[−][src]Crate rustc

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

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 u128 stabilized in Rust 1.26:

#[rustc::since(1.26)]
impl MyTrait for u128 {
    /* ... */
}

Similar but for language features; the #[must_use] attribute stabilized in Rust 1.27:

#[rustc::attr(since(1.27), must_use)]
fn f() -> MyReturn {
    /* ... */
}

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
}

Attribute Macros

all
any
attr
before
beta
nightly
not
since
stable