&JsValue -> &T where T is an exported Rust struct

[WilliamVenner]

Hi, is there a way to convert a &JsValue to a &T where T is an exported Rust struct?

The TryFromJsValue trait consumes a JsValue and even frees the underlying exported Rust struct from the heap.

JsCast isn't implemented for exported Rust structs.

[Liamolucko]

No, there isn't at the moment.

A literal &JsValue -> &T transformation wouldn't work. The reason is that the way that borrowing Rust types works is that wasm-bindgen wraps them in an internal RefCell which it borrows every time a reference is passed to an exported Rust function, and then at the end of the function the Ref / RefMut is dropped to mark the RefCell as non-borrowed again. Otherwise JavaScript could easily pass the same T in for two different &mut T parameters of a function and cause undefined behaviour.

With a &T, there's no way to run some code when it's dropped; so, such an API would require creating some kind of Ref<T> or Handle<T> type which you can then turn &JsValue into.

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Also, TryFromJsValue is a private trait (for now, anyway) - it's in wasm_bindgen::convert, the docs for which say:

This is mostly an internal module, no stability guarantees are provided. Use at your own risk.

It probably should be public, but it's a bit of a mess: see #3709 for the history here.

[WilliamVenner]

Thank you!