Rollup merge of #136205 - compiler-errors:len-3, r=BoxyUwU

Properly check that array length is valid type during built-in unsizing in index

This results in duplicated errors, but this class of errors is not new; in general, we aren't really equipped to detect cases where a WF error due to a field type would be shadowed by the parent struct of that field also not being WF.

This also adds a note for these types of mismatches to make it clear that this is due to an array type.

Fixes #134352

r? boxyuwu

compiler/rustc_middle/src/traits/mod.rs

@@ -194,6 +194,9 @@ pub enum ObligationCauseCode<'tcx> {
     /// A slice or array is WF only if `T: Sized`.
     SliceOrArrayElem,
 
+    /// An array `[T; N]` can only be indexed (and is only well-formed if) `N` has type usize.
+    ArrayLen(Ty<'tcx>),
+
     /// A tuple is WF only if its middle elements are `Sized`.
     TupleElem,