Can't mutate state from Funcs passed as wasm imports

[J0eCool]

Trying to build a graphical application using the wasmtime Rust API, using the sdl2 crate, I run in to the issue that SDL's drawing methods take a mutable reference to the screen, and using those from a closure passed to Func::wrap gives me the error

error[E0525]: expected a closure that implements the Fn trait, but this closure only implements FnMut

Feature

Let the closure argument to Func::wrap derive from FnMut instead of Fn

Benefit

This should allow embedder applications more freedom to build wasm modules that interact with the outside world in stateful ways

[alexcrichton]

Thanks for the report! This is required for safety when using wasm modules, however, so it's not something that we can fix. Host functions can be called recursively if they, for example, reenter wasm and then get called again.

You can get mutation through interior mutability in Rust, however, for example through RefCell or Cell.