Make InferIsolatedConformances a migratable upcoming feature

When migrating, provide warnings that add 'nonisolated' to nonisolated
conformances that don't already have it and would end up being inferred
to be isolated under the upcoming feature.

include/swift/AST/DiagnosticsSema.def

@@ -8619,6 +8619,9 @@ GROUPED_ERROR(isolated_conformance_with_sendable_simple,IsolatedConformances,
 GROUPED_ERROR(isolated_conformance_wrong_domain,IsolatedConformances,none,
       "%0 conformance of %1 to %2 cannot be used in %3 context",
       (ActorIsolation, Type, DeclName, ActorIsolation))
+GROUPED_WARNING(isolated_conformance_will_become_nonisolated,IsolatedConformances,none,
+      "conformance of %0 to %1 should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'",
+      (const ValueDecl *, const ValueDecl *))
 
 //===----------------------------------------------------------------------===//
 // MARK: @_inheritActorContext

include/swift/Basic/Features.def

@@ -283,7 +283,7 @@ UPCOMING_FEATURE(GlobalActorIsolatedTypesUsability, 0434, 6)
 MIGRATABLE_UPCOMING_FEATURE(ExistentialAny, 335, 7)
 UPCOMING_FEATURE(InternalImportsByDefault, 409, 7)
 UPCOMING_FEATURE(MemberImportVisibility, 444, 7)
-UPCOMING_FEATURE(InferIsolatedConformances, 470, 7)
+MIGRATABLE_UPCOMING_FEATURE(InferIsolatedConformances, 470, 7)
 MIGRATABLE_UPCOMING_FEATURE(NonisolatedNonsendingByDefault, 461, 7)
 
 // Optional language features / modes

lib/Sema/TypeCheckProtocol.cpp

@@ -6675,6 +6675,49 @@ void TypeChecker::checkConformancesInContext(IterableDeclContext *idc) {
     }
   }
 
+  // If we are migrating to InferIsolatedConformances, and the
+  // nominal type is global-actor-isolated, look for conformances
+  // that are nonisolated but were not explicitly marked as such.
+  // These conformances will need to be marked 'nonisolated' to
+  // retain their current behavior.
+  if (Context.LangOpts
+          .getFeatureState(Feature::InferIsolatedConformances)
+            .isEnabledForMigration() &&
+      getActorIsolation(const_cast<NominalTypeDecl *>(nominal))
+          .isGlobalActor()) {
+    for (auto conformance : conformances) {
+      auto normal = dyn_cast<NormalProtocolConformance>(conformance);
+      if (!normal)
+        continue;
+
+      // Explicit nonisolated and @preconcurrency suppress this.
+      auto options = normal->getOptions();
+      if (options.contains(ProtocolConformanceFlags::Nonisolated) ||
+          options.contains(ProtocolConformanceFlags::Preconcurrency))
+        continue;
+
+      // Only consider conformances that were explicitly written in the source.
+      if (normal->getSourceKind() != ConformanceEntryKind::Explicit)
+        continue;
+
+      // Only consider conformances to non-marker, nonisolated protocols.
+      auto proto = normal->getProtocol();
+      if (proto->isMarkerProtocol() || getActorIsolation(proto).isActorIsolated())
+        continue;
+
+      // Only nonisolated conformances can be affected.
+      if (!conformance->getIsolation().isNonisolated())
+        continue;
+
+      auto nameLoc = normal->getProtocolNameLoc();
+      if (nameLoc.isValid()) {
+        Context.Diags.diagnose(
+            nameLoc, diag::isolated_conformance_will_become_nonisolated, nominal, proto)
+          .fixItInsert(nameLoc, "nonisolated ");
+      }
+    }
+  }
+
   if (Context.TypeCheckerOpts.DebugGenericSignatures &&
       !conformances.empty()) {
     // Now that they're filled out, print out information about the conformances

test/Concurrency/isolated_conformance_migrate.swift

@@ -0,0 +1,33 @@
+// RUN: %target-swift-frontend -typecheck -verify -target %target-swift-5.1-abi-triple -swift-version 6 -enable-upcoming-feature InferIsolatedConformances:migrate %s
+
+// REQUIRES: concurrency
+
+
+protocol P { }
+protocol Q: P { }
+
+struct S: P { }
+
+struct S2: Q { }
+
+@MainActor
+struct MA1: P { }
+// expected-warning@-1{{conformance of 'MA1' to 'P' should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'}}{{13-13=nonisolated }}
+
+@MainActor
+struct MA2: Q { }
+// expected-warning@-1{{conformance of 'MA2' to 'Q' should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'}}{{13-13=nonisolated }}
+
+@MainActor
+struct MA3 { }
+
+extension MA3: P, Q { }
+// expected-warning@-1{{conformance of 'MA3' to 'P' should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'}}{{16-16=nonisolated }}
+// expected-warning@-2{{conformance of 'MA3' to 'Q' should be marked 'nonisolated' to retain its behavior with upcoming feature 'InferIsolatedConformances'}}{{19-19=nonisolated }}
+
+@MainActor
+struct MA4: @MainActor P { }
+
+@MainActor
+struct MA5: nonisolated P { }
+