Add Symbol.prototype.description

Summary:
This diff add a new getter `description` to `Symbol.prototype`,
which returned the [[Description]] slot w/o wrapping into `Symbol(...)`.

Noted that this is NOT spec-conformant as for the `Symbol(undefined)`.

Reviewed By: avp

Differential Revision: D25002865

fbshipit-source-id: 25bddb8366c447f3995dd822dac3bb37099f3d84

doc/Features.md

@@ -31,6 +31,7 @@ Hermes plans to target ECMAScript 2015 (ES6), with some carefully considered exc
 - `Intl` API glue for Android has been added, in order to enable community contribution of a complete, spec-compliant implementation.
 - Promise is opt-in (via `-Xes6-promise` flag or `withES6Promise` runtime config).
 - Async function (`async` and `await`).
+- `Symbol.prototype.description` (it's not fully spec-conformant yet. `Symbol().description` should be `undefined` but it's currently `''`).
 
 ## Excluded From Support
 

include/hermes/VM/NativeFunctions.def

@@ -330,6 +330,7 @@ NATIVE_FUNCTION(symbolConstructor)
 NATIVE_FUNCTION(symbolFor)
 NATIVE_FUNCTION(symbolKeyFor)
 NATIVE_FUNCTION(symbolPrototypeToString)
+NATIVE_FUNCTION(symbolPrototypeDescriptionGetter)
 NATIVE_FUNCTION(symbolPrototypeValueOf)
 NATIVE_FUNCTION(throwTypeError)
 NATIVE_FUNCTION(typedArrayBaseConstructor)

include/hermes/VM/PredefinedStrings.def

@@ -28,6 +28,7 @@ STR(symbol, "symbol")
 STR(object, "object")
 STR(function, "function")
 STR(toString, "toString")
+STR(description, "description")
 STR(toLocaleString, "toLocaleString")
 STR(hasOwnProperty, "hasOwnProperty")
 STR(isPrototypeOf, "isPrototypeOf")

lib/VM/JSLib/Symbol.cpp

@@ -128,6 +128,15 @@ Handle<JSObject> createSymbolConstructor(Runtime *runtime) {
 
   // Symbol.prototype.xxx methods.
   void *ctx = nullptr;
+  defineAccessor(
+      runtime,
+      symbolPrototype,
+      Predefined::getSymbolID(Predefined::description),
+      ctx,
+      symbolPrototypeDescriptionGetter,
+      nullptr,
+      false,
+      true);
   defineMethod(
       runtime,
       symbolPrototype,
@@ -223,6 +232,27 @@ symbolKeyFor(void *, Runtime *runtime, NativeArgs args) {
   return HermesValue::encodeUndefinedValue();
 }
 
+/// ES10.0 19.4.3.2 get Symbol.prototype.description
+/// TODO(T79770380): make the Symbol(undefined) case spec-conformant.
+CallResult<HermesValue>
+symbolPrototypeDescriptionGetter(void *, Runtime *runtime, NativeArgs args) {
+  MutableHandle<SymbolID> sym{runtime};
+  // 1. Let s be the this value.
+  // 2. Let sym be ? thisSymbolValue(s).
+  if (args.getThisArg().isSymbol()) {
+    sym = args.vmcastThis<SymbolID>().get();
+  } else if (auto symHandle = args.dyncastThis<JSSymbol>()) {
+    sym = JSSymbol::getPrimitiveSymbol(*symHandle, runtime).get();
+  } else {
+    return runtime->raiseTypeError(
+        "Symbol.prototype.description can only be called on Symbol");
+  }
+
+  // 3. Return sym.[[Description]].
+  StringPrimitive *desc = runtime->getStringPrimFromSymbolID(*sym);
+  return HermesValue::encodeStringValue(desc);
+}
+
 CallResult<HermesValue>
 symbolPrototypeToString(void *, Runtime *runtime, NativeArgs args) {
   MutableHandle<SymbolID> sym{runtime};

test/hermes/symbol.js

@@ -21,6 +21,26 @@ gc();
 print(String(sym));
 // CHECK-NEXT: Symbol(44912)
 
+print('description');
+// CHECK-LABEL: description
+print(Symbol().description);
+// CHECK-EMPTY:
+print(Symbol('asdf').description);
+// CHECK-NEXT: asdf
+print(Symbol({toString: function() {return 'asdf'}}).description)
+// CHECK-NEXT: asdf
+print(Object(Symbol('asdf')).description)
+// CHECK-NEXT: asdf
+try { print(Symbol.prototype.description.call(3)); }
+catch (e) { print('caught', e.name); }
+// CHECK-NEXT: caught TypeError
+print(Symbol.iterator.description)  // well-known Symbols
+// CHECK-NEXT: Symbol.iterator
+
+// TODO(The below cases need to return `undefined` to be fully conformant.)
+// print(Symbol().description)
+// print(Symbol(undefined).description)
+
 print('toString');
 // CHECK-LABEL: toString
 print(Symbol().toString());