Provides wrappers around closures which allows them to be called through context-free unsafe bare functions.

Context-free bare functions are not needed very often, as properly designed C APIs typically allow the user to specify an opaque pointer to a context object which will be provided to the function pointer. However, this is not always the case, and may be impossible in less common scenarios, e.g. function hooking for game modding/hacking.

Features

The crate comes with the following feature flags:

• no_std: Makes the crate compatible with #![no_std]. A dependency on alloc and spin is still required.
• bundled_jit_alloc: Provides a global JIT allocator through the jit-allocator crate. This is enabled by default.
• custom_jit_alloc: Allows providing a global JIT allocator through the global_jit_alloc! macro. This is incompatible with bundled_jit_alloc.
• proc_macros: Provides the cc::hrtb proc macro which is necessary for creating bare functions with signatures that involve higher-kinded lifetimes (i.e. for<'a, ...> statements), as well as the bare_dyn proc macro for writing BareFn* types of boxed closures (i.e. Box<dyn Fn()>) more concisely.
• full: Enables all features except for no_std.

Examples

Passing a closure to a C API taking a contextless function pointer:

use closure_ffi::{cc, BareFnMut};
// Imagine we have an foreign C API for reigstering and unregistering some callback function.
// Notably, the API does not let the user provide a context object to the callback.
unsafe extern "C" fn ffi_register_callback(cb: unsafe extern "C" fn(u32)) {
    // ...
}
unsafe extern "C" fn ffi_unregister_callback(cb: unsafe extern "C" fn(u32)) {
    // ...
}

#[cfg(feature = "bundled_jit_alloc")]
{
    // We want to keep track of sum of callback arguments without using 
    // statics. This is where closure-ffi comes in:
    let mut sum = 0; // Non-'static closures work too!
    let wrapped = BareFnMut::new(cc::C, |x: u32| {
        sum += x;
    });

    // Safety: Here, we assert that the foreign API won't use the callback
    // in ways that break Rust's safety rules. Namely:
    // - The exclusivity of the FnMut's borrow is respected.
    // - If the calls are made from a different thread, the closure is Sync.
    // - We unregister the callback before the BareFnMut is dropped.
    unsafe {
        ffi_register_callback(wrapped.bare());
        // Do something that triggers the callback...
        ffi_unregister_callback(wrapped.bare());
    }

    drop(wrapped);
    println!("{sum}");
}