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Rollup merge of #127054 - compiler-errors:bound-ordering, r=fmease

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Reorder trait bound modifiers *after* `for<...>` binder in trait bounds

This PR suggests changing the grammar of trait bounds from:

```
[CONSTNESS] [ASYNCNESS] [?] [BINDER] [TRAIT_PATH]

const async ? for<'a> Sized
```

to

```
([BINDER] [CONSTNESS] [ASYNCNESS] | [?]) [TRAIT_PATH]
```

i.e., either

```
? Sized
```

or

```
for<'a> const async Sized
```

(but not both)

### Why?

I think it's strange that the binder applies "more tightly" than the `?` trait polarity. This becomes even weirder when considering that we (or at least, I) want to have `async` trait bounds expressed like:

```
where T: for<'a> async Fn(&'a ()) -> i32,
```

and not:

```
where T: async for<'a> Fn(&'a ()) -> i32,
```

### Fallout

No crates on crater use this syntax, presumably because it's literally useless. This will require modifying the reference grammar, though.

### Alternatives

If this is not desirable, then we can alternatively keep parsing `for<'a>` after the `?` but deprecate it with either an FCW (or an immediate hard error), and begin parsing `for<'a>` *before* the `?`.
This commit is contained in:
Matthias Krüger 2024-07-25 04:43:18 +02:00 • committed by GitHub
commit cfc5f25b3d
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20 changed files with 201 additions and 68 deletions
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11
compiler/rustc_ast_passes/src/ast_validation.rs
View file

@ -1366,17 +1366,6 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
{
self.dcx().emit_err(errors::TildeConstDisallowed { span, reason });
}
(
_,
BoundConstness::Always(_) | BoundConstness::Maybe(_),
BoundPolarity::Negative(_) | BoundPolarity::Maybe(_),
) => {
self.dcx().emit_err(errors::IncompatibleTraitBoundModifiers {
span: bound.span(),
left: modifiers.constness.as_str(),
right: modifiers.polarity.as_str(),
});
}
_ => {}
}

9
compiler/rustc_ast_passes/src/errors.rs
View file

@ -656,15 +656,6 @@ pub enum TildeConstReason {
Item,
}
#[derive(Diagnostic)]
#[diag(ast_passes_incompatible_trait_bound_modifiers)]
pub struct IncompatibleTraitBoundModifiers {
#[primary_span]
pub span: Span,
pub left: &'static str,
pub right: &'static str,
}
#[derive(Diagnostic)]
#[diag(ast_passes_const_and_async)]
pub struct ConstAndAsync {