use std::env;
use std::process::Command;
use std::str::{self, FromStr};

fn main() {
    let compiler = match rustc_version() {
        Some(compiler) => compiler,
        None => return,
    };

    if compiler.minor >= 40 && compiler.nightly {
        println!("cargo:rustc-cfg=backtrace");
    }
}

struct Compiler {
    minor: u32,
    nightly: bool,
}

fn rustc_version() -> Option<Compiler> {
    let rustc = env::var_os("RUSTC")?;
    let output = Command::new(rustc).arg("--version").output().ok()?;
    let version = str::from_utf8(&output.stdout).ok()?;

    let mut pieces = version.split('.');
    if pieces.next() != Some("rustc 1") {
        return None;
    }

    let next = pieces.next()?;
    let minor = u32::from_str(next).ok()?;
    let nightly = version.contains("nightly") || version.contains("dev");
    Some(Compiler { minor, nightly })
}

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

// This code exercises the surface area that we expect of the std Backtrace
// type. If the current toolchain is able to compile it, we go ahead and use
// backtrace in anyhow.
const PROBE: &str = r#"
    #![feature(backtrace)]
    #![allow(dead_code)]

    use std::backtrace::{Backtrace, BacktraceStatus};
    use std::error::Error;
    use std::fmt::{self, Display};

    #[derive(Debug)]
    struct E;

    impl Display for E {
        fn fmt(&self, _formatter: &mut fmt::Formatter) -> fmt::Result {
            unimplemented!()
        }
    }

    impl Error for E {
        fn backtrace(&self) -> Option<&Backtrace> {
            let backtrace = Backtrace::capture();
            match backtrace.status() {
                BacktraceStatus::Captured | BacktraceStatus::Disabled | _ => {}
            }
            unimplemented!()
        }
    }
"#;

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("lib.rs");
    fs::write(&probefile, PROBE).ok()?;
    Command::new(rustc)
        .arg("--edition=2018")
        .arg("--crate-name=anyhow_build")
        .arg("--crate-type=lib")
        .arg("--emit=metadata")
        .arg("--out-dir")
        .arg(out_dir)
        .arg(probefile)
        .status()
        .ok()
}

Replace version check with probing Backtrace api
Replace version check with probing Backtrace api
