Runtime: Reinstate _stdlib_demangleName hook.

This is needed for Xcode support, even though it appeared dead within the Swift repo itself.

Author: jckarter

stdlib/public/core/Misc.swift

@@ -83,6 +83,30 @@ func _typeName(type: Any.Type, qualified: Bool = true) -> String {
     input: UnsafeBufferPointer(start: stringPtr, count: count))
 }
 
+@warn_unused_result
+@_silgen_name("swift_stdlib_demangleName")
+func _stdlib_demangleNameImpl(
+  mangledName: UnsafePointer<UInt8>,
+  _ mangledNameLength: UInt,
+  _ demangledName: UnsafeMutablePointer<String>)
+
+// NB: This function is not used directly in the Swift codebase, but is
+// exported for Xcode support. Please coordinate before changing.
+@warn_unused_result
+public // @testable
+func _stdlib_demangleName(mangledName: String) -> String {
+  let mangledNameUTF8 = Array(mangledName.utf8)
+  return mangledNameUTF8.withUnsafeBufferPointer {
+    (mangledNameUTF8) in
+    let (_, demangledName) = _withUninitializedString {
+      _stdlib_demangleNameImpl(
+        mangledNameUTF8.baseAddress, UInt(mangledNameUTF8.endIndex),
+        $0)
+    }
+    return demangledName
+  }
+}
+
 /// Returns `floor(log(x))`.  This equals to the position of the most
 /// significant non-zero bit, or 63 - number-of-zeros before it.
 ///

stdlib/public/runtime/Reflection.mm

@@ -1264,3 +1264,17 @@ static Mirror ObjC_getMirrorForSuperclass(Class sup,
     T->vw_destroy(value);
   return MirrorReturn(result);
 }
+
+// NB: This function is not used directly in the Swift codebase, but is
+// exported for Xcode support. Please coordinate before changing.
+extern "C" void swift_stdlib_demangleName(const char *mangledName,
+                                          size_t mangledNameLength,
+                                          String *demangledName) {
+  auto options = Demangle::DemangleOptions();
+  options.DisplayDebuggerGeneratedModule = false;
+  auto result =
+      Demangle::demangleSymbolAsString(mangledName,
+                                       mangledNameLength,
+                                       options);
+  new (demangledName) String(result.data(), result.size());
+}

test/1_stdlib/Runtime.swift

@@ -665,6 +665,16 @@ Runtime.test("typeName") {
   expectEqual("protocol<>.Protocol", _typeName(a.dynamicType))
 }
 
+Runtime.test("demangleName") {
+  expectEqual("", _stdlib_demangleName(""))
+  expectEqual("abc", _stdlib_demangleName("abc"))
+  expectEqual("\0", _stdlib_demangleName("\0"))
+  expectEqual("Swift.Double", _stdlib_demangleName("_TtSd"))
+  expectEqual("x.a : x.Foo<x.Foo<x.Foo<Swift.Int, Swift.Int>, x.Foo<Swift.Int, Swift.Int>>, x.Foo<x.Foo<Swift.Int, Swift.Int>, x.Foo<Swift.Int, Swift.Int>>>",
+      _stdlib_demangleName("_Tv1x1aGCS_3FooGS0_GS0_SiSi_GS0_SiSi__GS0_GS0_SiSi_GS0_SiSi___"))
+  expectEqual("Foobar", _stdlib_demangleName("_TtC13__lldb_expr_46Foobar"))
+}
+
 Runtime.test("_stdlib_atomicCompareExchangeStrongPtr") {
   typealias IntPtr = UnsafeMutablePointer<Int>
   var origP1 = IntPtr(bitPattern: 0x10101010)