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    #![cfg_attr(not(feature = "std"), no_std)]
    #![doc = include_str!("../README.md")]

    mod mutex;
    #[cfg(feature = "std")]
    mod std;

    use core::marker::PhantomData;

    pub use self::mutex::Mutex;

    /// Critical section token.
    ///
    /// An instance of this type indicates that the current thread is executing code within a critical
    /// section.
    #[derive(Clone, Copy, Debug)]
    pub struct CriticalSection<'cs> {
        _private: PhantomData<&'cs ()>,
    }

    impl<'cs> CriticalSection<'cs> {
        /// Creates a critical section token.
        ///
        /// This method is meant to be used to create safe abstractions rather than being directly used
        /// in applications.
        ///
        /// # Safety
        ///
        /// This must only be called when the current thread is in a critical section. The caller must
        /// ensure that the returned instance will not live beyond the end of the critical section.
        ///
        /// The caller must use adequate fences to prevent the compiler from moving the
        /// instructions inside the critical section to the outside of it. Sequentially consistent fences are
        /// suggested immediately after entry and immediately before exit from the critical section.
        ///
        /// Note that the lifetime `'cs` of the returned instance is unconstrained. User code must not
        /// be able to influence the lifetime picked for this type, since that might cause it to be
        /// inferred to `'static`.
        #[inline(always)]
        pub unsafe fn new() -> Self {
            CriticalSection {
                _private: PhantomData,
            }
        }
    }

    #[cfg(any(
        all(feature = "restore-state-none", feature = "restore-state-bool"),
        all(feature = "restore-state-none", feature = "restore-state-u8"),
        all(feature = "restore-state-none", feature = "restore-state-u16"),
        all(feature = "restore-state-none", feature = "restore-state-u32"),
        all(feature = "restore-state-none", feature = "restore-state-u64"),
        all(feature = "restore-state-bool", feature = "restore-state-u8"),
        all(feature = "restore-state-bool", feature = "restore-state-u16"),
        all(feature = "restore-state-bool", feature = "restore-state-u32"),
        all(feature = "restore-state-bool", feature = "restore-state-u64"),
        all(feature = "restore-state-u8", feature = "restore-state-u16"),
        all(feature = "restore-state-u8", feature = "restore-state-u32"),
        all(feature = "restore-state-u8", feature = "restore-state-u64"),
        all(feature = "restore-state-u16", feature = "restore-state-u32"),
        all(feature = "restore-state-u16", feature = "restore-state-u64"),
        all(feature = "restore-state-u32", feature = "restore-state-u64"),
    ))]
    compile_error!("You must set at most one of these Cargo features: restore-state-none, restore-state-bool, restore-state-u8, restore-state-u16, restore-state-u32, restore-state-u64");

    #[cfg(not(any(
        feature = "restore-state-bool",
        feature = "restore-state-u8",
        feature = "restore-state-u16",
        feature = "restore-state-u32",
        feature = "restore-state-u64"
    )))]
    type RawRestoreStateInner = ();

    #[cfg(feature = "restore-state-bool")]
    type RawRestoreStateInner = bool;

    #[cfg(feature = "restore-state-u8")]
    type RawRestoreStateInner = u8;

    #[cfg(feature = "restore-state-u16")]
    type RawRestoreStateInner = u16;

    #[cfg(feature = "restore-state-u32")]
    type RawRestoreStateInner = u32;

    #[cfg(feature = "restore-state-u64")]
    type RawRestoreStateInner = u64;

    // We have RawRestoreStateInner and RawRestoreState so that we don't have to copypaste the docs 5 times.
    // In the docs this shows as `pub type RawRestoreState = u8` or whatever the selected type is, because
    // the "inner" type alias is private.

    /// Raw, transparent "restore state".
    ///
    /// This type changes based on which Cargo feature is selected, out of
    /// - `restore-state-none` (default, makes the type be `()`)
    /// - `restore-state-bool`
    /// - `restore-state-u8`
    /// - `restore-state-u16`
    /// - `restore-state-u32`
    /// - `restore-state-u64`
    ///
    /// See [`RestoreState`].
    ///
    /// User code uses [`RestoreState`] opaquely, critical section implementations
    /// use [`RawRestoreState`] so that they can use the inner value.
    pub type RawRestoreState = RawRestoreStateInner;

    /// Opaque "restore state".
    ///
    /// Implementations use this to "carry over" information between acquiring and releasing
    /// a critical section. For example, when nesting two critical sections of an
    /// implementation that disables interrupts globally, acquiring the inner one won't disable
    /// the interrupts since they're already disabled. The impl would use the restore state to "tell"
    /// the corresponding release that it does *not* have to reenable interrupts yet, only the
    /// outer release should do so.
    ///
    /// User code uses [`RestoreState`] opaquely, critical section implementations
    /// use [`RawRestoreState`] so that they can use the inner value.
    #[derive(Clone, Copy, Debug)]
    pub struct RestoreState(RawRestoreState);

    impl RestoreState {
        /// Create an invalid, dummy  `RestoreState`.
        ///
        /// This can be useful to avoid `Option` when storing a `RestoreState` in a
        /// struct field, or a `static`.
        ///
        /// Note that due to the safety contract of [`acquire`]/[`release`], you must not pass
        /// a `RestoreState` obtained from this method to [`release`].
        pub const fn invalid() -> Self {
            #[cfg(not(any(
                feature = "restore-state-bool",
                feature = "restore-state-u8",
                feature = "restore-state-u16",
                feature = "restore-state-u32",
                feature = "restore-state-u64"
            )))]
            return Self(());

            #[cfg(feature = "restore-state-bool")]
            return Self(false);

            #[cfg(feature = "restore-state-u8")]
            return Self(0);

            #[cfg(feature = "restore-state-u16")]
            return Self(0);

            #[cfg(feature = "restore-state-u32")]
            return Self(0);

            #[cfg(feature = "restore-state-u64")]
            return Self(0);
        }
    }

    /// Acquire a critical section in the current thread.
    ///
    /// This function is extremely low level. Strongly prefer using [`with`] instead.
    ///
    /// Nesting critical sections is allowed. The inner critical sections
    /// are mostly no-ops since they're already protected by the outer one.
    ///
    /// # Safety
    ///
    /// - Each `acquire` call must be paired with exactly one `release` call in the same thread.
    /// - `acquire` returns a "restore state" that you must pass to the corresponding `release` call.
    /// - `acquire`/`release` pairs must be "properly nested", ie it's not OK to do `a=acquire(); b=acquire(); release(a); release(b);`.
    /// - It is UB to call `release` if the critical section is not acquired in the current thread.
    /// - It is UB to call `release` with a "restore state" that does not come from the corresponding `acquire` call.
    /// - It must provide ordering guarantees at least equivalent to a [`core::sync::atomic::Ordering::Acquire`]
    ///   on a memory location shared by all critical sections, on which the `release` call will do a
    ///   [`core::sync::atomic::Ordering::Release`] operation.
    #[inline(always)]
    pub unsafe fn acquire() -> RestoreState {
        extern "Rust" {
            fn _critical_section_1_0_acquire() -> RawRestoreState;
        }

        #[allow(clippy::unit_arg)]
        RestoreState(_critical_section_1_0_acquire())
    }

    /// Release the critical section.
    ///
    /// This function is extremely low level. Strongly prefer using [`with`] instead.
    ///
    /// # Safety
    ///
    /// See [`acquire`] for the safety contract description.
    #[inline(always)]
    pub unsafe fn release(restore_state: RestoreState) {
        extern "Rust" {
            fn _critical_section_1_0_release(restore_state: RawRestoreState);
        }

        #[allow(clippy::unit_arg)]
        _critical_section_1_0_release(restore_state.0)
    }

    /// Execute closure `f` in a critical section.
    ///
    /// Nesting critical sections is allowed. The inner critical sections
    /// are mostly no-ops since they're already protected by the outer one.
    ///
    /// # Panics
    ///
    /// This function panics if the given closure `f` panics. In this case
    /// the critical section is released before unwinding.
    #[inline]
    pub fn with<R>(f: impl FnOnce(CriticalSection) -> R) -> R {
        // Helper for making sure `release` is called even if `f` panics.
        struct Guard {
            state: RestoreState,
        }

        impl Drop for Guard {
            #[inline(always)]
            fn drop(&mut self) {
                unsafe { release(self.state) }
            }
        }

        let state = unsafe { acquire() };
        let _guard = Guard { state };

        unsafe { f(CriticalSection::new()) }
    }

    /// Methods required for a critical section implementation.
    ///
    /// This trait is not intended to be used except when implementing a critical section.
    ///
    /// # Safety
    ///
    /// Implementations must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
    pub unsafe trait Impl {
        /// Acquire the critical section.
        ///
        /// # Safety
        ///
        /// Callers must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
        unsafe fn acquire() -> RawRestoreState;

        /// Release the critical section.
        ///
        /// # Safety
        ///
        /// Callers must uphold the contract specified in [`crate::acquire`] and [`crate::release`].
        unsafe fn release(restore_state: RawRestoreState);
    }

    /// Set the critical section implementation.
    ///
    /// # Example
    ///
    /// ```
    /// # #[cfg(not(feature = "std"))] // needed for `cargo test --features std`
    /// # mod no_std {
    /// use critical_section::RawRestoreState;
    ///
    /// struct MyCriticalSection;
    /// critical_section::set_impl!(MyCriticalSection);
    ///
    /// unsafe impl critical_section::Impl for MyCriticalSection {
    ///     unsafe fn acquire() -> RawRestoreState {
    ///         // ...
    ///     }
    ///
    ///     unsafe fn release(restore_state: RawRestoreState) {
    ///         // ...
    ///     }
    /// }
    /// # }
    #[macro_export]
    macro_rules! set_impl {
        ($t: ty) => {
            #[no_mangle]
            unsafe fn _critical_section_1_0_acquire() -> $crate::RawRestoreState {
                <$t as $crate::Impl>::acquire()
            }
            #[no_mangle]
            unsafe fn _critical_section_1_0_release(restore_state: $crate::RawRestoreState) {
                <$t as $crate::Impl>::release(restore_state)
            }
        };
    }