coverage-helper 0.2.0

Helper for <https://github.com/taiki-e/cargo-llvm-cov/issues/123>.
Documentation 
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use std::{
    env,
    io::Write,
    process::{Command, Stdio},
};

fn main() {
    if probe_feature("coverage_attribute").unwrap_or(false) {
        println!("cargo:rustc-cfg=coverage_helper_has_coverage_attribute")
    }
}

// Refs:
// - https://github.com/cuviper/autocfg/blob/e75324cfa5fa0bec4f06f9609ab169bb7747f984/src/lib.rs#L205
// - https://github.com/dtolnay/anyhow/blob/7ecec7a3799b94c3b58a6b4cd050ff60cba99129/build.rs#L70
fn probe_feature(feature_name: &str) -> Option<bool> {
    // https://github.com/dtolnay/thiserror/pull/248
    if env::var_os("RUSTC_STAGE").is_some() {
        return None;
    }

    let rustc = env::var_os("RUSTC")?;
    let out_dir = env::var_os("OUT_DIR")?;

    let mut cmd = if let Some(wrapper) = env::var_os("RUSTC_WRAPPER") {
        let mut cmd = Command::new(wrapper);
        cmd.arg(rustc);
        cmd
    } else {
        Command::new(rustc)
    };
    cmd.stderr(Stdio::null())
        .arg("--edition=2018")
        .arg("--crate-name")
        .arg(format!("coverage_helper_build_{}", feature_name))
        .arg("--crate-type=lib")
        .arg("--emit=metadata")
        .arg("--out-dir")
        .arg(out_dir);

    cmd.arg("-").stdin(Stdio::piped());
    let mut child = cmd.spawn().ok()?;
    let mut stdin = child.stdin.take().expect("rustc stdin");

    // There is no need to respect TARGET since #[coverage] is platform-independent,
    // just check for host. And host always has std.
    stdin
        .write_all(
            br"#![allow(stable_features)]
#![feature(",
        )
        .ok()?;
    stdin.write_all(feature_name.as_bytes()).ok()?;
    stdin.write_all(br")]").ok()?;

    drop(stdin);

    let status = child.wait().ok()?;
    Some(status.success())
}