Compiler version cfg
====================

[github](https://github.com/dtolnay/rustversion)
[crates.io](https://crates.io/crates/rustversion)
[docs.rs](https://docs.rs/rustversion)
[build status](https://github.com/dtolnay/rustversion/actions?query=branch%3Amaster)

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"
```


## Selectors

- `#[rustversion::stable]`
  —
  True on any stable compiler.

- `#[rustversion::stable(1.34)]`
  —
  True on exactly the specified stable compiler.

- `#[rustversion::beta]`
  —
  True on any beta compiler.

- `#[rustversion::nightly]`
  —
  True on any nightly compiler or dev build.

- `#[rustversion::nightly(2019-01-01)]`
  —
  True on exactly one nightly.

- `#[rustversion::since(1.34)]`
  —
  True on that stable release and any later compiler, including beta and
  nightly.

- `#[rustversion::since(2019-01-01)]`
  —
  True on that nightly and all newer ones.

- `#[rustversion::before(`version or date`)]`
  —
  Negative of *#[rustversion::since(...)]*.

- `#[rustversion::not(`selector`)]`
  —
  Negative of any selector; for example *#[rustversion::not(nightly)]*.

- `#[rustversion::any(`selectors...`)]`
  —
  True if any of the comma-separated selectors is true; for example
  *#[rustversion::any(stable, beta)]*.

- `#[rustversion::all(`selectors...`)]`
  —
  True if all of the comma-separated selectors are true; for example
  *#[rustversion::all(since(1.31), before(1.34))]*.

- `#[rustversion::attr(`selector`, `attribute`)]`
  —
  For conditional inclusion of attributes; analogous to `cfg_attr`.

## 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
}
```


#### License

Licensed under either of Apache License, Version
2.0 or MIT license at your option.

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.

