compiletest: warn if we see a `// @<directive-name>`

[jieyouxu]

Realized in #125808 (comment).

A test writer might accidentally write

// @ignore-cross-compile

which is parsed as a normal comment not a compiletest directive because compiletest directives require that the line starts with //@.

This typo is hard to spot: both by PR authors and the reviewers. It might not fail in PR CI -- it might not even fail in full build CI, but then explode when the conditions are just right.

We should detect these and issue at least warnings from compiletest to help test writers realize the problem.

[GuillaumeGomez]

Gonna take a look at it.

[fmease]

This should obviously skip tests/rustdoc/ and tests/rustdoc-json/ as // @ (\s@ to be precise) mark the start of {Html,Json}DocCk directives.

[jieyouxu]

Gonna take a look at it.

It might be worth waiting on some directive handling reworks (not concrete design and plan just yet unfortunately) I've been wanting to do

[jieyouxu]

This should obviously skip tests/rustdoc/ and tests/rustdoc-json/ as // @ (\s@ to be precise) mark the start of {Html,Json}DocCk directives.

Huh, many thanks for the heads up, I was not aware that we had these kind of directives lol

[ChrisDenton]

That seems confusing? Could that be changed?