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

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 `?`.

1 files changed, 30 insertions, 0 deletions

diff --git a/compiler/rustc_parse/src/errors.rs b/compiler/rustc_parse/src/errors.rs
index 109d36fe689..2e81d2a876b 100644
--- a/compiler/rustc_parse/src/errors.rs
+++ b/compiler/rustc_parse/src/errors.rs
@@ -3212,3 +3212,33 @@ pub struct UnsafeAttrOutsideUnsafeSuggestion {
     #[suggestion_part(code = ")")]
     pub right: Span,
 }
+
+#[derive(Diagnostic)]
+#[diag(parse_binder_before_modifiers)]
+pub struct BinderBeforeModifiers {
+    #[primary_span]
+    pub binder_span: Span,
+    #[label]
+    pub modifiers_span: Span,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_binder_and_polarity)]
+pub struct BinderAndPolarity {
+    #[primary_span]
+    pub polarity_span: Span,
+    #[label]
+    pub binder_span: Span,
+    pub polarity: &'static str,
+}
+
+#[derive(Diagnostic)]
+#[diag(parse_modifiers_and_polarity)]
+pub struct PolarityAndModifiers {
+    #[primary_span]
+    pub polarity_span: Span,
+    #[label]
+    pub modifiers_span: Span,
+    pub polarity: &'static str,
+    pub modifiers_concatenated: String,
+}