#rustc #select #cfg #compiler #macros #conditional #compilation #version

macro select-rustc

Conditional compilation according to rustc compiler version

3 releases

0.1.2 May 7, 2019
0.1.1 May 3, 2019
0.1.0 Apr 29, 2019
Download history 265/week @ 2022-04-24 77/week @ 2022-05-01 85/week @ 2022-05-08 110/week @ 2022-05-15 76/week @ 2022-05-22 71/week @ 2022-05-29 81/week @ 2022-06-05 86/week @ 2022-06-12 69/week @ 2022-06-19 66/week @ 2022-06-26 238/week @ 2022-07-03 235/week @ 2022-07-10 181/week @ 2022-07-17 355/week @ 2022-07-24 628/week @ 2022-07-31 230/week @ 2022-08-07

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Used in 8 crates (2 directly)


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Compiler version cfg

Build Status Latest Version Rust Documentation

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

select-rustc = "0.1"


  • #[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:

use std::pin::Pin;

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


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.


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