[AST] Allow @preconcurrency attribute in inheritance clause

Author: xedin

include/swift/AST/ASTBridging.h

@@ -1161,6 +1161,7 @@ enum ENUM_EXTENSIBILITY_ATTR(closed) BridgedTypeAttrKind : size_t {
   BridgedTypeAttrKind_Sendable,
   BridgedTypeAttrKind_retroactive,
   BridgedTypeAttrKind_unchecked,
+  BridgedTypeAttrKind_preconcurrency,
   BridgedTypeAttrKind__local,
   BridgedTypeAttrKind__noMetadata,
   BridgedTypeAttrKind__opaqueReturnTypeOf,

include/swift/AST/Attr.def

@@ -57,6 +57,7 @@ TYPE_ATTR(async)
 TYPE_ATTR(Sendable)
 TYPE_ATTR(retroactive)
 TYPE_ATTR(unchecked)
+TYPE_ATTR(preconcurrency)
 TYPE_ATTR(_local)
 TYPE_ATTR(_noMetadata)
 TYPE_ATTR(_opaqueReturnTypeOf)

include/swift/AST/DiagnosticsSema.def

@@ -5626,6 +5626,11 @@ ERROR(unchecked_not_inheritance_clause,none,
 ERROR(unchecked_not_existential,none,
       "'unchecked' attribute cannot apply to non-protocol type %0", (Type))
 
+ERROR(preconcurrency_not_inheritance_clause,none,
+      "'preconcurrency' attribute only applies in inheritance clauses", ())
+ERROR(preconcurrency_not_existential,none,
+      "'preconcurrency' attribute cannot apply to non-protocol type %0", (Type))
+
 ERROR(redundant_any_in_existential,none,
        "redundant 'any' in type %0",
        (Type))

lib/ASTGen/Sources/ASTGen/Types.swift

@@ -419,8 +419,8 @@ extension ASTGenVisitor {
           fallthrough
 
         case .autoclosure, .escaping, .noescape, .noDerivative, .async,
-          .sendable, .retroactive, .unchecked, ._local, ._noMetadata,
-          .pack_owned, .pack_guaranteed, .pack_inout, .pack_out,
+          .sendable, .retroactive, .unchecked, .preconcurrency, ._local,
+          ._noMetadata, .pack_owned, .pack_guaranteed, .pack_inout, .pack_out,
           .pseudogeneric, .yields, .yield_once, .yield_many, .thin, .thick,
           .count, .unimplementable:
           typeAttributes.addSimpleAttr(kind: typeAttrKind, atLoc: atLoc, attrLoc: attrLoc)

lib/Sema/TypeCheckType.cpp

@@ -3250,6 +3250,25 @@ TypeResolver::resolveAttributedType(TypeAttributes &attrs, TypeRepr *repr,
     attrs.clearAttribute(TAK_unchecked);
   }
 
+  if (attrs.has(TAK_preconcurrency)) {
+    ty = resolveType(repr, options);
+    if (!ty || ty->hasError()) return ty;
+
+    if (!options.is(TypeResolverContext::Inherited) ||
+        getDeclContext()->getSelfProtocolDecl()) {
+      diagnoseInvalid(repr, attrs.getLoc(TAK_preconcurrency),
+                      diag::preconcurrency_not_inheritance_clause);
+      ty = ErrorType::get(getASTContext());
+    } else if (!ty->isConstraintType()) {
+      diagnoseInvalid(repr, attrs.getLoc(TAK_preconcurrency),
+                      diag::preconcurrency_not_existential, ty);
+      ty = ErrorType::get(getASTContext());
+    }
+
+    // Nothing to record in the type. Just clear the attribute.
+    attrs.clearAttribute(TAK_preconcurrency);
+  }
+
   if (attrs.has(TAK_retroactive)) {
     ty = resolveType(repr, options);
     if (!ty || ty->hasError()) return ty;

test/Concurrency/predates_concurrency.swift

@@ -249,3 +249,28 @@ extension MainActorPreconcurrency: NotIsolated {
     }
   }
 }
+
+do {
+  class K {}
+
+  struct A : @preconcurrency Q {} // Ok
+  struct B : @preconcurrency K {
+    // expected-error@-1 {{'preconcurrency' attribute cannot apply to non-protocol type 'K'}}
+    var x: @preconcurrency Int
+    // expected-error@-1 {{'preconcurrency' attribute only applies in inheritance clauses}}
+  }
+
+  typealias T = @preconcurrency Q
+  // expected-error@-1 {{'preconcurrency' attribute only applies in inheritance clauses}}
+
+  func test(_: @preconcurrency K) {}
+  // expected-error@-1 {{'preconcurrency' attribute only applies in inheritance clauses}}
+}
+
+protocol InvalidUseOfPreconcurrencyAttr : @preconcurrency Q {
+  // expected-error@-1 {{'preconcurrency' attribute only applies in inheritance clauses}}
+}
+
+struct TestPreconcurrencyAttr {}
+extension TestPreconcurrencyAttr : @preconcurrency Q { // Ok
+}