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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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9
compiler/rustc_parse/messages.ftl
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@ -53,6 +53,12 @@ parse_bare_cr = {$double_quotes ->
parse_bare_cr_in_raw_string = bare CR not allowed in raw string
parse_binder_and_polarity = `for<...>` binder not allowed with `{$polarity}` trait polarity modifier
.label = there is not a well-defined meaning for a higher-ranked `{$polarity}` trait
parse_binder_before_modifiers = `for<...>` binder should be placed before trait bound modifiers
.label = place the `for<...>` binder before any modifiers
parse_bounds_not_allowed_on_trait_aliases = bounds are not allowed on trait aliases
parse_box_not_pat = expected pattern, found {$descr}
@ -577,6 +583,9 @@ parse_missing_trait_in_trait_impl = missing trait in a trait impl
parse_modifier_lifetime = `{$modifier}` may only modify trait bounds, not lifetime bounds
.suggestion = remove the `{$modifier}`
parse_modifiers_and_polarity = `{$modifiers_concatenated}` trait not allowed with `{$polarity}` trait polarity modifier
.label = there is not a well-defined meaning for a `{$modifiers_concatenated} {$polarity}` trait
parse_more_than_one_char = character literal may only contain one codepoint
.followed_by = this `{$chr}` is followed by the combining {$len ->
[one] mark