regression: macro expansion/resolution change of some kind

[Mark-Simulacrum]

• https://crater-reports.s3.amazonaws.com/beta-1.76-3/beta-2024-01-09/gh/fishfight.FishFightThePrequel/log.txt

[INFO] [stdout] error[E0424]: expected value, found module `self`
[INFO] [stdout]   --> src/capabilities/weapon.rs:13:23
[INFO] [stdout]    |
[INFO] [stdout] 13 | #[derive(Clone, Copy, CapabilityTrait)]
[INFO] [stdout]    |                       ^^^^^^^^^^^^^^^
[INFO] [stdout]    |                       |
[INFO] [stdout]    |                       `self` value is a keyword only available in methods with a `self` parameter
[INFO] [stdout]    |                       this function doesn't have a `self` parameter
[INFO] [stdout]    |
[INFO] [stdout]    = note: this error originates in the derive macro `CapabilityTrait` (in Nightly builds, run with -Z macro-backtrace for more info)
[INFO] [stdout] help: add a `self` receiver parameter to make the associated `fn` a method
[INFO] [stdout]    |
[INFO] [stdout] 13 | #[derive(Clone, Copy, &self, CapabilityTrait)]

• https://crater-reports.s3.amazonaws.com/beta-1.76-3/beta-2024-01-09/reg/repeated-0.1.2/log.txt
• https://crater-reports.s3.amazonaws.com/beta-1.76-3/beta-2024-01-09/reg/movie-0.1.2/log.txt
  • https://crater-reports.s3.amazonaws.com/beta-1.76-3/beta-2024-01-09/reg/movie_example-0.1.0/log.txt

[INFO] [stdout] error: expected one of `->`, `<`, `where`, or `{`, found `%`
[INFO] [stdout]   --> tests/smoke.rs:5:5
[INFO] [stdout]    |
[INFO] [stdout] 5  | /     repeated!(for x in [0;9;3] {
[INFO] [stdout] 6  | |         fn Welcome_%%x%%() {
[INFO] [stdout] 7  | |             repeated!(for y in [1;%%x%%;2] {
[INFO] [stdout] 8  | |                 println!("From within the macro %%x%%:%%y%%!");
[INFO] [stdout] 9  | |             });
[INFO] [stdout] 10 | |         }
[INFO] [stdout] 11 | |     });
[INFO] [stdout]    | |______^ expected one of `->`, `<`, `where`, or `{`
[INFO] [stdout]    |
[INFO] [stdout]    = note: this error originates in the macro `repeated` (in Nightly builds, run with -Z macro-backtrace for more info)

[compiler-errors]

This regressed in #114571, cc @nnethercote @petrochenkov.

This breakage seems to be acknowledged here: #114571 (comment)

I'm surprised this was just r+'d rather than going through some team consensus or comment period given that it breaks crates in the wild.

[nnethercote]

There are a small number of crates that implement proc macros by calling .to_string() on the input token stream and then doing some half-assed "parsing" (i.e. regex or substring matching) of the output. This is not the right way to do it, because it relies on the output of .to_string() having a particular form, something that isn't guaranteed.

We judged that the number of affected crates was small, and their use was small, and therefore that this was acceptable. I'm happy to listen if others disagree.