The UI test chapter should explain reference annotations

[mejrs]

I want to update a test that has a //@ reference: attributes.diagnostic.namespace.unknown-invalid-syntax at the top, and I had a lot of trouble figuring out what that does (I found rust-lang/compiler-team#783 eventually).

In particular if I want to add to or change a test that has this, should I care?

cc @ehuss

[ehuss]

Did you see https://rustc-dev-guide.rust-lang.org/tests/directives.html#misc-directives which links to https://github.com/rust-lang/reference/blob/master/docs/authoring.md#test-rule-annotations which should explain what they mean?

[mejrs]

I did not, my bad. I was only looking at https://rustc-dev-guide.rust-lang.org/tests/ui.html and my brain skipped over the test directives chapter...