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

2 files changed, 72 insertions, 1 deletions

diff --git a/tests/ui/const-generics/generic_arg_infer/parend_infer.nogate.stderr b/tests/ui/const-generics/generic_arg_infer/parend_infer.nogate.stderr
new file mode 100644
index 00000000000..d0a5da9676d
--- /dev/null
+++ b/tests/ui/const-generics/generic_arg_infer/parend_infer.nogate.stderr
@@ -0,0 +1,53 @@
+error[E0658]: const arguments cannot yet be inferred with `_`
+  --> $DIR/parend_infer.rs:24:16
+   |
+LL |     let c: Foo<_> = Foo::<1>;
+   |                ^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: const arguments cannot yet be inferred with `_`
+  --> $DIR/parend_infer.rs:26:16
+   |
+LL |     let c: Foo<(_)> = Foo::<1>;
+   |                ^^^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: const arguments cannot yet be inferred with `_`
+  --> $DIR/parend_infer.rs:28:16
+   |
+LL |     let c: Foo<(((_)))> = Foo::<1>;
+   |                ^^^^^^^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/parend_infer.rs:17:17
+   |
+LL |     let b: [u8; (_)] = [1; (((((_)))))];
+   |                 ^^^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error[E0658]: using `_` for array lengths is unstable
+  --> $DIR/parend_infer.rs:17:28
+   |
+LL |     let b: [u8; (_)] = [1; (((((_)))))];
+   |                            ^^^^^^^^^^^
+   |
+   = note: see issue #85077 <https://github.com/rust-lang/rust/issues/85077> for more information
+   = help: add `#![feature(generic_arg_infer)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 5 previous errors
+
+For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/const-generics/generic_arg_infer/parend_infer.rs b/tests/ui/const-generics/generic_arg_infer/parend_infer.rs
index 81c42183b38..3dc27a702de 100644
--- a/tests/ui/const-generics/generic_arg_infer/parend_infer.rs
+++ b/tests/ui/const-generics/generic_arg_infer/parend_infer.rs
@@ -1,7 +1,9 @@
-//@ check-pass
+//@[gate] check-pass
 //@ revisions: gate nogate
 #![cfg_attr(gate, feature(generic_arg_infer))]
 
+struct Foo<const N: usize>;
+
 fn main() {
     // AST Types preserve parens for pretty printing reasons. This means
     // that this is parsed as a `TyKind::Paren(TyKind::Infer)`. Generic
@@ -9,4 +11,20 @@ fn main() {
     // but `TyKind::Infer` wrapped in arbitrarily many `TyKind::Paren`.
     let a: Vec<(_)> = vec![1_u8];
     let a: Vec<(((((_)))))> = vec![1_u8];
+
+    // AST Exprs similarly preserve parens for pretty printing reasons.
+    #[rustfmt::skip]
+    let b: [u8; (_)] = [1; (((((_)))))];
+    //[nogate]~^ error: using `_` for array lengths is unstable
+    //[nogate]~| error: using `_` for array lengths is unstable
+    let b: [u8; 2] = b;
+
+    // This is the same case as AST types as the parser doesn't distinguish between const
+    // and type args when they share syntax
+    let c: Foo<_> = Foo::<1>;
+    //[nogate]~^ error: const arguments cannot yet be inferred with `_`
+    let c: Foo<(_)> = Foo::<1>;
+    //[nogate]~^ error: const arguments cannot yet be inferred with `_`
+    let c: Foo<(((_)))> = Foo::<1>;
+    //[nogate]~^ error: const arguments cannot yet be inferred with `_`
 }