AST: Introduce the SwiftLanguageMode availability domain spelling.

Require `-enable-experimental-feature SwiftRuntimeAvailability` to use this new
spelling.

include/swift/AST/DiagnosticsSema.def

@@ -6986,6 +6986,12 @@ WARNING(availability_unsupported_version_number, none,
 WARNING(availability_invalid_version_number_for_domain, none,
         "'%0' is not a valid version number for %1",
         (llvm::VersionTuple, AvailabilityDomain))
+WARNING(availability_domain_renamed, none,
+        "%0 has been renamed to '%1'",
+        (Identifier, StringRef))
+ERROR(availability_domain_requires_feature, none,
+      "%0 requires '-enable-experimental-feature %1'",
+      (AvailabilityDomain, StringRef))
 
 WARNING(attr_availability_expected_deprecated_version, none,
        "expected version number with 'deprecated' in '%0' attribute for %1",
@@ -6996,9 +7002,6 @@ WARNING(attr_availability_cannot_be_used_for_domain, none,
 WARNING(attr_availability_expected_version_spec, none,
        "expected 'introduced', 'deprecated', or 'obsoleted' in '%0' attribute "
        "for %1", (const DeclAttribute, AvailabilityDomain))
-ERROR(attr_availability_requires_custom_availability, none,
-      "%0 requires '-enable-experimental-feature CustomAvailability'",
-      (AvailabilityDomain))
 ERROR(attr_availability_domain_access, none,
       "availability domain '%0' is "
       "%select{private|fileprivate|internal|package|%error|%error}1 "

include/swift/Basic/Features.def

@@ -556,6 +556,9 @@ EXPERIMENTAL_FEATURE(BorrowAndMutateAccessors, false)
 /// Allow use of @inline(always) attribute
 SUPPRESSIBLE_EXPERIMENTAL_FEATURE(InlineAlways, false)
 
+/// Allow use of 'Swift' (Swift runtime version) in @available attributes
+EXPERIMENTAL_FEATURE(SwiftRuntimeAvailability, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/AvailabilityDomain.cpp

@@ -107,12 +107,14 @@ AvailabilityDomain::builtinDomainForString(StringRef string,
   // This parameter is used in downstream forks, do not remove.
   (void)declContext;
 
-  auto domain = llvm::StringSwitch<std::optional<AvailabilityDomain>>(string)
-                    .Case("*", AvailabilityDomain::forUniversal())
-                    .Case("swift", AvailabilityDomain::forSwiftLanguageMode())
-                    .Case("_PackageDescription",
-                          AvailabilityDomain::forPackageDescription())
-                    .Default(std::nullopt);
+  auto domain =
+      llvm::StringSwitch<std::optional<AvailabilityDomain>>(string)
+          .Case("*", AvailabilityDomain::forUniversal())
+          .Case("swift", AvailabilityDomain::forSwiftLanguageMode())
+          .Case("SwiftLanguageMode", AvailabilityDomain::forSwiftLanguageMode())
+          .Case("_PackageDescription",
+                AvailabilityDomain::forPackageDescription())
+          .Default(std::nullopt);
 
   if (domain)
     return domain;
@@ -480,6 +482,8 @@ AvailabilityDomainOrIdentifier::lookUpInDeclContext(
 
   bool hasCustomAvailability =
       ctx.LangOpts.hasFeature(Feature::CustomAvailability);
+  bool hasSwiftRuntimeAvailability =
+      ctx.LangOpts.hasFeature(Feature::SwiftRuntimeAvailability);
 
   if (!domain) {
     auto domainString = identifier.str();
@@ -500,11 +504,32 @@ AvailabilityDomainOrIdentifier::lookUpInDeclContext(
     return std::nullopt;
   }
 
-  if (domain->isCustom() && !hasCustomAvailability &&
-      !declContext->isInSwiftinterface()) {
-    diags.diagnose(loc, diag::attr_availability_requires_custom_availability,
-                   *domain);
-    return std::nullopt;
+  if (!declContext->isInSwiftinterface()) {
+    if (domain->isCustom() && !hasCustomAvailability) {
+      diags.diagnose(loc, diag::availability_domain_requires_feature, *domain,
+                     "CustomAvailability");
+      return std::nullopt;
+    }
+
+    if (domain->isSwiftLanguageMode()) {
+      // 'swift' -> 'SwiftLanguageMode'
+      if (hasSwiftRuntimeAvailability && identifier.str() == "swift") {
+        diags
+            .diagnose(loc, diag::availability_domain_renamed, identifier,
+                      "SwiftLanguageMode")
+            .fixItReplace(SourceRange(loc), "SwiftLanguageMode");
+      }
+
+      if (!hasSwiftRuntimeAvailability &&
+          identifier.str() == "SwiftLanguageMode") {
+        // This diagnostic ("Swift requires '-enable-experimental-feature
+        // SwiftRuntimeAvailability'") is confusing but it's also temporary,
+        // assuming the experimental feature becomes official.
+        diags.diagnose(loc, diag::availability_domain_requires_feature, *domain,
+                       "SwiftRuntimeAvailability");
+        return std::nullopt;
+      }
+    }
   }
 
   return domain;

lib/AST/FeatureSet.cpp

@@ -448,6 +448,8 @@ static bool usesFeatureInlineAlways(Decl *decl) {
   return false;
 }
 
+UNINTERESTING_FEATURE(SwiftRuntimeAvailability)
+
 // ----------------------------------------------------------------------------
 // MARK: - FeatureSet
 // ----------------------------------------------------------------------------

lib/Serialization/ModuleFormat.h

@@ -771,7 +771,7 @@ using GenericParamKindField = BCFixed<2>;
 // the module version.
 enum class AvailabilityDomainKind : uint8_t {
   Universal = 0,
-  SwiftLanguage,
+  SwiftLanguageMode,
   PackageDescription,
   Embedded,
   Platform,

test/Availability/availability_swift_runtime.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift -parse-as-library -enable-experimental-feature SwiftRuntimeAvailability
+
+// REQUIRES: swift_feature_SwiftRuntimeAvailability
+
+@available(swift 99) // expected-warning {{'swift' has been renamed to 'SwiftLanguageMode'}}{{12-17=SwiftLanguageMode}}
+func availableInSwift99() { }
+// expected-note@-1 {{'availableInSwift99()' was introduced in Swift 99}}
+
+@available(SwiftLanguageMode 99)
+func availableInSwift99LanguageMode() { }
+// expected-note@-1 {{'availableInSwift99LanguageMode()' was introduced in Swift 99}}
+
+func testUses() {
+  availableInSwift99() // expected-error {{'availableInSwift99()' is unavailable in Swift}}
+  availableInSwift99LanguageMode() // expected-error {{'availableInSwift99LanguageMode()' is unavailable in Swift}}
+}

test/attr/attr_availability_swift_language_mode.swift

@@ -58,10 +58,13 @@ func testMemberAvailability() {
   TestStruct().doAnotherThing() // expected-error {{'doAnotherThing()' is unavailable}}
 }
 
-@available(swift 400) // FIXME: This has no effect and should be complained about.
+@available(swift 400)
 @available(macOS 10.11, *)
 extension TestStruct {}
 
 @available(macOS 10.11, *)
-@available(swift 400) // FIXME: This has no effect and should be complained about.
+@available(swift 400)
+extension TestStruct {}
+
+@available(SwiftLanguageMode 400) // expected-error {{Swift requires '-enable-experimental-feature SwiftRuntimeAvailability'}}
 extension TestStruct {}