AST: Introduce the AllowRuntimeSymbolDeclarations experimental feature

include/swift/Basic/Features.def

@@ -517,6 +517,9 @@ EXPERIMENTAL_FEATURE(CompileTimeValues, true)
 /// Allow function body macros applied to closures.
 EXPERIMENTAL_FEATURE(ClosureBodyMacro, true)
 
+/// Allow declarations of Swift runtime symbols using @_silgen_name.
+EXPERIMENTAL_FEATURE(AllowRuntimeSymbolDeclarations, true)
+
 #undef EXPERIMENTAL_FEATURE_EXCLUDED_FROM_MODULE_INTERFACE
 #undef EXPERIMENTAL_FEATURE
 #undef UPCOMING_FEATURE

lib/AST/FeatureSet.cpp

@@ -405,6 +405,7 @@ UNINTERESTING_FEATURE(SafeInteropWrappers)
 UNINTERESTING_FEATURE(AssumeResilientCxxTypes)
 UNINTERESTING_FEATURE(ImportNonPublicCxxMembers)
 UNINTERESTING_FEATURE(CoroutineAccessorsUnwindOnCallerError)
+UNINTERESTING_FEATURE(AllowRuntimeSymbolDeclarations)
 
 static bool usesFeatureSwiftSettings(const Decl *decl) {
   // We just need to guard `#SwiftSettings`.

lib/Sema/TypeCheckAttr.cpp

@@ -2336,6 +2336,12 @@ static bool canDeclareSymbolName(StringRef symbol, ModuleDecl *fromModule) {
     return true;
   }
 
+  // Allow reserved symbols to be declared if the AllowRuntimeSymbolDeclarations
+  // experimental feature is enabled.
+  auto &ctx = fromModule->getASTContext();
+  if (ctx.LangOpts.hasFeature(Feature::AllowRuntimeSymbolDeclarations))
+    return true;
+
   // Swift runtime functions are a private contract between the compiler and
   // runtime, and attempting to access them directly without going through
   // builtins or proper language features breaks the compiler in various hard

stdlib/public/RegexParser/CMakeLists.txt

@@ -47,6 +47,7 @@ add_swift_target_library(swift_RegexParser ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
     # Workaround until `_RegexParser` is imported as implementation-only
     # by `_StringProcessing`.
     -Xfrontend -disable-implicit-string-processing-module-import
+    -enable-experimental-feature AllowRuntimeSymbolDeclarations
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
 
   INSTALL_IN_COMPONENT stdlib

test/attr/attr_silgen_name.swift

@@ -1,6 +1,8 @@
-// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern -verify-additional-prefix runtime-symbols-
+// RUN: %target-typecheck-verify-swift -enable-experimental-feature Extern -enable-experimental-feature AllowRuntimeSymbolDeclarations
 
 // REQUIRES: swift_feature_Extern
+// REQUIRES: swift_feature_AllowRuntimeSymbolDeclarations
 
 @_silgen_name("foo") // expected-note {{attribute already specified here}}
 @_silgen_name("bar") // expected-error {{duplicate attribute}}
@@ -19,14 +21,14 @@ func func_with_nested__silgen_name() {
 // Ensure that magic runtime symbol names can't be declared or defined through
 // various symbol-assigning attributes
 
-@_silgen_name("swift_retain") // expected-warning{{reserved}}
+@_silgen_name("swift_retain") // expected-runtime-symbols-warning {{reserved}}
 func liveDangerously() {}
 
-@_silgen_name("swift_retain") // expected-warning{{reserved}}
+@_silgen_name("swift_retain") // expected-runtime-symbols-warning {{reserved}}
 func liveRecklessly();
 
-@_extern(c, "swift_retain") // expected-warning{{reserved}}
+@_extern(c, "swift_retain") // expected-runtime-symbols-warning {{reserved}}
 func liveEphemerally()
 
-@_cdecl("swift_retain") // expected-warning{{reserved}}
+@_cdecl("swift_retain") // expected-runtime-symbols-warning {{reserved}}
 func liveFrivolously() {}