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diff --git a/rustversion/README.md b/rustversion/README.md new file mode 100644 index 0000000..f7fdab1 --- /dev/null +++ b/rustversion/README.md @@ -0,0 +1,138 @@ +Compiler version cfg +==================== + +[](https://travis-ci.com/dtolnay/rustversion) +[](https://crates.io/crates/rustversion) +[](https://docs.rs/rustversion) + +This crate provides macros for conditional compilation according to rustc +compiler version, analogous to [`#[cfg(...)]`][cfg] and +[`#[cfg_attr(...)]`][cfg_attr]. + +[cfg]: https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg-attribute +[cfg_attr]: https://doc.rust-lang.org/reference/conditional-compilation.html#the-cfg_attr-attribute + +```toml +[dependencies] +rustversion = "1.0" +``` + +<br> + +## Selectors + +- <b>`#[rustversion::stable]`</b> + —<br> + True on any stable compiler. + +- <b>`#[rustversion::stable(1.34)]`</b> + —<br> + True on exactly the specified stable compiler. + +- <b>`#[rustversion::beta]`</b> + —<br> + True on any beta compiler. + +- <b>`#[rustversion::nightly]`</b> + —<br> + True on any nightly compiler or dev build. + +- <b>`#[rustversion::nightly(2019-01-01)]`</b> + —<br> + True on exactly one nightly. + +- <b>`#[rustversion::since(1.34)]`</b> + —<br> + True on that stable release and any later compiler, including beta and + nightly. + +- <b>`#[rustversion::since(2019-01-01)]`</b> + —<br> + True on that nightly and all newer ones. + +- <b>`#[rustversion::before(`</b><i>version or date</i><b>`)]`</b> + —<br> + Negative of *#[rustversion::since(...)]*. + +- <b>`#[rustversion::not(`</b><i>selector</i><b>`)]`</b> + —<br> + Negative of any selector; for example *#[rustversion::not(nightly)]*. + +- <b>`#[rustversion::any(`</b><i>selectors...</i><b>`)]`</b> + —<br> + True if any of the comma-separated selectors is true; for example + *#[rustversion::any(stable, beta)]*. + +- <b>`#[rustversion::all(`</b><i>selectors...</i><b>`)]`</b> + —<br> + True if all of the comma-separated selectors are true; for example + *#[rustversion::all(since(1.31), before(1.34))]*. + +- <b>`#[rustversion::attr(`</b><i>selector</i><b>`, `</b><i>attribute</i><b>`)]`</b> + —<br> + For conditional inclusion of attributes; analogous to `cfg_attr`. + +<br> + +## Use cases + +Providing additional trait impls as types are stabilized in the standard library +without breaking compatibility with older compilers; in this case Pin\<P\> +stabilized in [Rust 1.33][pin]: + +[pin]: https://blog.rust-lang.org/2019/02/28/Rust-1.33.0.html#pinning + +```rust +#[rustversion::since(1.33)] +use std::pin::Pin; + +#[rustversion::since(1.33)] +impl<P: MyTrait> MyTrait for Pin<P> { + /* ... */ +} +``` + +Similar but for language features; the ability to control alignment greater than +1 of packed structs was stabilized in [Rust 1.33][packed]. + +[packed]: https://github.com/rust-lang/rust/blob/master/RELEASES.md#version-1330-2019-02-28 + +```rust +#[rustversion::attr(before(1.33), repr(packed))] +#[rustversion::attr(since(1.33), repr(packed(2)))] +struct Six(i16, i32); + +fn main() { + println!("{}", std::mem::align_of::<Six>()); +} +``` + +Augmenting code with `const` as const impls are stabilized in the standard +library. This use of `const` as an attribute is recognized as a special case by +the rustversion::attr macro. + +```rust +use std::time::Duration; + +#[rustversion::attr(since(1.32), const)] +fn duration_as_days(dur: Duration) -> u64 { + dur.as_secs() / 60 / 60 / 24 +} +``` + +<br> + +#### License + +<sup> +Licensed under either of <a href="LICENSE-APACHE">Apache License, Version +2.0</a> or <a href="LICENSE-MIT">MIT license</a> at your option. +</sup> + +<br> + +<sub> +Unless you explicitly state otherwise, any contribution intentionally submitted +for inclusion in this crate by you, as defined in the Apache-2.0 license, shall +be dual licensed as above, without any additional terms or conditions. +</sub> |