libffi 0.7.0

# libffi-rs: Rust bindings for [libffi](https://sourceware.org/libffi/)

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The C libffi library provides two main facilities: assembling calls
to functions dynamically, and creating closures that can be called
as ordinary C functions. In Rust, the latter means that we can turn
a Rust lambda (or any object implementing `Fn`/`FnMut`) into an
ordinary C function pointer that we can pass as a callback to C.

## Usage

Building libffi will build lifbffi-sys, which will in turn build the
libffi C library [from github](https://github.com/libffi/libffi), which
requires that you have a working make, C compiler, automake, autoconf,
and texinfo first. It’s [on crates.io](https://crates.io/crates/libffi),
so you can add

```toml
[dependencies]
libffi = "0.7.0"
```

to your `Cargo.toml` and

```rust
extern crate libffi;
```

to your crate root.

This crate supports Rust version 1.31 and later.

### Examples

In this example, we convert a Rust lambda containing a free variable
into an ordinary C code pointer. The type of `fun` below is
`extern "C" fn(u64, u64) -> u64`.

```
use libffi::high::Closure2;

let x = 5u64;
let f = |y: u64, z: u64| x + y + z;

let closure = Closure2::new(&f);
let fun     = closure.code_ptr();

assert_eq!(18, fun(6, 7));
```