Crate rustflags

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This is one of the environment variables provided by Cargo to build scripts. It synthesizes several sources of flags affecting Cargo’s rustc invocations that build scripts might care about:

  • Flags passed via the RUSTFLAGS environment variable,
  • Cargo config entries under target.<triple>.rustflags and target.<cfg>.rustflags and build.rustflags, including from the project-specific Cargo config file and the Cargo config in the user’s CARGO_HOME.

If a build script needs to make some rustc invocations, or needs to characterize aspects of the upcoming rustc invocation, it likely needs these flags.


This build script wants to know whether it is okay to enable #![feature(proc_macro_span)]. If the user is building with -Zallow-features with a feature list that does not include proc_macro_span, we need to not enable the feature or the build will fail.


use rustflags::Flag;

fn main() {
    if is_nightly() && feature_allowed("proc_macro_span") {

// Look for `-Z allow-features=feature1,feature2`
fn feature_allowed(feature: &str) -> bool {
    for flag in rustflags::from_env() {
        if let Flag::Z(option) = flag {
            if option.starts_with("allow-features=") {
                return option["allow-features=".len()..]
                    .any(|allowed| allowed == feature);

    // No allow-features= flag, allowed by default.

This build scripts wants to try compiling a source file to figure out whether an unstable API is supported in the expected form by the current toolchain.


use rustflags::Flag;
use std::env;
use std::fs;
use std::path::Path;
use std::process::{Command, ExitStatus, Stdio};

// This code exercises the surface area that we expect of the unstable
// feature. If the current toolchain is able to compile it, we go ahead and
// enable the feature.
const PROBE: &str = r#"

    use std::backtrace::{Backtrace, BacktraceStatus};

    fn probe() {
        let backtrace = Backtrace::capture();
        match backtrace.status() {
            BacktraceStatus::Captured | BacktraceStatus::Disabled | _ => {}

fn main() {
    match compile_probe() {
        Some(status) if status.success() => println!("cargo:rustc-cfg=backtrace"),
        _ => {}

fn compile_probe() -> Option<ExitStatus> {
    let rustc = env::var_os("RUSTC")?;
    let out_dir = env::var_os("OUT_DIR")?;
    let probefile = Path::new(&out_dir).join("");
    fs::write(&probefile, PROBE).ok()?;

    // Make sure to pick up Cargo rustc configuration.
    let mut cmd = if let Some(wrapper) = env::var_os("CARGO_RUSTC_WRAPPER") {
        let mut cmd = Command::new(wrapper);
        // The wrapper's first argument is supposed to be the path to rustc.
    } else {

            .filter(|flag| matches!(flag, Flag::Cfg{..} | Flag::Z(_)))




  • Parse flags from CARGO_ENCODED_RUSTFLAGS environment variable.