //@ {compile,run}-flags.
• Or try to merge rustdoc-gui-test into compiletest, but it runs tests with node in quite a different way versus other test suites.

Start with src/tools/rustdoc-gui-test/src/main.rs around lines 114-137, then inspect compiletest Config construction and directive parsing. Review the listed rustdoc-gui test files and determine whether the tool should parse only compile/run flags or be integrated with compiletest; done means avoiding an invalid dummy configuration and making unsupported directives unambiguous.