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    //! This module implements run-time feature detection.
    //!
    //! The `is_{arch}_feature_detected!("feature-name")` macros take the name of a
    //! feature as a string-literal, and return a boolean indicating whether the
    //! feature is enabled at run-time or not.
    //!
    //! These macros do two things:
    //! * map the string-literal into an integer stored as a `Feature` enum,
    //! * call a `os::check_for(x: Feature)` function that returns `true` if the
    //! feature is enabled.
    //!
    //! The `Feature` enums are also implemented in the `arch/{target_arch}.rs`
    //! modules.
    //!
    //! The `check_for` functions are, in general, Operating System dependent. Most
    //! architectures do not allow user-space programs to query the feature bits
    //! due to security concerns (x86 is the big exception). These functions are
    //! implemented in the `os/{target_os}.rs` modules.

    use cfg_if::cfg_if;

    #[macro_use]
    mod error_macros;

    #[macro_use]
    mod macros;

    cfg_if! {
        if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
            #[path = "arch/x86.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "arm")] {
            #[path = "arch/arm.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "aarch64")] {
            #[path = "arch/aarch64.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "powerpc")] {
            #[path = "arch/powerpc.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "powerpc64")] {
            #[path = "arch/powerpc64.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "mips")] {
            #[path = "arch/mips.rs"]
            #[macro_use]
            mod arch;
        } else if #[cfg(target_arch = "mips64")] {
            #[path = "arch/mips64.rs"]
            #[macro_use]
            mod arch;
        } else {
            // Unimplemented architecture:
            #[allow(dead_code)]
            mod arch {
                #[doc(hidden)]
                pub(crate) enum Feature {
                    Null
                }
                #[doc(hidden)]
                pub mod __is_feature_detected {}

                impl Feature {
                    #[doc(hidden)]
                    pub(crate) fn from_str(_s: &str) -> Result<Feature, ()> { Err(()) }
                    #[doc(hidden)]
                    pub(crate) fn to_str(self) -> &'static str { "" }
                }
            }
        }
    }

    // This module needs to be public because the `is_{arch}_feature_detected!`
    // macros expand calls to items within it in user crates.
    #[doc(hidden)]
    pub use self::arch::__is_feature_detected;

    pub(crate) use self::arch::Feature;

    mod bit;
    mod cache;

    cfg_if! {
        if #[cfg(miri)] {
            // When running under miri all target-features that are not enabled at
            // compile-time are reported as disabled at run-time.
            //
            // For features for which `cfg(target_feature)` returns true,
            // this run-time detection logic is never called.
            #[path = "os/other.rs"]
            mod os;
        } else if #[cfg(any(target_arch = "x86", target_arch = "x86_64"))] {
            // On x86/x86_64 no OS specific functionality is required.
            #[path = "os/x86.rs"]
            mod os;
        } else if #[cfg(all(target_os = "linux", feature = "use_std"))] {
            #[path = "os/linux/mod.rs"]
            mod os;
        } else if #[cfg(all(target_os = "freebsd", feature = "use_std"))] {
            #[cfg(target_arch = "aarch64")]
            #[path = "os/aarch64.rs"]
            mod aarch64;
            #[path = "os/freebsd/mod.rs"]
            mod os;
        } else if #[cfg(all(target_os = "windows", target_arch = "aarch64"))] {
            #[path = "os/windows/aarch64.rs"]
            mod os;
        } else {
            #[path = "os/other.rs"]
            mod os;
        }
    }

    /// Performs run-time feature detection.
    #[inline]
    #[allow(dead_code)]
    fn check_for(x: Feature) -> bool {
        cache::test(x as u32, self::os::detect_features)
    }

    /// Returns an `Iterator<Item=(&'static str, bool)>` where
    /// `Item.0` is the feature name, and `Item.1` is a `bool` which
    /// is `true` if the feature is supported by the host and `false` otherwise.
    #[unstable(feature = "stdsimd", issue = "27731")]
    pub fn features() -> impl Iterator<Item = (&'static str, bool)> {
        cfg_if! {
            if #[cfg(any(
                target_arch = "x86",
                target_arch = "x86_64",
                target_arch = "arm",
                target_arch = "aarch64",
                target_arch = "powerpc",
                target_arch = "powerpc64",
                target_arch = "mips",
                target_arch = "mips64",
            ))] {
                (0_u8..Feature::_last as u8).map(|discriminant: u8| {
                    let f: Feature = unsafe { crate::mem::transmute(discriminant) };
                    let name: &'static str = f.to_str();
                    let enabled: bool = check_for(f);
                    (name, enabled)
                })
            } else {
                None.into_iter()
            }
        }
    }