Identify non-function annotated by contract in macro expand

[xizheyin]

Closing #137129

[rustbot]

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[Noratrieb]

Hm, I'm not entirely sure if this is the right fix. The contracts::ensures attribute must only be applied to functions, that the builtin macro expands when applied in this position in the first place is a bit strange.. it looks like this logic here is wrong:

rust/compiler/rustc_builtin_macros/src/contracts.rs

Line 71 in 4510e86

.span_err(attr_span, "contract annotations can only be used on functions"));

I'd prefer to see the contract macros do proper parsing by fixing this FIXME here:

rust/compiler/rustc_builtin_macros/src/contracts.rs

Line 42 in 4510e86

// FIXME: this kind of manual token tree munging does not have significant precedent among

instead of adding special cases for them

[xizheyin]

Thank you, I will try to fix it in this way. @Noratrieb

[xizheyin]

@rustbot label -S-waiting-on-author +S-waiting-on-review

[Noratrieb]

I don't think we should make this token parsing even more complicated than it already is. Either we do a proper AST macro like the other built-in attribute macros, or we'll leave this issue open. It's a very weird edge case, so it's not very important.
Doing this as a proper AST macro will probably not be very easy as it requires adding more features to the rustc expansion infrastructure (but it's certainly not impossible).

[xizheyin]

I might have a try. Do you have any suggestions on implementation (such as things to pay attention to)? Or are there some codes and documents to refer to? I have no experience in builtin atrribute macro. @Noratrieb

[Noratrieb]

I haven't looked much at the code myself, no. Something like the rustc_autodiff macro might be worth looking at, and how it works end-to-end. The rustc dev guide also has some docs on macro expansion. But as I said, it's probably not easy.

[xizheyin]

Should I open an issue to track this?