Tracking issue for RFC 2383, "Lint Reasons RFC"

[Centril]

This is a tracking issue for the RFC "Lint Reasons RFC" (rust-lang/rfcs#2383).

Steps:

• Implement the RFC (cc @rust-lang/compiler -- can anyone write up mentoring instructions?)
  • Implement reason = lint reasons (RFC 2383, part 1) #54683
  • Implement #[expect(lint)] -- see Implement expect attribute from RFC 2383, "Lint Reasons RFC" #85549
• Adjust documentation (see instructions on forge)
• Stabilization PR: Let's #[expect] some lints: Stabilize lint_reasons (RFC 2383)  #120924

Unresolved questions:

• The use sites of the reason parameter.

[zackmdavis]

I started looking at this tonight. (Briefly; I regret that my time is limited.)

The example output in the RFC puts a reason: line immediately below the top-level error: line, but from a consilience-with-the-existing-implementation perspective, I'm inclined to think it would make more sense to use an ordinary note: (just as the existing "#[level(lint)] on by default" messages and must-use rationales do).

The reason should probably be stored as a third field inside of LintSource::Node.

[zackmdavis]

PR for reason = is #54683.

Unresolved questions:

• The use sites of the reason parameter.

@myrrlyn I'm not sure what "use sites" means in this context?

[zackmdavis]

It's a little bit sad/awkward that reason comments are most useful in practice for allow (rather than warn, deny, or forbid, which authors are more likely to regard as self-explanatory), and yet allow is the only case for which the new reason = functionality doesn't actually do anything. 😿

(This observation prompted by my thought that we should be able to dogfood reason = in this repo now that that functionality is in the beta bootstrap compiler, but ag '//.*\n\w*#\[(warn|deny|forbid|allow)' --ignore tools --ignore test only turns up instances of allow.)

[oli-obk]

We could add a lint that suggests turning allow with reason into expect because $reasons

[myrrlyn]

I apologize for my absence on this issue.

I'm not sure what "use sites" means in this context?

My standing question was "how should the reason value be used in the diagnostic", and it appears that the simplest answer is the one you provided: "put it in a note item". I have no firm attachment to rendering it as reason: User text here, and if prior art exists for note: User text here, then that is perfectly fine.