allocator_fallback/

lib.rs

/*
 * Copyright 2022, 2026 taylor.fish <contact@taylor.fish>
 *
 * This file is part of allocator-fallback.
 *
 * allocator-fallback is licensed under the Apache License, Version 2.0
 * (the "License"); you may not use allocator-fallback except in compliance
 * with the License. You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#![cfg_attr(not(any(feature = "std", doc)), no_std)]
#![cfg_attr(has_allocator_api, feature(allocator_api))]
#![cfg_attr(feature = "doc_cfg", feature(doc_cfg))]
#![deny(unsafe_op_in_unsafe_fn)]
#![allow(clippy::needless_doctest_main)]

//! This crate provides a minimal fallback for the standard library’s allocator
//! API, which is currently unstable.
//!
//! Usage
//! -----
//!
//! Because allocator-fallback can be configured to re-export the real
//! unstable allocator API via the [`allocator_api` crate feature][features],
//! your crate must conditionally enable `#![feature(allocator_api)]` in
//! preparation for this occurrence; otherwise, compilation errors may occur.
//! This is true even if your crate never directly enables the `allocator_api`
//! feature, because a different crate that also depends on allocator-fallback
//! could enable it.
//!
//! [features]: #crate-features
//!
//! To accomplish this, in `Cargo.toml`, add allocator-fallback as both a
//! regular dependency and a build dependency:
//!
//! ```toml
//! [dependencies]
//! allocator-fallback = "0.1.9"
//!
//! [build-dependencies]
//! allocator-fallback = "0.1.9"
//! ```
//!
//! **Note:** It is very important that the two dependencies are identical. Do
//! not enable a feature in one without enabling it in the other.
//!
//! Then add a [build script][build] (`build.rs`) with the following
//! contents:[^1]
//!
//! [build]: https://doc.rust-lang.org/cargo/reference/build-scripts.html
//!
//! ```rust
//! fn main() {
//!    if allocator_fallback::HAS_ALLOCATOR_API {
//!        println!("cargo:rustc-cfg=has_allocator_api");
//!    }
//!    println!("cargo:rerun-if-changed=build.rs");
//! }
//! ```
//!
//! Finally, at the top of your crate root (likely `lib.rs` or `main.rs`), add
//! the following:
//!
//! ```rust
//! #![cfg_attr(has_allocator_api, feature(allocator_api))]
//! ```
//!
//! Rust may show a warning about an “unexpected `cfg` condition name”; you can
//! silence it by adding the following to Cargo.toml:
//!
//! ```toml
//! [lints.rust.unexpected_cfgs]
//! level = "warn"
//! check-cfg = ["cfg(has_allocator_api)"]
//! ```
//!
//! ### Use as an optional dependency
//!
//! If you’d like allocator-fallback to be an optional dependency, first add
//! `optional = true` to both of its declarations as a dependency:
//!
//! ```toml
//! [dependencies.allocator-fallback]
//! version = "0.1.9"
//! optional = true
//!
//! [build-dependencies.allocator-fallback]
//! version = "0.1.9"
//! optional = true
//! ```
//!
//! Then adjust `build.rs` as follows:[^1]
//!
//! ```rust
//! fn main() {
//!    #[cfg(feature = "allocator-fallback")]
//!    if allocator_fallback::HAS_ALLOCATOR_API {
//!        println!("cargo:rustc-cfg=has_allocator_api");
//!    }
//!    println!("cargo:rerun-if-changed=build.rs");
//! }
//! ```
//!
//! Finally, make sure you still have the following in your crate root:
//!
//! ```rust
//! #![cfg_attr(has_allocator_api, feature(allocator_api))]
//! ```
//!
//! [^1]: These build script code snippets have been released to the public
//!       domain using the [CC0 1.0 Universal Public Domain Dedication][CC0].
//!
//! [CC0]: https://creativecommons.org/publicdomain/zero/1.0/legalcode.txt
//!
//! ### Exposing an `allocator_api` feature in your crate
//!
//! If you want your crate to directly provide an `allocator_api` feature that
//! enables the real allocator API, add the following to your Cargo.toml:
//!
//! ```toml
//! [features]
//! allocator_api = ["allocator-fallback/allocator_api"]
//! ```
//!
//! If you declared allocator-fallback as an optional dependency, add the
//! following instead:
//!
//! ```toml
//! [features]
//! allocator_api = ["allocator-fallback?/allocator_api"]
//! ```
//!
//! Crate features
//! --------------
//!
//! If the crate feature `allocator_api` is enabled, this crate will simply
//! re-export the real allocator API in the standard library. Of course, this
//! requires Rust nightly.
//!
//! If the crate feature `std` is enabled (the default), the crate will use
//! [`std`]; otherwise, it will be `no_std`. Using [`std`] allows
//! [`AllocError`] to implement [`std::error::Error`].

extern crate alloc;

#[cfg(not(has_allocator_api))]
mod fallback;

#[cfg(not(has_allocator_api))]
pub use fallback::{AllocError, Allocator, Global};

#[cfg(has_allocator_api)]
pub use alloc::alloc::{AllocError, Allocator, Global};

/// For use in build scripts. See [Usage](crate#usage).
pub const HAS_ALLOCATOR_API: bool = cfg!(has_allocator_api);

#[test]
fn test() {
    use alloc::alloc::Layout;
    let _: &dyn Allocator = &Global;
    let ptr = Global.allocate(Layout::new::<u64>()).unwrap();
    unsafe {
        ptr.cast::<u64>().as_ptr().write(0x123456789abcdef);
        (&&&Global).deallocate(ptr.cast(), Layout::new::<u64>());
    }
}