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Rollup merge of #139641 - BoxyUwU:allow_parend_array_len_infer, r=compiler-errors

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Allow parenthesis around inferred array lengths

In #135272 it was noticed that we weren't handling `Vec<(((((_)))))>` correctly under the new desugaring for `generic_arg_infer`, this had to be fixed in order to not regress stable code for types that should continue working. This has the side effect of *also* allowing the following to work:
```rust
#![feature(generic_arg_infer)]
struct Bar<const N: usize>;
fn main() {
    let a: Bar<((_))> = Bar::<10>;
}
```

However I did not make the same change for array lengths resulting in the following not compiling:
```rust
#![feature(generic_arg_infer)]
fn main() {
    let a: [u8; (((_)))] = [2; 2];
    let a: [u8; 2] = [2; (((((_)))))];
}
```

This is rather inconsistent as parenthesis around `_` *are* supported for const args to non-arrays, and type args. This PR fixes this allowing the above example to compile. No stable impact.

r? compiler-errors
This commit is contained in:
Stuart Cook 2025-04-11 13:31:49 +10:00 • committed by GitHub
commit 25d282efd4
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3 changed files with 75 additions and 2 deletions
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4
compiler/rustc_ast_lowering/src/lib.rs
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@ -2034,7 +2034,9 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
}
fn lower_array_length_to_const_arg(&mut self, c: &AnonConst) -> &'hir hir::ConstArg<'hir> {
match c.value.kind {
// We cannot just match on `ExprKind::Underscore` as `(_)` is represented as
// `ExprKind::Paren(ExprKind::Underscore)` and should also be lowered to `GenericArg::Infer`
match c.value.peel_parens().kind {
ExprKind::Underscore => {
if !self.tcx.features().generic_arg_infer() {
feature_err(