[Sema] Produce better diagnostics when using inout on parameter types that cannot be 'inout'. Resolves swiftlang#68417

Author: mininny

include/swift/AST/DiagnosticsSema.def

@@ -3685,6 +3685,9 @@ ERROR(attr_not_on_variadic_parameters,none,
       "'%0' must not be used on variadic parameters", (StringRef))
 ERROR(attr_not_on_subscript_parameters,none,
       "'%0' must not be used on subscript parameters", (StringRef))
+ERROR(attr_not_on_enum_case_parameters,none,
+      "'%0' must not be used on enum case parameters", (StringRef))
+
 ERROR(attr_not_on_stored_properties,none,
       "'%0' must not be used on stored properties", (DeclAttribute))
 

lib/Sema/TypeCheckType.cpp

@@ -4294,19 +4294,26 @@ TypeResolver::resolveOwnershipTypeRepr(OwnershipTypeRepr *repr,
       options.hasBase(TypeResolverContext::SubscriptDecl) ||
       options.hasBase(TypeResolverContext::EnumElementDecl)) {
 
-    decltype(diag::attr_only_on_parameters) diagID;
-    if (options.getBaseContext() == TypeResolverContext::SubscriptDecl) {
+    auto diagID = diag::attr_only_on_parameters;
+    bool removeRepr = true;
+    if (options.hasBase(TypeResolverContext::SubscriptDecl)) {
       diagID = diag::attr_not_on_subscript_parameters;
     } else if (options.is(TypeResolverContext::VariadicFunctionInput)) {
       diagID = diag::attr_not_on_variadic_parameters;
+    } else if (options.hasBase(TypeResolverContext::EnumElementDecl)) {
+      diagID = diag::attr_not_on_enum_case_parameters;
     } else {
       diagID = diag::attr_only_on_parameters;
+      removeRepr = false;
     }
     StringRef name;
     if (ownershipRepr) {
       name = ownershipRepr->getSpecifierSpelling();
     }
-    diagnoseInvalid(repr, repr->getSpecifierLoc(), diagID, name);
+    auto diag = diagnoseInvalid(repr, repr->getSpecifierLoc(), diagID, name);
+    if (removeRepr)
+      diag.fixItRemove(repr->getLoc());
+
     return ErrorType::get(getASTContext());
   }
 

test/type/types.swift

@@ -205,3 +205,10 @@ func rdar94888357() {
 
   let _ = S<String, String>("") // expected-error {{generic type 'S' specialized with too many type parameters (got 2, but expected 1)}}
 }
+
+// https://github.com/apple/swift/issues/68417
+enum E {
+  subscript(x: inout Int) -> Bool { true } // expected-error {{'inout' must not be used on subscript parameters}} {{16-22=}}
+  case c(x: inout Int) // expected-error {{'inout' must not be used on enum case parameters}} {{13-19=}}
+  func d(x: inout Int ...) // expected-error {{'inout' must not be used on variadic parameters}} {{13-19=}}
+}