Compiler version cfg

This crate provides macros for conditional compilation according to rustc compiler version, analogous to #[cfg(...)] and #[cfg_attr(...)].

[dependencies]
select-rustc = "0.1"

Selectors

• #[rustc::stable] —
  True on any stable compiler.

• #[rustc::stable(1.34)] —
  True on exactly the specified stable compiler.

• #[rustc::beta] —
  True on any beta compiler.

• #[rustc::nightly] —
  True on any nightly compiler or dev build.

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

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

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

• #[rustc::before(version or date)] —
  Negative of #[rustc::since(...)].

• #[rustc::not(selector)] —
  Negative of any selector; for example #[rustc::not(nightly)].

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

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

• #[rustc::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:

#[rustc::since(1.33)]
use std::pin::Pin;

#[rustc::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.

#[rustc::attr(before(1.33), repr(packed))]
#[rustc::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 rustc::attr macro.

use std::time::Duration;

#[rustc::attr(since(1.32), const)]
fn duration_as_days(dur: Duration) -> u64 {
    dur.as_secs() / 60 / 60 / 24
}

License