[SE-0368] StaticBigInt: support older compilers (swiftlang#62541)

Author: benrimmington

include/swift/Basic/Features.def

@@ -90,6 +90,7 @@ SUPPRESSIBLE_LANGUAGE_FEATURE(UnsafeInheritExecutor, 0, "@_unsafeInheritExecutor
 SUPPRESSIBLE_LANGUAGE_FEATURE(PrimaryAssociatedTypes2, 346, "Primary associated types", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(UnavailableFromAsync, 0, "@_unavailableFromAsync", true)
 SUPPRESSIBLE_LANGUAGE_FEATURE(NoAsyncAvailability, 340, "@available(*, noasync)", true)
+LANGUAGE_FEATURE(BuiltinIntLiteralAccessors, 368, "Builtin.IntLiteral accessors", true)
 
 UPCOMING_FEATURE(ConciseMagicFile, 274, 6)
 UPCOMING_FEATURE(ForwardTrailingClosures, 286, 6)

lib/AST/ASTPrinter.cpp

@@ -3042,6 +3042,10 @@ static bool usesFeatureNoAsyncAvailability(Decl *decl) {
    return decl->getAttrs().getNoAsync(decl->getASTContext()) != nullptr;
 }
 
+static bool usesFeatureBuiltinIntLiteralAccessors(Decl *decl) {
+  return false;
+}
+
 static bool usesFeatureConciseMagicFile(Decl *decl) {
   return false;
 }

stdlib/public/core/StaticBigInt.swift

@@ -73,7 +73,11 @@ extension StaticBigInt {
   @available(SwiftStdlib 5.8, *)
   @inlinable
   internal var _isNegative: Bool {
+#if compiler(>=5.8) && $BuiltinIntLiteralAccessors
     Bool(Builtin.isNegative_IntLiteral(_value))
+#else
+    fatalError("Swift compiler is incompatible with this SDK version")
+#endif
   }
 
   /// Returns the minimal number of bits in this value's binary representation,
@@ -94,7 +98,11 @@ extension StaticBigInt {
   @available(SwiftStdlib 5.8, *)
   @inlinable
   public var bitWidth: Int {
+#if compiler(>=5.8) && $BuiltinIntLiteralAccessors
     Int(Builtin.bitWidth_IntLiteral(_value))
+#else
+    fatalError("Swift compiler is incompatible with this SDK version")
+#endif
   }
 
   /// Returns a 32-bit or 64-bit word of this value's binary representation.
@@ -125,9 +133,13 @@ extension StaticBigInt {
     guard !bitIndex.overflow, bitIndex.partialValue < bitWidth else {
       return _isNegative ? ~0 : 0
     }
+#if compiler(>=5.8) && $BuiltinIntLiteralAccessors
     return UInt(
       Builtin.wordAtIndex_IntLiteral(_value, wordIndex._builtinWordValue)
     )
+#else
+    fatalError("Swift compiler is incompatible with this SDK version")
+#endif
   }
 }