Conditional compilation for 'nightly' vs 'stable' Rust or compiler version

Question

I am using some Rust unstable features, but I still want to be able to compile a reduced version of my library with stable Rust. I am happy to only include those unstable features when the compiler supports them, and exclude them when they are not supported.

I thought it would be easy to achieve this goal using conditional compilation like #[cfg(rust_version = "nightly")], but it seems like 'stable' vs 'nightly' are not cfg options.

How do you guys perform conditional compilation based on 'stable' vs 'nightly', or based on the compiler version?

Answer 1

I recommend creating a feature for your nightly-only code that is disabled by default, for example

Cargo.toml

[features]
default = []
nightly-features = []

Since the nightly-features feature is not default, compilation with the stable toolchain works out of the box. You can use attributes #[cfg(feature = "nightly-features")] and #[cfg(not(feature = "nightly-features"))] to include or exclude code from nightly-specialized versions. This method has the added benefit of allowing testing of the nightly features independently of the compiler (i.e. answer the question: did the compiler break my code, or does code enabled by nightly-features contain bugs?).

Despite the risks, I want to enable nightly features automatically

Use build scripts, sometimes called build.rs in addition to the nightly feature described above. (note: the following should NEVER be used in a library, otherwise switching compilers could become a breaking change. prefer the solution explained above)

build.rs (goes in package root)

use std::env;

fn main() {
    let rust_toolchain = env::var("RUSTUP_TOOLCHAIN").unwrap();
    if rust_toolchain.starts_with("stable") {
        // do nothing
    } else if rust_toolchain.starts_with("nightly") {
        //enable the 'nightly-features' feature flag
        println!("cargo:rustc-cfg=feature=\"nightly-features\"");
    } else {
        panic!("Unexpected value for rustc toolchain")
    }
}

this build script checks the toolchain environment variable set by rustup (some rust installations do not use rustup) and enables the nightly feature flag if the compiler is nightly.

src/main.rs

fn main() {
    #[cfg(feature = "nightly-features")]
    println!("Hello, nightly!");
    #[cfg(not(feature = "nightly-features"))]
    println!("Hello, stable!");
}

now, running

➜  cargo +stable run      
Hello, stable!
➜  cargo +nightly run
Hello, nightly!

Is it possible to turn this feature off when build.rs turns it on?

As far as I can tell, no. Running cargo +nightly run --no-default-features leaves the feature on, due to how cargo passes flags to rustc. A programmer could create a specific environmental variable that build.rs checks for to skip the automatic version detection, but that is more complicated than the alternative with no build script - cargo build --features=nightly-features

Crate alternative

Instead of the proposed solution, you can use the rustversion crate, which works in a very similar way (but parses the output of rustc --version).

fn main() {
    #[rustversion(nightly)]
    println!("Hello, nightly!");
    #[rustversion::not(nightly)]
    println!("Hello, stable! (or beta)");
}