Suggest global actor isolation rather than 'isolated' for conformances

Update diagnostic and Fix-It to suggest global actor isolation on a
conformance (e.g, `@MainActor P`) rather than `isolated`.

Author: DougGregor

include/swift/AST/DiagnosticsSema.def

@@ -8332,8 +8332,8 @@ ERROR(isolated_conformance_mismatch_with_associated_isolation,none,
       "%0 conformance of %1 to %2 cannot depend on %3 conformance of %4 to %5",
       (ActorIsolation, Type, DeclName, ActorIsolation, Type, DeclName))
 NOTE(add_isolated_to_conformance,none,
-     "add 'isolated' to the %0 conformance to restrict it to %1 code",
-    (DeclName, ActorIsolation))
+     "add '%0' to the %1 conformance to restrict it to %2 code",
+    (StringRef, DeclName, ActorIsolation))
 ERROR(isolated_conformance_with_sendable,none,
       "isolated conformance of %0 to %1 cannot be used to satisfy conformance "
       "requirement for a %select{`Sendable`|`SendableMetatype`}2 type "

lib/Sema/TypeCheckProtocol.cpp

@@ -3590,17 +3590,22 @@ ConformanceChecker::checkActorIsolation(ValueDecl *requirement,
     }
 
     // Another way to address the issue is to mark the conformance as
-    // "isolated" or "@preconcurrency".
+    // isolated to the global actor or "@preconcurrency".
     if (Conformance->getSourceKind() == ConformanceEntryKind::Explicit &&
         !Conformance->isGlobalActorIsolated() &&
         !Conformance->isPreconcurrency() &&
         !suggestedPreconcurrencyOrIsolated &&
-        !requirementIsolation.isActorIsolated()) {
+        !requirementIsolation.isActorIsolated() &&
+        refResult.isolation.isGlobalActor()) {
       if (Context.LangOpts.hasFeature(Feature::IsolatedConformances)) {
+        std::string globalActorStr = "@" +
+            refResult.isolation.getGlobalActor().getString();
         Context.Diags.diagnose(Conformance->getProtocolNameLoc(),
                                diag::add_isolated_to_conformance,
+                               globalActorStr,
                                Proto->getName(), refResult.isolation)
-            .fixItInsert(Conformance->getProtocolNameLoc(), "isolated ");
+            .fixItInsert(Conformance->getProtocolNameLoc(),
+                         globalActorStr + " ");
       }
 
       Context.Diags.diagnose(Conformance->getProtocolNameLoc(),

test/Concurrency/isolated_conformance.swift

@@ -12,7 +12,7 @@ protocol P {
 // ----------------------------------------------------------------------------
 
 // expected-note@+3{{add '@preconcurrency' to the 'P' conformance to defer isolation checking to run time}}{{25-25=@preconcurrency }}
-// expected-note@+2{{add 'isolated' to the 'P' conformance to restrict it to main actor-isolated code}}{{25-25=isolated }}
+// expected-note@+2{{add '@MainActor' to the 'P' conformance to restrict it to main actor-isolated code}}{{25-25=@MainActor }}
 @MainActor
 class CWithNonIsolated: P {
   func f() { } // expected-error{{main actor-isolated instance method 'f()' cannot be used to satisfy nonisolated requirement from protocol 'P'}}