README.md
1autocfg
2=======
3
4[](https://crates.io/crates/autocfg)
5[](https://docs.rs/autocfg)
6
7
8
9A Rust library for build scripts to automatically configure code based on
10compiler support. Code snippets are dynamically tested to see if the `rustc`
11will accept them, rather than hard-coding specific version support.
12
13
14## Usage
15
16Add this to your `Cargo.toml`:
17
18```toml
19[build-dependencies]
20autocfg = "1"
21```
22
23Then use it in your `build.rs` script to detect compiler features. For
24example, to test for 128-bit integer support, it might look like:
25
26```rust
27extern crate autocfg;
28
29fn main() {
30 let ac = autocfg::new();
31 ac.emit_has_type("i128");
32
33 // (optional) We don't need to rerun for anything external.
34 autocfg::rerun_path("build.rs");
35}
36```
37
38If the type test succeeds, this will write a `cargo:rustc-cfg=has_i128` line
39for Cargo, which translates to Rust arguments `--cfg has_i128`. Then in the
40rest of your Rust code, you can add `#[cfg(has_i128)]` conditions on code that
41should only be used when the compiler supports it.
42
43
44## Release Notes
45
46- 1.0.1 (2020-08-20)
47 - Apply `RUSTFLAGS` for more `--target` scenarios, by @adamreichold.
48
49- 1.0.0 (2020-01-08)
50 - �� Release 1.0! �� (no breaking changes)
51 - Add `probe_expression` and `emit_expression_cfg` to test arbitrary expressions.
52 - Add `probe_constant` and `emit_constant_cfg` to test arbitrary constant expressions.
53
54- 0.1.7 (2019-10-20)
55 - Apply `RUSTFLAGS` when probing `$TARGET != $HOST`, mainly for sysroot, by @roblabla.
56
57- 0.1.6 (2019-08-19)
58 - Add `probe`/`emit_sysroot_crate`, by @leo60228.
59
60- 0.1.5 (2019-07-16)
61 - Mask some warnings from newer rustc.
62
63- 0.1.4 (2019-05-22)
64 - Relax `std`/`no_std` probing to a warning instead of an error.
65 - Improve `rustc` bootstrap compatibility.
66
67- 0.1.3 (2019-05-21)
68 - Auto-detects if `#![no_std]` is needed for the `$TARGET`.
69
70- 0.1.2 (2019-01-16)
71 - Add `rerun_env(ENV)` to print `cargo:rerun-if-env-changed=ENV`.
72 - Add `rerun_path(PATH)` to print `cargo:rerun-if-changed=PATH`.
73
74
75## Minimum Rust version policy
76
77This crate's minimum supported `rustc` version is `1.0.0`. Compatibility is
78its entire reason for existence, so this crate will be extremely conservative
79about raising this requirement. If this is ever deemed necessary, it will be
80treated as a major breaking change for semver purposes.
81
82
83## License
84
85This project is licensed under either of
86
87 * Apache License, Version 2.0, ([LICENSE-APACHE](LICENSE-APACHE) or
88 http://www.apache.org/licenses/LICENSE-2.0)
89 * MIT license ([LICENSE-MIT](LICENSE-MIT) or
90 http://opensource.org/licenses/MIT)
91
92at your option.
93
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