Remove the @_unsafeSendable/@_unsafeMainActor attributes.

These have been subsumed by @_predatesConcurrency. Leave in minimal
parser support that warns about the removal, so we don't fully break
existing source code.

Author: DougGregor

include/swift/AST/Attr.def

@@ -635,15 +635,8 @@ CONTEXTUAL_SIMPLE_DECL_ATTR(nonisolated, Nonisolated,
   APIBreakingToAdd | APIStableToRemove,
   112)
 
-CONTEXTUAL_SIMPLE_DECL_ATTR(_unsafeSendable, UnsafeSendable,
-  OnParam | UserInaccessible |
-  ABIStableToAdd | ABIStableToRemove | APIBreakingToAdd | APIStableToRemove,
-  113)
-
-CONTEXTUAL_SIMPLE_DECL_ATTR(_unsafeMainActor, UnsafeMainActor,
-  OnParam | UserInaccessible |
-  ABIStableToAdd | ABIStableToRemove | APIStableToAdd | APIBreakingToRemove,
-  114)
+// 113 was experimental _unsafeSendable and is now unused
+// 114 was experimental _unsafeMainActor and is now unused
 
 SIMPLE_DECL_ATTR(_implicitSelfCapture, ImplicitSelfCapture,
   OnParam | UserInaccessible |

include/swift/AST/DiagnosticsParse.def

@@ -1719,6 +1719,10 @@ ERROR(sil_inst_autodiff_invalid_witness_generic_signature,PointsToFirstBadToken,
       "parameters as original function generic signature '%1'",
       (StringRef, StringRef))
 
+WARNING(warn_attr_unsafe_removed,none,
+        "'%0' attribute has been removed in favor of @_predatesConcurrency",
+        (StringRef))
+
 //------------------------------------------------------------------------------
 // MARK: Generics parsing diagnostics
 //------------------------------------------------------------------------------

lib/Parse/ParseDecl.cpp

@@ -3080,6 +3080,17 @@ ParserStatus Parser::parseDeclAttribute(
     return makeParserSuccess();
   }
 
+  // @_unsafeSendable and @_unsafeMainActor have been removed; warn about them.
+  if (DK == DAK_Count &&
+      (Tok.getText() == "_unsafeSendable" ||
+       Tok.getText() == "_unsafeMainActor")) {
+    StringRef attrName = Tok.getText();
+    SourceLoc attrLoc = consumeToken();
+    diagnose(AtLoc, diag::warn_attr_unsafe_removed, attrName)
+      .fixItRemove(SourceRange(AtLoc, attrLoc));
+    return makeParserSuccess();
+  }
+
   if (DK != DAK_Count && !DeclAttribute::shouldBeRejectedByParser(DK)) {
     parseNewDeclAttribute(Attributes, AtLoc, DK, isFromClangAttribute);
     return makeParserSuccess();

lib/Sema/TypeCheckAttr.cpp

@@ -114,8 +114,6 @@ class AttributeChecker : public AttributeVisitor<AttributeChecker> {
   IGNORED_ATTR(NonSendable)
   IGNORED_ATTR(AtRethrows)
   IGNORED_ATTR(AtReasync)
-  IGNORED_ATTR(UnsafeSendable)
-  IGNORED_ATTR(UnsafeMainActor)
   IGNORED_ATTR(ImplicitSelfCapture)
   IGNORED_ATTR(InheritActorContext)
   IGNORED_ATTR(Isolated)

lib/Sema/TypeCheckConcurrency.cpp

@@ -4129,13 +4129,9 @@ static AnyFunctionType *applyUnsafeConcurrencyToFunctionType(
     // @MainActor. @Sendable occurs only in concurrency contents, while
     // @MainActor occurs in concurrency contexts or those where we have an
     // application.
-    bool addSendable =
-        (paramDecl->getAttrs().hasAttribute<UnsafeSendableAttr>() ||
-         knownUnsafeParams) &&
-        inConcurrencyContext;
+    bool addSendable = knownUnsafeParams && inConcurrencyContext;
     bool addMainActor =
-        (paramDecl->getAttrs().hasAttribute<UnsafeMainActorAttr>() ||
-         (isMainDispatchQueue && knownUnsafeParams)) &&
+        (isMainDispatchQueue && knownUnsafeParams) &&
         (inConcurrencyContext || numApplies >= 1);
     Type newParamType = param.getPlainType();
     if (addSendable || addMainActor) {

lib/Sema/TypeCheckDeclOverride.cpp

@@ -1555,8 +1555,6 @@ namespace  {
 
     UNINTERESTING_ATTR(AtReasync)
     UNINTERESTING_ATTR(Nonisolated)
-    UNINTERESTING_ATTR(UnsafeSendable)
-    UNINTERESTING_ATTR(UnsafeMainActor)
     UNINTERESTING_ATTR(ImplicitSelfCapture)
     UNINTERESTING_ATTR(InheritActorContext)
     UNINTERESTING_ATTR(Isolated)

test/Concurrency/predates_concurrency.swift

@@ -95,3 +95,12 @@ struct S2: Q {
 struct S3: Q, Sendable {
   var ns: NS // expected-error{{stored property 'ns' of 'Sendable'-conforming struct 'S3' has non-sendable type 'NS'}}
 }
+
+// ---------------------------------------------------------------------------
+// Historical attribute names do nothing (but are permitted)
+// ---------------------------------------------------------------------------
+func aFailedExperiment(@_unsafeSendable _ body: @escaping () -> Void) { }
+// expected-warning@-1{{'_unsafeSendable' attribute has been removed in favor of @_predatesConcurrency}}
+
+func anothingFailedExperiment(@_unsafeMainActor _ body: @escaping () -> Void) { }
+// expected-warning@-1{{'_unsafeMainActor' attribute has been removed in favor of @_predatesConcurrency}}