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    /// Common code for printing the backtrace in the same way across the different
    /// supported platforms.

    use crate::env;
    use crate::fmt;
    use crate::io;
    use crate::borrow::Cow;
    use crate::io::prelude::*;
    use crate::path::{self, Path, PathBuf};
    use crate::sys::mutex::Mutex;

    use backtrace_rs::{BacktraceFmt, BytesOrWideString, PrintFmt};

    /// Max number of frames to print.
    const MAX_NB_FRAMES: usize = 100;

    pub fn lock() -> impl Drop {
        struct Guard;
        static LOCK: Mutex = Mutex::new();

        impl Drop for Guard {
            fn drop(&mut self) {
                unsafe {
                    LOCK.unlock();
                }
            }
        }

        unsafe {
            LOCK.lock();
            return Guard;
        }
    }

    /// Prints the current backtrace.
    pub fn print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
        // There are issues currently linking libbacktrace into tests, and in
        // general during libstd's own unit tests we're not testing this path. In
        // test mode immediately return here to optimize away any references to the
        // libbacktrace symbols
        if cfg!(test) {
            return Ok(());
        }

        // Use a lock to prevent mixed output in multithreading context.
        // Some platforms also requires it, like `SymFromAddr` on Windows.
        unsafe {
            let _lock = lock();
            _print(w, format)
        }
    }

    unsafe fn _print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
        struct DisplayBacktrace {
            format: PrintFmt,
        }
        impl fmt::Display for DisplayBacktrace {
            fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
                unsafe {
                    _print_fmt(fmt, self.format)
                }
            }
        }
        write!(w, "{}", DisplayBacktrace { format })
    }

    unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::Result {
        let cwd = env::current_dir().ok();
        let mut print_path = move |fmt: &mut fmt::Formatter<'_>, bows: BytesOrWideString<'_>| {
            output_filename(fmt, bows, print_fmt, cwd.as_ref())
        };
        let mut bt_fmt = BacktraceFmt::new(fmt, print_fmt, &mut print_path);
        bt_fmt.add_context()?;
        let mut idx = 0;
        let mut res = Ok(());
        backtrace_rs::trace_unsynchronized(|frame| {
            if print_fmt == PrintFmt::Short && idx > MAX_NB_FRAMES {
                return false;
            }

            let mut hit = false;
            let mut stop = false;
            backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
                hit = true;
                if print_fmt == PrintFmt::Short {
                    if let Some(sym) = symbol.name().and_then(|s| s.as_str()) {
                        if sym.contains("__rust_begin_short_backtrace") {
                            stop = true;
                            return;
                        }
                    }
                }

                res = bt_fmt.frame().symbol(frame, symbol);
            });
            if stop {
                return false;
            }
            if !hit {
                res = bt_fmt.frame().print_raw(frame.ip(), None, None, None);
            }

            idx += 1;
            res.is_ok()
        });
        res?;
        bt_fmt.finish()?;
        if print_fmt == PrintFmt::Short {
            writeln!(
                fmt,
                "note: Some details are omitted, \
                 run with `RUST_BACKTRACE=full` for a verbose backtrace."
            )?;
        }
        Ok(())
    }

    /// Fixed frame used to clean the backtrace with `RUST_BACKTRACE=1`.
    #[inline(never)]
    pub fn __rust_begin_short_backtrace<F, T>(f: F) -> T
    where
        F: FnOnce() -> T,
        F: Send,
        T: Send,
    {
        f()
    }

    // For now logging is turned off by default, and this function checks to see
    // whether the magical environment variable is present to see if it's turned on.
    pub fn log_enabled() -> Option<PrintFmt> {
        use crate::sync::atomic::{self, Ordering};

        // Setting environment variables for Fuchsia components isn't a standard
        // or easily supported workflow. For now, always display backtraces.
        if cfg!(target_os = "fuchsia") {
            return Some(PrintFmt::Full);
        }

        static ENABLED: atomic::AtomicIsize = atomic::AtomicIsize::new(0);
        match ENABLED.load(Ordering::SeqCst) {
            0 => {}
            1 => return None,
            2 => return Some(PrintFmt::Short),
            _ => return Some(PrintFmt::Full),
        }

        let val = env::var_os("RUST_BACKTRACE").and_then(|x| {
            if &x == "0" {
                None
            } else if &x == "full" {
                Some(PrintFmt::Full)
            } else {
                Some(PrintFmt::Short)
            }
        });
        ENABLED.store(
            match val {
                Some(v) => v as isize,
                None => 1,
            },
            Ordering::SeqCst,
        );
        val
    }

    /// Prints the filename of the backtrace frame.
    ///
    /// See also `output`.
    pub fn output_filename(
        fmt: &mut fmt::Formatter<'_>,
        bows: BytesOrWideString<'_>,
        print_fmt: PrintFmt,
        cwd: Option<&PathBuf>,
    ) -> fmt::Result {
        let file: Cow<'_, Path> = match bows {
            #[cfg(unix)]
            BytesOrWideString::Bytes(bytes) => {
                use crate::os::unix::prelude::*;
                Path::new(crate::ffi::OsStr::from_bytes(bytes)).into()
            }
            #[cfg(not(unix))]
            BytesOrWideString::Bytes(bytes) => {
                Path::new(crate::str::from_utf8(bytes).unwrap_or("<unknown>")).into()
            }
            #[cfg(windows)]
            BytesOrWideString::Wide(wide) => {
                use crate::os::windows::prelude::*;
                Cow::Owned(crate::ffi::OsString::from_wide(wide).into())
            }
            #[cfg(not(windows))]
            BytesOrWideString::Wide(_wide) => {
                Path::new("<unknown>").into()
            }
        };
        if print_fmt == PrintFmt::Short && file.is_absolute() {
            if let Some(cwd) = cwd {
                if let Ok(stripped) = file.strip_prefix(&cwd) {
                    if let Some(s) = stripped.to_str() {
                        return write!(fmt, ".{}{}", path::MAIN_SEPARATOR, s);
                    }
                }
            }
        }
        fmt::Display::fmt(&file.display(), fmt)
    }

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