Shrink JS-allocated strings down to the correct size before passing t…

…hem to Rust (#3808)

* Shrink JS-allocated strings down to the correct size before passing them to Rust

Fixes #3801.

I opted to solve it this way rather than just pass the capacity to Rust
as well because it means the allocation isn't up to 3x bigger than it
needs to be anymore. I also removed a TODO about fixing that.

* Update reference tests

* Add changelog entry

CHANGELOG.md

@@ -28,6 +28,11 @@
 * Deprecate `wasm_bindgen_test_configure!`s `run_in_worker` in favor of `run_in_dedicated_worker`.
   [#3804](https://github.com/rustwasm/wasm-bindgen/pull/3804)
 
+### Fixed
+
+* Fixed UB when freeing strings received from JS if not using the default allocator.
+  [#3808](https://github.com/rustwasm/wasm-bindgen/pull/3808)
+
 ## [0.2.90](https://github.com/rustwasm/wasm-bindgen/compare/0.2.89...0.2.90)
 
 Released 2024-01-06

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

@@ -1296,11 +1296,6 @@ impl<'a> Context<'a> {
             mem = mem,
         );
 
-        // TODO:
-        // When converting a JS string to UTF-8, the maximum size is `arg.length * 3`,
-        // so we just allocate that. This wastes memory, so we should investigate
-        // looping over the string to calculate the precise size, or perhaps using
-        // `shrink_to_fit` on the Rust side.
         self.global(&format!(
             "function {name}(arg, malloc, realloc) {{
                 {debug}
@@ -1314,6 +1309,7 @@ impl<'a> Context<'a> {
                     const ret = encodeString(arg, view);
                     {debug_end}
                     offset += ret.written;
+                    ptr = realloc(ptr, len, offset, 1) >>> 0;
                 }}
 
                 WASM_VECTOR_LEN = offset;

crates/cli/tests/reference/string-arg.js

@@ -75,6 +75,7 @@ function passStringToWasm0(arg, malloc, realloc) {
         const ret = encodeString(arg, view);
 
         offset += ret.written;
+        ptr = realloc(ptr, len, offset, 1) >>> 0;
     }
 
     WASM_VECTOR_LEN = offset;