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The ability to interoperate with C code via FFI is not limited to crates using std; this allows using these types without std. The existing types in `std::os::raw` become type aliases for the ones in `core::ffi`. This uses type aliases rather than re-exports, to allow the std types to remain stable while the core types are unstable. This also moves the currently unstable `NonZero_` variants and `c_size_t`/`c_ssize_t`/`c_ptrdiff_t` types to `core::ffi`, while leaving them unstable.
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Equivalent to C's void type when used as a [pointer].
In essence, *const c_void is equivalent to C's const void*
and *mut c_void is equivalent to C's void*. That said, this is
not the same as C's void return type, which is Rust's () type.
To model pointers to opaque types in FFI, until extern type is
stabilized, it is recommended to use a newtype wrapper around an empty
byte array. See the Nomicon for details.
One could use std::os::raw::c_void if they want to support old Rust
compiler down to 1.1.0. After Rust 1.30.0, it was re-exported by
this definition. For more information, please read RFC 2521.