Negative UI test error annotations

[jieyouxu]

I wish there is a negative assertion, saying that here one shall not expect a HELP note.

Originally posted by @dingxiangfei2009 in #107251 (comment)

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Something that might be nice-to-have is negative ui test error annotations, i.e. that we explicitly want to make sure some diagnostic is not emitted (analogous to FileCheck check-NOT annotations).

Obviously, you can rely on checking the blessed stderr, but a negative ui test annotation could help communicate intent in the test itself and is less prone to accidentally letting the blessed stderr though.

Note: we should probably try to make the directive parser more robust and less hacky before attempting to add more syntax.

[dingxiangfei2009]

Let me know if we would like to go ahead with this feature. I am getting my hands dirty enough with compiletest that I feel like I know how to. 😄

[jieyouxu]

@dingxiangfei2009 Thanks for the interest. I would be happy to aid and review this feature, but I feel like we want to make compiletest a bit more maintainable first, see e.g. #128070. I think we should not hack on more syntax for directives and error annotations before we figure out how to make the directive and annotation parser itself a bit more robust, lol.

[fmease]

Related: #46667 (2017).