ui_test: more robust comment parsing

[RalfJung]

Currently, if we fail to parse a //~ line or an annotation like // ignore-target, we just go on. That's a pretty bad default for infrastructure that is meant to test our code! For both of these I think we should hard error if we cannot parse the line.

For //~ that is fairly easy; if any comment starting //~ eludes us we can just error.

For the other annotations it is a lot harder since that might just be a regular comment. I think we need to change their syntax so that we can tell that this line is meant to be special. //@ might work? We should ideally sync this with compiletest, in case they have similar plans.

[oli-obk]

Compiletest wants to become more robust/explicit here, too: rust-lang/compiler-team#513

[RalfJung]

That sounds somewhat orthogonal -- that is about adding extra annotations that compiletest leaves uninterpreted, which is different from being able to tell when the user has made a typo in their annotation.

[oli-obk]

Hmm... seems the same to me. If we don't have a sigil like @ making sure that we must successfully parse, anything like // ingore-x86 will just silently be ignored (pun intended)

[RalfJung]

In my understanding, that issue is about // annotations. As in, 'annotations' is a keyword, not a stand-in for "any kind of annotation".

[RalfJung]

Ah, this is the compiletest proposal you probably meant: rust-lang/compiler-team#512. That's probably where I got //@ from.

[RalfJung]

Reopening since the transition is still ongoing. :)

[RalfJung]

Oh wait, you have ported all tests. I didn't realize that. Wow :)