Fix crash in DowncastError Display impl, remove String member

src/handle/mod.rs

@@ -52,7 +52,6 @@ impl<'a, T: Managed + 'a> Handle<'a, T> {
 pub struct DowncastError<F: Value, T: Value> {
     phantom_from: PhantomData<F>,
     phantom_to: PhantomData<T>,
-    description: String
 }
 
 impl<F: Value, T: Value> Debug for DowncastError<F, T> {
@@ -66,22 +65,17 @@ impl<F: Value, T: Value> DowncastError<F, T> {
         DowncastError {
             phantom_from: PhantomData,
             phantom_to: PhantomData,
-            description: format!("failed downcast to {}", T::name())
         }
     }
 }
 
 impl<F: Value, T: Value> Display for DowncastError<F, T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        write!(f, "{}", self.to_string())
+        write!(f, "failed to downcast {} to {}", F::name(), T::name())
     }
 }
 
-impl<F: Value, T: Value> Error for DowncastError<F, T> {
-    fn description(&self) -> &str {
-        &self.description
-    }
-}
+impl<F: Value, T: Value> Error for DowncastError<F, T> {}
 
 /// The result of a call to `Handle::downcast()`.
 pub type DowncastResult<'a, F, T> = Result<Handle<'a, T>, DowncastError<F, T>>;
@@ -90,7 +84,7 @@ impl<'a, F: Value, T: Value> JsResultExt<'a, T> for DowncastResult<'a, F, T> {
     fn or_throw<'b, C: Context<'b>>(self, cx: &mut C) -> JsResult<'a, T> {
         match self {
             Ok(v) => Ok(v),
-            Err(e) => cx.throw_type_error(&e.description)
+            Err(e) => cx.throw_type_error(&e.to_string())
         }
     }
 }

test/dynamic/lib/functions.js

@@ -60,6 +60,11 @@ describe('JsFunction', function() {
     assert.throw(addon.unexpected_throw_and_catch, Error, /^internal error in Neon module: try_catch: unexpected Err\(Throw\) when VM is not in a throwing state$/);
   })
 
+  it('should be able to stringify a downcast error', function () {
+    let msg = addon.downcast_error();
+    assert.strictEqual(msg, "failed to downcast string to number");
+  });
+
   it('computes the right number of arguments', function() {
     assert.equal(addon.num_arguments(), 0);
     assert.equal(addon.num_arguments('a'), 1);

test/dynamic/native/src/js/functions.rs

@@ -127,3 +127,12 @@ pub fn unexpected_throw_and_catch(mut cx: FunctionContext) -> JsResult<JsValue>
     Ok(cx.try_catch(|_| { Err(Throw) })
          .unwrap_or_else(|err| err))
 }
+
+pub fn downcast_error(mut cx: FunctionContext) -> JsResult<JsString> {
+    let s = cx.string("hi");
+    if let Err(e) = s.downcast::<JsNumber>() {
+        Ok(cx.string(format!("{}", e)))
+    } else {
+        panic!()
+    }
+}

test/dynamic/native/src/lib.rs

@@ -75,6 +75,7 @@ register_module!(mut cx, {
     cx.export_function("call_and_catch", call_and_catch)?;
     cx.export_function("panic_and_catch", panic_and_catch)?;
     cx.export_function("unexpected_throw_and_catch", unexpected_throw_and_catch)?;
+    cx.export_function("downcast_error", downcast_error)?;
 
     cx.export_class::<JsEmitter>("Emitter")?;
     cx.export_class::<JsTestEmitter>("TestEmitter")?;

test/napi/lib/errors.js

@@ -33,4 +33,10 @@ describe('errors', function() {
 
     assert.throws(() => addon.throw_error(msg), msg);
   });
+
+  it('should be able to stringify a downcast error', function () {
+    let msg = addon.downcast_error();
+    assert.strictEqual(msg, "failed to downcast string to number");
+  });
+
 });

test/napi/native/src/js/errors.rs

@@ -23,3 +23,12 @@ pub fn throw_error(mut cx: FunctionContext) -> JsResult<JsUndefined> {
 
     cx.throw_error(msg)
 }
+
+pub fn downcast_error(mut cx: FunctionContext) -> JsResult<JsString> {
+    let s = cx.string("hi");
+    if let Err(e) = s.downcast::<JsNumber, _>(&mut cx) {
+        Ok(cx.string(format!("{}", e)))
+    } else {
+        panic!()
+    }
+}

test/napi/native/src/lib.rs

@@ -158,6 +158,7 @@ register_module!(|mut cx| {
     cx.export_function("new_type_error", new_type_error)?;
     cx.export_function("new_range_error", new_range_error)?;
     cx.export_function("throw_error", throw_error)?;
+    cx.export_function("downcast_error", downcast_error)?;
 
     cx.export_function("panic", panic)?;
     cx.export_function("panic_after_throw", panic_after_throw)?;