Allow conversion from Rust errors to JS throws

Fixes rustwasm#1017.

crates/cli-support/src/js/mod.rs

@@ -460,16 +460,17 @@ impl<'a> Context<'a> {
         self.export_table();
         self.gc();
 
-        // Note that it's important `throw` comes last *after* we gc. The
+        // Note that it's important `error_new` comes last *after* we gc. The
         // `__wbindgen_malloc` function may call this but we only want to
         // generate code for this if it's actually live (and __wbindgen_malloc
         // isn't gc'd).
-        self.bind("__wbindgen_throw", &|me| {
+        self.bind("__wbindgen_error_new", &|me| {
+            me.expose_add_heap_object();
             me.expose_get_string_from_wasm();
             Ok(String::from(
                 "
                 function(ptr, len) {
-                    throw new Error(getStringFromWasm(ptr, len));
+                    return addHeapObject(new Error(getStringFromWasm(ptr, len)));
                 }
                 ",
             ))

src/convert/impls.rs

@@ -418,3 +418,16 @@ impl<T: IntoWasmAbi> ReturnWasmAbi for Result<T, JsValue> {
         }
     }
 }
+
+if_std! {
+    use std::string::ToString;
+
+    impl<T: IntoWasmAbi, E: std::error::Error> ReturnWasmAbi for Result<T, E> {
+        type Abi = T::Abi;
+
+        fn return_abi(self, extra: &mut Stack) -> Self::Abi {
+            self.map_err(|err| JsValue::error(&err.to_string()))
+                .return_abi(extra)
+        }
+    }
+}

src/describe.rs

@@ -154,6 +154,14 @@ impl<T: WasmDescribe> WasmDescribe for Result<T, JsValue> {
     }
 }
 
+if_std! {
+    impl<T: WasmDescribe, E: std::error::Error> WasmDescribe for Result<T, E> {
+        fn describe() {
+            T::describe()
+        }
+    }
+}
+
 impl<T: WasmDescribe> WasmDescribe for Clamped<T> {
     fn describe() {
         inform(CLAMPED);

src/lib.rs

@@ -162,6 +162,13 @@ impl JsValue {
         }
     }
 
+    /// Creates an instance of JS `Error` with the provided message.
+    ///
+    /// The stacktrace generated by the JS engine will end at this invocation.
+    pub fn error(msg: &str) -> JsValue {
+        JsValue::_new(unsafe { __wbindgen_error_new(msg.as_ptr(), msg.len()) })
+    }
+
     /// Creates a new `JsValue` from the JSON serialization of the object `t`
     /// provided.
     ///
@@ -477,8 +484,8 @@ externs! {
     fn __wbindgen_is_function(idx: u32) -> u32;
     fn __wbindgen_is_string(idx: u32) -> u32;
     fn __wbindgen_string_get(idx: u32, len: *mut usize) -> *mut u8;
-    fn __wbindgen_throw(a: *const u8, b: usize) -> !;
-    fn __wbindgen_rethrow(a: u32) -> !;
+    fn __wbindgen_error_new(ptr: *const u8, len: usize) -> u32;
+    fn __wbindgen_rethrow(err_idx: u32) -> !;
 
     fn __wbindgen_cb_drop(idx: u32) -> u32;
     fn __wbindgen_cb_forget(idx: u32) -> ();
@@ -610,9 +617,7 @@ pub fn throw(s: &str) -> ! {
 #[cold]
 #[inline(never)]
 pub fn throw_str(s: &str) -> ! {
-    unsafe {
-        __wbindgen_throw(s.as_ptr(), s.len());
-    }
+    throw_val(JsValue::error(s));
 }
 
 /// Rethrow a JS exception

tests/wasm/rethrow.js

@@ -12,3 +12,8 @@ exports.call_throw_one = function() {
 exports.call_ok = function() {
   wasm.nothrow();
 };
+
+exports.call_parse_or_throw_rust_error = function() {
+  assert.strictEqual(wasm.parse_or_throw_rust_error("12"), 12);
+  assert.throws(() => wasm.parse_or_throw_rust_error("12#"), new Error('invalid digit found in string'));
+};

tests/wasm/rethrow.rs

@@ -1,10 +1,13 @@
+use std::num::ParseIntError;
+
 use wasm_bindgen::prelude::*;
 use wasm_bindgen_test::*;
 
 #[wasm_bindgen(module = "tests/wasm/rethrow.js")]
 extern "C" {
     fn call_throw_one();
     fn call_ok();
+    fn call_parse_or_throw_rust_error();
 }
 
 #[wasm_bindgen_test]
@@ -26,3 +29,13 @@ fn ok_works() {
 pub fn nothrow() -> Result<u32, JsValue> {
     Ok(1)
 }
+
+#[wasm_bindgen_test]
+fn parse_or_throw_rust_error_works() {
+    call_parse_or_throw_rust_error();
+}
+
+#[wasm_bindgen]
+pub fn parse_or_throw_rust_error(s: &str) -> Result<u32, ParseIntError> {
+    s.parse()
+}