allocator_api 0.4.0

This is a copy of the unstable allocator_api (https://github.com/rust-lang/rust/issues/32838) and of parts of the unstable alloc feature. Usable with stable rust, but requires 1.26.

Documentation

extern crate rustc_version;

use rustc_version::{version, Version};

fn main() {
    if version().unwrap() >= Version::parse("1.27.0-beta").unwrap() {
        println!("cargo:rustc-cfg=feature=\"nonnull_cast\"");
    }
}