Suggest adding return if the type of unused semi return value can coerce to the fn return type

compiler/rustc_errors/src/lib.rs

@@ -507,6 +507,7 @@ pub enum StashKey {
     CallAssocMethod,
     TraitMissingMethod,
     OpaqueHiddenTypeMismatch,
+    MaybeForgetReturn,
 }
 
 fn default_track_diagnostic(d: &mut Diagnostic, f: &mut dyn FnMut(&mut Diagnostic)) {