Rollup merge of #135044 - compiler-errors:better-infer-suggestions-in-const, r=oli-obk

Improve infer (`_`) suggestions in `const`s and `static`s

Fixes https://github.com/rust-lang/rust/issues/135010.

This PR does a few things to (imo) greatly improve the error message when users write something like `static FOO: [i32; _] = [1, 2, 3]`.

Firstly, it adapts the recovery code for when we encounter `_` in a const/static to work a bit more like `fn foo() -> _`, and removes the somewhat redundant query `diagnostic_only_typeck`.

Secondly, it changes the lowering for `[T; _]` to always lower under the `feature(generic_arg_infer)` logic to `ConstArgKind::Infer`. We still issue the feature error, so it's not doing anything *observable* on the good path, but it does mean that we no longer erroneously interpret `[T; _]`'s array length as a `_` **wildcard expression** (à la destructuring assignment, like `(_, y) = expr`).

Lastly it makes the suggestions verbose and fixes (well, suppresses) a bug with stashing and suggestions.

r? oli-obk

1 files changed, 11 insertions, 14 deletions

diff --git a/tests/ui/array-slice-vec/suggest-array-length.fixed b/tests/ui/array-slice-vec/suggest-array-length.fixed
index 29f85da56e5..2eacc2517d3 100644
--- a/tests/ui/array-slice-vec/suggest-array-length.fixed
+++ b/tests/ui/array-slice-vec/suggest-array-length.fixed
@@ -3,24 +3,21 @@
 
 fn main() {
     const Foo: [i32; 3] = [1, 2, 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
     const REF_FOO: &[u8; 1] = &[1];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for constants
+    static Statik: [i32; 3] = [1, 2, 3];
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for static variables
+    static REF_STATIK: &[u8; 1] = &[1];
+    //~^ ERROR the placeholder `_` is not allowed within types on item signatures for static variables
     let foo: [i32; 3] = [1, 2, 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR using `_` for array lengths is unstable
     let bar: [i32; 3] = [0; 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR using `_` for array lengths is unstable
     let ref_foo: &[i32; 3] = &[1, 2, 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR using `_` for array lengths is unstable
     let ref_bar: &[i32; 3] = &[0; 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR using `_` for array lengths is unstable
     let multiple_ref_foo: &&[i32; 3] = &&[1, 2, 3];
-    //~^ ERROR in expressions, `_` can only be used on the left-hand side of an assignment
-    //~| ERROR using `_` for array lengths is unstable
+    //~^ ERROR using `_` for array lengths is unstable
 }