Suggest @preconcurrency on conformances it could help

When diagnosing a case where an actor-isolated witness cannot satisfy
a non-isolated requirement, also suggest that the conformance could be
annotated with `@preconcurrency`.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -2644,6 +2644,8 @@ WARNING(add_predates_concurrency_import,none,
      "%select{| as warnings}0", (bool, Identifier))
 WARNING(remove_predates_concurrency_import,none,
      "'@preconcurrency' attribute on module %0 has no effect", (Identifier))
+NOTE(add_preconcurrency_to_conformance,none,
+     "add '@preconcurrency' to the %0 conformance to suppress isolation-related diagnostics", (DeclName))
 WARNING(remove_public_import,none,
         "public import of %0 was not used in public declarations or inlinable code",
         (const ModuleDecl *))

lib/AST/ConformanceLookupTable.cpp

@@ -969,10 +969,11 @@ ConformanceLookupTable::getConformance(NominalTypeDecl *nominal,
     assert(!isa<ProtocolDecl>(conformingDC->getSelfNominalTypeDecl()));
     Type conformingType = conformingDC->getSelfInterfaceType();
 
-    SourceLoc conformanceLoc
-      = conformingNominal == conformingDC
-          ? conformingNominal->getLoc()
-          : cast<ExtensionDecl>(conformingDC)->getLoc();
+    SourceLoc conformanceLoc =
+      entry->getLoc().isValid() ? entry->getLoc()
+        : (conformingNominal == conformingDC
+             ? conformingNominal->getLoc()
+             : cast<ExtensionDecl>(conformingDC)->getLoc());
 
     NormalProtocolConformance *implyingConf = nullptr;
     if (entry->Source.getKind() == ConformanceEntryKind::Implied) {

lib/Sema/TypeCheckProtocol.cpp

@@ -3297,13 +3297,28 @@ ConformanceChecker::checkActorIsolation(ValueDecl *requirement,
     missingOptions -= MissingFlags::WitnessDistributed;
   }
 
-  // One way to address the issue is to make the witness function nonisolated.
-  if ((isa<AbstractFunctionDecl>(witness) || isa<SubscriptDecl>(witness)) &&
-      !hasExplicitGlobalActorAttr(witness) &&
-      !isDistributedDecl(requirement) &&
-      !isDistributedDecl(witness)) {
-    witness->diagnose(diag::note_add_nonisolated_to_decl, witness)
-          .fixItInsert(witness->getAttributeInsertionLoc(true), "nonisolated ");
+  // If 'nonisolated' or 'preconcurrency' might help us, provide those as
+  // options.
+  if (!isDistributedDecl(requirement) && !isDistributedDecl(witness)) {
+    // One way to address the issue is to make the witness function nonisolated.
+    if ((isa<AbstractFunctionDecl>(witness) || isa<SubscriptDecl>(witness)) &&
+        !hasExplicitGlobalActorAttr(witness)) {
+      witness->diagnose(diag::note_add_nonisolated_to_decl, witness)
+            .fixItInsert(witness->getAttributeInsertionLoc(true), "nonisolated ");
+    }
+
+    // Another way to address the issue is to mark the conformance as
+    // "preconcurrency".
+    if (Conformance->getSourceKind() == ConformanceEntryKind::Explicit &&
+        !Conformance->isPreconcurrency() &&
+        !suggestedPreconcurrency &&
+        !requirementIsolation.isActorIsolated()) {
+      Context.Diags.diagnose(Conformance->getLoc(),
+                             diag::add_preconcurrency_to_conformance,
+                             Proto->getName())
+          .fixItInsert(Conformance->getLoc(), "@preconcurrency ");
+      suggestedPreconcurrency = true;
+    }
   }
 
   // If there are remaining options, they are missing async/throws on the

lib/Sema/TypeCheckProtocol.h

@@ -112,6 +112,9 @@ enum class ResolveWitnessResult {
 /// This helper class handles most of the details of checking whether a
 /// given type (\c Adoptee) conforms to a protocol (\c Proto).
 class ConformanceChecker : public WitnessChecker {
+  /// Whether we already suggested adding `@preconcurrency`.
+  bool suggestedPreconcurrency = false;
+
 public:
   NormalProtocolConformance *Conformance;
   SourceLoc Loc;

test/Concurrency/actor_isolation.swift

@@ -1560,6 +1560,8 @@ protocol NonisolatedProtocol {
 }
 
 actor ActorWithNonSendableLet: NonisolatedProtocol {
+  // expected-note@-1{{add '@preconcurrency' to the 'NonisolatedProtocol' conformance to suppress isolation-related diagnostics}}{{32-32=@preconcurrency }}
+
   // expected-warning@+1 {{actor-isolated property 'ns' cannot be used to satisfy nonisolated protocol requirement; this is an error in the Swift 6 language mode}}
   let ns = NonSendable()
 }

test/Concurrency/global_actor_inference.swift

@@ -117,6 +117,8 @@ protocol Interface {
 
 @MainActor
 class Object: Interface {
+  // expected-note@-1{{add '@preconcurrency' to the 'Interface' conformance to suppress isolation-related diagnostics}}{{15-15=@preconcurrency }}
+
   var baz: Int = 42 // expected-warning{{main actor-isolated property 'baz' cannot be used to satisfy nonisolated protocol requirement}}
 }
 

test/Concurrency/preconcurrency_conformances.swift

@@ -91,13 +91,21 @@ extension MyActor : @preconcurrency TestSendability {
 
 protocol Initializable {
   init()
+  // expected-note@-1{{mark the protocol requirement 'init()' 'async' to allow actor-isolated conformances}}
 }
 
 final class K : @preconcurrency Initializable {
   // expected-warning@-1 {{@preconcurrency attribute on conformance to 'Initializable' has no effect}}
   init() {} // Ok
 }
 
+@MainActor
+final class MainActorK: Initializable {
+  // expected-note@-1{{add '@preconcurrency' to the 'Initializable' conformance to suppress isolation-related diagnostics}}{{25-25=@preconcurrency }}
+  init() { } // expected-warning{{main actor-isolated initializer 'init()' cannot be used to satisfy nonisolated protocol requirement}}
+  // expected-note@-1{{add 'nonisolated' to 'init()' to make this initializer not isolated to the actor}}
+}
+
 protocol WithAssoc {
   associatedtype T
   func test() -> T

test/Concurrency/predates_concurrency.swift

@@ -230,6 +230,9 @@ protocol NotIsolated {
 }
 
 extension MainActorPreconcurrency: NotIsolated {
+  // expected-complete-note@-1{{add '@preconcurrency' to the 'NotIsolated' conformance to suppress isolation-related diagnostics}}{{36-36=@preconcurrency }}
+  // expected-complete-tns-note@-2{{add '@preconcurrency' to the 'NotIsolated' conformance to suppress isolation-related diagnostics}}{{36-36=@preconcurrency }}
+
   func requirement() {}
   // expected-complete-tns-warning@-1 {{main actor-isolated instance method 'requirement()' cannot be used to satisfy nonisolated protocol requirement}}
   // expected-complete-tns-note@-2 {{add 'nonisolated' to 'requirement()' to make this instance method not isolated to the actor}}

test/decl/class/actor/conformance.swift

@@ -36,6 +36,8 @@ protocol SyncProtocol {
 
 
 actor OtherActor: SyncProtocol {
+  // expected-note@-1{{add '@preconcurrency' to the 'SyncProtocol' conformance to suppress isolation-related diagnostics}}{{19-19=@preconcurrency }}
+
   var propertyB: Int = 17
   // expected-error@-1{{actor-isolated property 'propertyB' cannot be used to satisfy nonisolated protocol requirement}}
 
@@ -95,4 +97,4 @@ actor A2: Initializers {
   init(int: Int) { }
 
   func withBells() async -> A2 { self }
-}
+}

test/decl/class/actor/global_actor_conformance.swift

@@ -30,6 +30,8 @@ protocol P2 {
 }
 
 class C1 : P1, P2 {
+  // expected-note@-1{{add '@preconcurrency' to the 'P1' conformance to suppress isolation-related diagnostics}}
+
   typealias Assoc = String
 
   func method1() { }
@@ -52,6 +54,8 @@ protocol NonIsolatedRequirement {
 @MainActor class OnMain {}
 
 extension OnMain: NonIsolatedRequirement {
+  // expected-note@-1{{add '@preconcurrency' to the 'NonIsolatedRequirement' conformance to suppress isolation-related diagnostics}}
+
   // expected-warning@+2 {{main actor-isolated instance method 'requirement()' cannot be used to satisfy nonisolated protocol requirement}}
   // expected-note@+1 {{add 'nonisolated' to 'requirement()' to make this instance method not isolated to the actor}}
   func requirement() {}

test/decl/protocol/req/missing_conformance.swift

@@ -144,10 +144,10 @@ struct CountSteps1<T> : Collection {
   subscript(i: Int) -> Int { return i }
 }
 
-extension CountSteps1 // expected-error {{type 'CountSteps1<T>' does not conform to protocol 'RandomAccessCollection'}}
-  // expected-error@-1 {{conditional conformance of type 'CountSteps1<T>' to protocol 'RandomAccessCollection' does not imply conformance to inherited protocol 'BidirectionalCollection'}}
-  // expected-note@-2 {{did you mean to explicitly state the conformance like 'extension CountSteps1: BidirectionalCollection where ...'?}}
-  // expected-error@-3 {{type 'CountSteps1<T>' does not conform to protocol 'BidirectionalCollection'}}
+extension CountSteps1 // expected-error {{type 'CountSteps1<T>' does not conform to protocol 'BidirectionalCollection'}}
+  // expected-error@+3 {{conditional conformance of type 'CountSteps1<T>' to protocol 'RandomAccessCollection' does not imply conformance to inherited protocol 'BidirectionalCollection'}}
+  // expected-note@+2 {{did you mean to explicitly state the conformance like 'extension CountSteps1: BidirectionalCollection where ...'?}}
+  // expected-error@+1 {{type 'CountSteps1<T>' does not conform to protocol 'RandomAccessCollection'}}
   : RandomAccessCollection
      where T : Equatable
 {