The Rust team has announced a breaking change for WebAssembly targets: the --allow-undefined flag passed to wasm-ld will be removed, landing in nightly builds soon and shipping with Rust 1.96 on 2026-05-28. This flag has historically allowed undefined symbols to silently become imports rather than linker errors, causing subtle bugs like typos in symbol names producing broken binaries instead of compile-time errors. The change aligns WebAssembly behavior with native platforms, where undefined symbols are errors by default. In practice, most projects are not expected to break, and the change should improve diagnostics for misconfigured builds.
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