third_party_rust_syn/build.rs

use std::env;
use std::ffi::OsString;
use std::process::{self, Command, Stdio};

// The rustc-cfg strings below are *not* public API. Please let us know by
// opening a GitHub issue if your build environment requires some way to enable
// these cfgs other than by executing our build script.
fn main() {
    println!("cargo:rerun-if-changed=build.rs");

    // Note: add "/build.rs" to package.include in Cargo.toml if adding any
    // conditional compilation within the library.

    if !unstable() {
        println!("cargo:rustc-cfg=syn_disable_nightly_tests");
    }
}

fn unstable() -> bool {
    let rustc = cargo_env_var("RUSTC");

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

    cmd.stdin(Stdio::null());
    cmd.stdout(Stdio::null());
    cmd.stderr(Stdio::null());
    cmd.arg("-");

    // Find out whether this is a nightly or dev build.
    cmd.env_remove("RUSTC_BOOTSTRAP");
    cmd.arg("-Zcrate-attr=feature(rustc_private)");

    // Pass `-Zunpretty` to terminate earlier without writing out any "emit"
    // files. Use `expanded` to proceed far enough to actually apply crate
    // attrs. With `unpretty=normal` or `--print`, not enough compilation
    // happens to recognize that the feature attribute is unstable.
    cmd.arg("-Zunpretty=expanded");

    // Set #![no_std] to bypass loading libstd.rlib. This is a 7.5% speedup.
    cmd.arg("-Zcrate-attr=no_std");

    cmd.arg("--crate-type=lib");
    cmd.arg("--edition=2021");

    if let Some(target) = env::var_os("TARGET") {
        cmd.arg("--target").arg(target);
    }

    // If Cargo wants to set RUSTFLAGS, use that.
    if let Ok(rustflags) = env::var("CARGO_ENCODED_RUSTFLAGS") {
        if !rustflags.is_empty() {
            for arg in rustflags.split('\x1f') {
                cmd.arg(arg);
            }
        }
    }

    // This rustc invocation should take around 0.03 seconds.
    match cmd.status() {
        Ok(status) => status.success(),
        Err(_) => false,
    }
}

fn cargo_env_var(key: &str) -> OsString {
    env::var_os(key).unwrap_or_else(|| {
        eprintln!(
            "Environment variable ${} is not set during execution of build script",
            key,
        );
        process::exit(1);
    })
}