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-allocator2 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 bare_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.
• unstable: Enable the use of unstable Rust features for aspects of the crate that benefit from them without causing any API breaks. Unstable features that can cause breaking changes when enabled are gated separately.
• tuple_trait: Adds a core::marker::Tuple bound on FnPtr::Args. This allows downstream crates to easily integrate the library with closure-related nightly features such as unboxed_closures and fn_traits. Also enables the unstable feature.
• c_variadic: Adds partial (no invocation through call) FnPtr and Fn*Thunk implementations for variadic functions.
• full: Enables bundled_jit_alloc and proc_macros features.

Examples

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

use closure_ffi::{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_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}");
}