Compiler version cfg
This crate provides macros for conditional compilation according to rustc
compiler version, analogous to #[cfg(...)]
and
#[cfg_attr(...)]
.
[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 tocfg_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:
#[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.
#[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.
use std::time::Duration;
#[rustversion::attr(since(1.32), const)]
fn duration_as_days(dur: Duration) -> u64 {
dur.as_secs() / 60 / 60 / 24
}