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

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

1 files changed, 3 insertions, 1 deletions

diff --git a/compiler/rustc_ast_lowering/src/lib.rs b/compiler/rustc_ast_lowering/src/lib.rs
index 446e02e4024..22a293d9b50 100644
--- a/compiler/rustc_ast_lowering/src/lib.rs
+++ b/compiler/rustc_ast_lowering/src/lib.rs
@@ -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(