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    //! Platform-independent platform abstraction
    //!
    //! This is the platform-independent portion of the standard library's
    //! platform abstraction layer, whereas `std::sys` is the
    //! platform-specific portion.
    //!
    //! The relationship between `std::sys_common`, `std::sys` and the
    //! rest of `std` is complex, with dependencies going in all
    //! directions: `std` depending on `sys_common`, `sys_common`
    //! depending on `sys`, and `sys` depending on `sys_common` and `std`.
    //! Ideally `sys_common` would be split into two and the dependencies
    //! between them all would form a dag, facilitating the extraction of
    //! `std::sys` from the standard library.

    #![allow(missing_docs)]
    #![allow(missing_debug_implementations)]

    #[cfg(test)]
    mod tests;

    use crate::sync::Once;
    use crate::sys;

    macro_rules! rtabort {
        ($($t:tt)*) => (crate::sys_common::util::abort(format_args!($($t)*)))
    }

    macro_rules! rtassert {
        ($e:expr) => {
            if !$e {
                rtabort!(concat!("assertion failed: ", stringify!($e)));
            }
        };
    }

    #[allow(unused_macros)] // not used on all platforms
    macro_rules! rtunwrap {
        ($ok:ident, $e:expr) => {
            match $e {
                $ok(v) => v,
                ref err => {
                    let err = err.as_ref().map(drop); // map Ok/Some which might not be Debug
                    rtabort!(concat!("unwrap failed: ", stringify!($e), " = {:?}"), err)
                }
            }
        };
    }

    pub mod alloc;
    pub mod at_exit_imp;
    pub mod backtrace;
    pub mod bytestring;
    pub mod condvar;
    pub mod fs;
    pub mod io;
    pub mod mutex;
    // `doc` is required because `sys/mod.rs` imports `unix/ext/mod.rs` on Windows
    // when generating documentation.
    #[cfg(any(doc, not(windows)))]
    pub mod os_str_bytes;
    pub mod poison;
    pub mod process;
    pub mod remutex;
    pub mod rwlock;
    pub mod thread;
    pub mod thread_info;
    pub mod thread_local_dtor;
    pub mod thread_local_key;
    pub mod thread_parker;
    pub mod util;
    pub mod wtf8;

    cfg_if::cfg_if! {
        if #[cfg(any(target_os = "l4re",
                     target_os = "hermit",
                     feature = "restricted-std",
                     all(target_arch = "wasm32", not(target_os = "emscripten")),
                     all(target_vendor = "fortanix", target_env = "sgx")))] {
            pub use crate::sys::net;
        } else {
            pub mod net;
        }
    }

    // common error constructors

    /// A trait for viewing representations from std types
    #[doc(hidden)]
    pub trait AsInner<Inner: ?Sized> {
        fn as_inner(&self) -> &Inner;
    }

    /// A trait for viewing representations from std types
    #[doc(hidden)]
    pub trait AsInnerMut<Inner: ?Sized> {
        fn as_inner_mut(&mut self) -> &mut Inner;
    }

    /// A trait for extracting representations from std types
    #[doc(hidden)]
    pub trait IntoInner<Inner> {
        fn into_inner(self) -> Inner;
    }

    /// A trait for creating std types from internal representations
    #[doc(hidden)]
    pub trait FromInner<Inner> {
        fn from_inner(inner: Inner) -> Self;
    }

    /// Enqueues a procedure to run when the main thread exits.
    ///
    /// Currently these closures are only run once the main *Rust* thread exits.
    /// Once the `at_exit` handlers begin running, more may be enqueued, but not
    /// infinitely so. Eventually a handler registration will be forced to fail.
    ///
    /// Returns `Ok` if the handler was successfully registered, meaning that the
    /// closure will be run once the main thread exits. Returns `Err` to indicate
    /// that the closure could not be registered, meaning that it is not scheduled
    /// to be run.
    pub fn at_exit<F: FnOnce() + Send + 'static>(f: F) -> Result<(), ()> {
        if at_exit_imp::push(Box::new(f)) { Ok(()) } else { Err(()) }
    }

    /// One-time runtime cleanup.
    pub fn cleanup() {
        static CLEANUP: Once = Once::new();
        CLEANUP.call_once(|| unsafe {
            sys::args::cleanup();
            sys::stack_overflow::cleanup();
            at_exit_imp::cleanup();
        });
    }

    // Computes (value*numer)/denom without overflow, as long as both
    // (numer*denom) and the overall result fit into i64 (which is the case
    // for our time conversions).
    #[allow(dead_code)] // not used on all platforms
    pub fn mul_div_u64(value: u64, numer: u64, denom: u64) -> u64 {
        let q = value / denom;
        let r = value % denom;
        // Decompose value as (value/denom*denom + value%denom),
        // substitute into (value*numer)/denom and simplify.
        // r < denom, so (denom*numer) is the upper bound of (r*numer)
        q * numer + r * numer / denom
    }