Rollup merge of #103383 - compiler-errors:tait-scope, r=oli-obk

Note scope of TAIT more accurately

This maybe explains why the person was confused in #101897, since we say "same module" but really should've said "same impl".

r? ``@oli-obk``

compiler/rustc_hir_analysis/src/errors.rs

@@ -143,6 +143,7 @@ pub struct UnconstrainedOpaqueType {
     #[primary_span]
     pub span: Span,
     pub name: Symbol,
+    pub what: &'static str,
 }
 
 pub struct MissingTypeParams {