Speed up passing ASCII-only strings to WASM

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

@@ -1499,12 +1499,19 @@ impl<'a> Context<'a> {
                     arg = arg.slice(offset);
                     ptr = wasm.__wbindgen_realloc(ptr, size, size = offset + arg.length * 3);
                     const view = getUint8Memory().subarray(ptr + offset, ptr + size);
+                    const ret = cachedTextEncoder.encodeInto(arg, view);
+                    {}
                     offset += cachedTextEncoder.encodeInto(arg, view).written;
                 }}
                 WASM_VECTOR_LEN = offset;
                 return ptr;
             ",
-            start_encoding_as_ascii
+            start_encoding_as_ascii,
+            if self.config.debug {
+                "if (ret.read != arg.length) throw new Error('failed to pass whole string');"
+            } else {
+                ""
+            },
         );
 
         // Looks like `encodeInto` doesn't currently work when the memory passed

tests/headless/main.rs

@@ -50,3 +50,4 @@ pub fn import_export_same_name() {
 pub mod snippets;
 pub mod modules;
 pub mod anyref_heap_live_count;
+pub mod strings;

tests/headless/strings.js

@@ -0,0 +1,15 @@
+export function test_string_roundtrip(f) {
+  const test = expected => {
+    const actual = f(expected);
+    if (actual === expected)
+      return;
+    throw new Error(`string roundtrip "${actual}" != "${expected}"`);
+  };
+
+  test('');
+  test('a');
+  test('💖');
+
+  test('a longer string');
+  test('a longer 💖 string');
+}

tests/headless/strings.rs

@@ -0,0 +1,12 @@
+use wasm_bindgen::prelude::*;
+use wasm_bindgen_test::*;
+
+#[wasm_bindgen(module = "/tests/headless/strings.js")]
+extern "C" {
+    fn test_string_roundtrip(c: &Closure<Fn(String) -> String>);
+}
+
+#[wasm_bindgen_test]
+fn string_roundtrip() {
+    test_string_roundtrip(&Closure::wrap(Box::new(|s| s)));
+}

tests/wasm/simple.js

@@ -92,3 +92,16 @@ exports.RenamedInRust = class {};
 exports.new_renamed = () => new exports.RenamedInRust;
 
 exports.import_export_same_name = () => {};
+
+exports.test_string_roundtrip = () => {
+  const test = s => {
+    assert.strictEqual(wasm.do_string_roundtrip(s), s);
+  };
+
+  test('');
+  test('a');
+  test('💖');
+
+  test('a longer string');
+  test('a longer 💖 string');
+};

tests/wasm/simple.rs

@@ -27,6 +27,8 @@ extern "C" {
     #[wasm_bindgen(js_name = RenamedInRust)]
     type Renamed;
     fn new_renamed() -> Renamed;
+
+    fn test_string_roundtrip();
 }
 
 #[wasm_bindgen_test]
@@ -201,3 +203,13 @@ fn renaming_imports_and_instanceof() {
 pub fn import_export_same_name() {
     js_import_export_same_name();
 }
+
+#[wasm_bindgen_test]
+fn string_roundtrip() {
+    test_string_roundtrip();
+}
+
+#[wasm_bindgen]
+pub fn do_string_roundtrip(s: String) -> String {
+    s
+}