WebAssembly targets for Rust will soon face a change that could risk breaking existing projects, according to an April 4 bulletin in the official Rust blog. The bulletin notes that all WebAssembly targets in Rust have been linked using the --allow-undefined flag to wasm-ld, but this flag is being removed.
Removing --allow-undefined on wasm targets is being done in rust-lang/rust#149868. That change is slated to land in nightly builds soon and will be released with Rust 1.96 on 2026-05-28. The bulletin explains that all WebAssembly binaries in Rust are created by linking with wasm-ld, thus serving a similar purpose to ld, lld, and mold. Since the first introduction of WebAssembly targets in Rust, the --allow-undefined flag has been passed to wasm-ld.
However, by passing --allow-undefined on all WebAssembly targets, rustc introduces diverging behavior between other platforms and WebAssembly, the bulletin says. The main risk of --allow-undefined is that misconfiguration or mistakes in building can result in broken WebAssembly modules being produced, as opposed to compilation errors. The bulletin lists the following example problematic situations:
