fn probe_fmt<'a>(&self, source: Arguments<'a>) -> Result<(), Error> {
        let crate_name = self.new_crate_name();
        let mut command = self.rustc.command();
        command
            .arg("--crate-name")
            .arg(&crate_name)
            .arg("--crate-type=lib")
            .arg("--out-dir")
            .arg(&self.out_dir)
            .arg("--emit=llvm-ir");

        if let Some(edition) = self.edition.as_ref() {
            command.arg("--edition").arg(edition);
        }

        if let Some(target) = self.target.as_ref() {
            command.arg("--target").arg(target);
        }

        command.args(&self.rustflags);

        command.arg("-").stdin(Stdio::piped());
        let mut child = try!(command.spawn().map_err(error::from_io));
        let mut stdin = child.stdin.take().expect("rustc stdin");

        try!(stdin.write_fmt(source).map_err(error::from_io));
        drop(stdin);

        match child.wait() {
            Ok(status) if status.success() => {
                // Try to remove the output file so it doesn't look like a build product for
                // systems like bazel -- but this is best-effort, so we can ignore failure.
                // The probe itself is already considered successful at this point.
                let mut file = self.out_dir.join(crate_name);
                file.set_extension("ll");
                let _ = fs::remove_file(file);

                Ok(())
            }
            Ok(status) => Err(error::from_exit(status)),
            Err(error) => Err(error::from_io(error)),
        }
    }

fn rustflags(target: &Option<OsString>, dir: &Path) -> Vec<String> {
    // Starting with rust-lang/cargo#9601, shipped in Rust 1.55, Cargo always sets
    // CARGO_ENCODED_RUSTFLAGS for any host/target build script invocation. This
    // includes any source of flags, whether from the environment, toml config, or
    // whatever may come in the future. The value is either an empty string, or a
    // list of arguments separated by the ASCII unit separator (US), 0x1f.
    if let Ok(a) = env::var("CARGO_ENCODED_RUSTFLAGS") {
        return if a.is_empty() {
            Vec::new()
        } else {
            a.split('\x1f').map(str::to_string).collect()
        };
    }

    // Otherwise, we have to take a more heuristic approach, and we don't
    // support values from toml config at all.
    //
    // Cargo only applies RUSTFLAGS for building TARGET artifact in
    // cross-compilation environment. Sadly, we don't have a way to detect
    // when we're building HOST artifact in a cross-compilation environment,
    // so for now we only apply RUSTFLAGS when cross-compiling an artifact.
    //
    // See https://github.com/cuviper/autocfg/pull/10#issuecomment-527575030.
    if *target != env::var_os("HOST")
        || dir_contains_target(target, dir, env::var_os("CARGO_TARGET_DIR"))
    {
        if let Ok(rustflags) = env::var("RUSTFLAGS") {
            // This is meant to match how cargo handles the RUSTFLAGS environment variable.
            // See https://github.com/rust-lang/cargo/blob/69aea5b6f69add7c51cca939a79644080c0b0ba0/src/cargo/core/compiler/build_context/target_info.rs#L434-L441
            return rustflags
                .split(' ')
                .map(str::trim)
                .filter(|s| !s.is_empty())
                .map(str::to_string)
                .collect();
        }
    }

    Vec::new()
}

        let meta = try!(fs::metadata(&dir).map_err(error::from_io));
        if !meta.is_dir() || meta.permissions().readonly() {
            return Err(error::from_str("output path is not a writable directory"));
        }

    /// Returns a new (hopefully unique) crate name for probes.
    fn new_crate_name(&self) -> String {
        #[allow(deprecated)]
        static ID: AtomicUsize = ATOMIC_USIZE_INIT;

        let id = ID.fetch_add(1, Ordering::Relaxed);
        format!("autocfg_{:016x}_{}", self.uuid, id)
    }

//! ## Caution
//!
//! Many of the probing methods of `AutoCfg` document the particular template they
//! use, **subject to change**. The inputs are not validated to make sure they are
//! semantically correct for their expected use, so it's _possible_ to escape and
//! inject something unintended. However, such abuse is unsupported and will not
//! be considered when making changes to the templates.

    pub fn new() -> Self {
        Rustc {
            rustc: env::var_os("RUSTC")
                .unwrap_or_else(|| "rustc".into())
                .into(),
            rustc_wrapper: get_rustc_wrapper(false),
            rustc_workspace_wrapper: get_rustc_wrapper(true),
        }
    }