[Sema] Diagnose places where @preconcurrency attribute on conformance has no effect

Author: xedin

include/swift/AST/DiagnosticsSema.def

@@ -5626,6 +5626,8 @@ ERROR(unchecked_not_inheritance_clause,none,
 ERROR(unchecked_not_existential,none,
       "'unchecked' attribute cannot apply to non-protocol type %0", (Type))
 
+WARNING(preconcurrency_conformance_not_used,none,
+        "@preconcurrency attribute on conformance to %0 has no effect", (Type))
 ERROR(preconcurrency_not_inheritance_clause,none,
       "'preconcurrency' attribute only applies in inheritance clauses", ())
 ERROR(preconcurrency_not_existential,none,

lib/Sema/TypeCheckProtocol.cpp

@@ -5281,6 +5281,7 @@ hasInvalidTypeInConformanceContext(const ValueDecl *requirement,
 }
 
 void ConformanceChecker::resolveValueWitnesses() {
+  bool usesPreconcurrencyConformance = false;
   for (auto *requirement : Proto->getProtocolRequirements()) {
     // Associated type requirements handled elsewhere.
     if (isa<TypeDecl>(requirement))
@@ -5299,6 +5300,15 @@ void ConformanceChecker::resolveValueWitnesses() {
       // Check actor isolation. If we need to enter into the actor's
       // isolation within the witness thunk, record that.
       if (auto enteringIsolation = checkActorIsolation(requirement, witness)) {
+        // Only @preconcurrency conformances allow entering isolation
+        // when neither requirement nor witness are async by adding a
+        // runtime precondition that witness is always called on the
+        // expected executor.
+        if (Conformance->isPreconcurrency()) {
+          usesPreconcurrencyConformance =
+              !isAsyncDecl(requirement) && !isAsyncDecl(witness);
+        }
+
         Conformance->overrideWitness(
             requirement,
             Conformance->getWitnessUncached(requirement)
@@ -5432,6 +5442,12 @@ void ConformanceChecker::resolveValueWitnesses() {
     }
   }
 
+  if (Conformance->isPreconcurrency() && !usesPreconcurrencyConformance) {
+    DC->getASTContext().Diags.diagnose(
+        Conformance->getLoc(), diag::preconcurrency_conformance_not_used,
+        Proto->getDeclaredInterfaceType());
+  }
+
   // Finally, check some ad-hoc protocol requirements.
   //
   // These protocol requirements are not expressible in Swift today, but as

test/Concurrency/preconcurrency_conformances.swift

@@ -10,6 +10,8 @@ do {
   class K {}
 
   struct A : @preconcurrency Q {} // Ok
+  // expected-warning@-1 {{@preconcurrency attribute on conformance to 'Q' has no effect}}
+
   struct B : @preconcurrency K {
     // expected-error@-1 {{'preconcurrency' attribute cannot apply to non-protocol type 'K'}}
     var x: @preconcurrency Int
@@ -29,6 +31,7 @@ protocol InvalidUseOfPreconcurrencyAttr : @preconcurrency Q {
 
 struct TestPreconcurrencyAttr {}
 extension TestPreconcurrencyAttr : @preconcurrency Q { // Ok
+  // expected-warning@-1 {{@preconcurrency attribute on conformance to 'Q' has no effect}}
 }
 
 class NonSendable {}
@@ -91,6 +94,7 @@ protocol Initializable {
 }
 
 final class K : @preconcurrency Initializable {
+  // expected-warning@-1 {{@preconcurrency attribute on conformance to 'Initializable' has no effect}}
   init() {} // Ok
 }
 
@@ -122,6 +126,8 @@ protocol WithIndividuallyIsolatedRequirements {
 do {
   @MainActor
   struct TestExplicitGlobalActorAttrs : @preconcurrency WithIndividuallyIsolatedRequirements {
+    // expected-warning@-1 {{@preconcurrency attribute on conformance to 'WithIndividuallyIsolatedRequirements' has no effect}}
+
     var a: Int = 42
 
     @MainActor var b: Int {
@@ -146,10 +152,20 @@ protocol WithNonIsolated {
 
 do {
   class TestExplicitOtherIsolation : @preconcurrency WithNonIsolated {
+    // expected-warning@-1 {{@preconcurrency attribute on conformance to 'WithNonIsolated' has no effect}}
+
     @GlobalActor var prop: Int = 42
     // expected-warning@-1 {{global actor 'GlobalActor'-isolated property 'prop' cannot be used to satisfy main actor-isolated protocol requirement}}
 
     @MainActor func test() {}
     // expected-warning@-1 {{main actor-isolated instance method 'test()' cannot be used to satisfy nonisolated protocol requirement}}
   }
 }
+
+do {
+  class InferredGlobalActorAttrs : @preconcurrency WithNonIsolated {
+    // expected-warning@-1 {{@preconcurrency attribute on conformance to 'WithNonIsolated' has no effect}}
+    var prop: Int = 42
+    func test() {}
+  }
+}