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    //! Encapsulation for system call arguments and return values.
    //!
    //! The inline-asm and outline-asm code paths do some amount of reordering
    //! of arguments; to ensure that we don't accidentally misroute an argument
    //! or return value, we use distinct types for each argument index and
    //! return value.
    //!
    //! # Safety
    //!
    //! The `ToAsm` and `FromAsm` traits are unsafe to use; they should only be
    //! used by the syscall code which executes actual syscall machine
    //! instructions.

    #![allow(unsafe_code)]

    use super::c;
    use super::fd::RawFd;
    use core::marker::PhantomData;

    pub(super) trait ToAsm: private::Sealed {
        /// Convert `self` to a `usize` ready to be passed to a syscall
        /// machine instruction.
        ///
        /// # Safety
        ///
        /// This should be used immediately before the syscall instruction, and
        /// the returned value shouldn't be used for any other purpose.
        #[must_use]
        unsafe fn to_asm(self) -> *mut Opaque;
    }

    pub(super) trait FromAsm: private::Sealed {
        /// Convert `raw` from a value produced by a syscall machine instruction
        /// into a `Self`.
        ///
        /// # Safety
        ///
        /// This should be used immediately after the syscall instruction, and
        /// the operand value shouldn't be used for any other purpose.
        #[must_use]
        unsafe fn from_asm(raw: *mut Opaque) -> Self;
    }

    /// To preserve provenance, syscall arguments and return values are passed as
    /// pointer types. They need a type to point to, so we define a custom private
    /// type, to prevent it from being used for anything else.
    #[repr(transparent)]
    pub(super) struct Opaque(c::c_void);

    // Argument numbers.
    pub(super) struct A0(());
    pub(super) struct A1(());
    pub(super) struct A2(());
    pub(super) struct A3(());
    pub(super) struct A4(());
    pub(super) struct A5(());
    #[cfg(target_arch = "mips")]
    pub(super) struct A6(());
    #[cfg(target_arch = "x86")]
    pub(super) struct SocketArg;

    pub(super) trait ArgNumber: private::Sealed {}
    impl ArgNumber for A0 {}
    impl ArgNumber for A1 {}
    impl ArgNumber for A2 {}
    impl ArgNumber for A3 {}
    impl ArgNumber for A4 {}
    impl ArgNumber for A5 {}
    #[cfg(target_arch = "mips")]
    impl ArgNumber for A6 {}
    #[cfg(target_arch = "x86")]
    impl ArgNumber for SocketArg {}

    // Return value numbers.
    pub(super) struct R0(());

    pub(super) trait RetNumber: private::Sealed {}
    impl RetNumber for R0 {}

    /// Syscall arguments use register-sized types. We use a newtype to
    /// discourage accidental misuse of the raw integer values.
    ///
    /// This type doesn't implement `Clone` or `Copy`; it should be used exactly
    /// once. And it has a lifetime to ensure that it doesn't outlive any resources
    /// it might be pointing to.
    #[repr(transparent)]
    #[must_use]
    pub(super) struct ArgReg<'a, Num: ArgNumber> {
        raw: *mut Opaque,
        _phantom: PhantomData<(&'a (), Num)>,
    }

    impl<'a, Num: ArgNumber> ToAsm for ArgReg<'a, Num> {
        #[inline]
        unsafe fn to_asm(self) -> *mut Opaque {
            self.raw
        }
    }

    /// Syscall return values use register-sized types. We use a newtype to
    /// discourage accidental misuse of the raw integer values.
    ///
    /// This type doesn't implement `Clone` or `Copy`; it should be used exactly
    /// once.
    #[repr(transparent)]
    #[must_use]
    pub(super) struct RetReg<Num: RetNumber> {
        raw: *mut Opaque,
        _phantom: PhantomData<Num>,
    }

    impl<Num: RetNumber> RetReg<Num> {
        #[inline]
        pub(super) fn decode_usize(self) -> usize {
            debug_assert!(!(-4095..0).contains(&(self.raw as isize)));
            self.raw as usize
        }

        #[inline]
        pub(super) fn decode_raw_fd(self) -> RawFd {
            let bits = self.decode_usize();
            let raw_fd = bits as RawFd;

            // Converting `raw` to `RawFd` should be lossless.
            debug_assert_eq!(raw_fd as usize, bits);

            raw_fd
        }

        #[inline]
        pub(super) fn decode_c_int(self) -> c::c_int {
            let bits = self.decode_usize();
            let c_int_ = bits as c::c_int;

            // Converting `raw` to `c_int` should be lossless.
            debug_assert_eq!(c_int_ as usize, bits);

            c_int_
        }

        #[inline]
        pub(super) fn decode_c_uint(self) -> c::c_uint {
            let bits = self.decode_usize();
            let c_uint_ = bits as c::c_uint;

            // Converting `raw` to `c_uint` should be lossless.
            debug_assert_eq!(c_uint_ as usize, bits);

            c_uint_
        }

        #[inline]
        pub(super) fn decode_void_star(self) -> *mut c::c_void {
            self.raw.cast()
        }

        #[cfg(target_pointer_width = "64")]
        #[inline]
        pub(super) fn decode_u64(self) -> u64 {
            self.decode_usize() as u64
        }

        #[inline]
        pub(super) fn decode_void(self) {
            let ignore = self.decode_usize();
            debug_assert_eq!(ignore, 0);
        }

        #[inline]
        pub(super) fn decode_error_code(self) -> u16 {
            let bits = self.raw as usize;

            // `raw` must be in `-4095..0`. Linux always returns errors in
            // `-4095..0`, and we double-check it here.
            debug_assert!((-4095..0).contains(&(bits as isize)));

            bits as u16
        }

        #[inline]
        pub(super) fn is_nonzero(&self) -> bool {
            !self.raw.is_null()
        }

        #[inline]
        pub(super) fn is_negative(&self) -> bool {
            (self.raw as isize) < 0
        }

        #[inline]
        pub(super) fn is_in_range(&self, range: core::ops::Range<isize>) -> bool {
            range.contains(&(self.raw as isize))
        }
    }

    impl<Num: RetNumber> FromAsm for RetReg<Num> {
        #[inline]
        unsafe fn from_asm(raw: *mut Opaque) -> Self {
            Self {
                raw,
                _phantom: PhantomData,
            }
        }
    }

    #[repr(transparent)]
    pub(super) struct SyscallNumber<'a> {
        nr: usize,
        _phantom: PhantomData<&'a ()>,
    }

    impl<'a> ToAsm for SyscallNumber<'a> {
        #[inline]
        unsafe fn to_asm(self) -> *mut Opaque {
            self.nr as usize as *mut Opaque
        }
    }

    /// Encode a system call argument as an `ArgReg`.
    #[inline]
    pub(super) fn raw_arg<'a, Num: ArgNumber>(raw: *mut Opaque) -> ArgReg<'a, Num> {
        ArgReg {
            raw,
            _phantom: PhantomData,
        }
    }

    /// Encode a system call number (a `__NR_*` constant) as a `SyscallNumber`.
    #[inline]
    pub(super) const fn nr<'a>(nr: u32) -> SyscallNumber<'a> {
        SyscallNumber {
            nr: nr as usize,
            _phantom: PhantomData,
        }
    }

    /// Seal our various traits using the technique documented [here].
    ///
    /// [here]: https://rust-lang.github.io/api-guidelines/future-proofing.html
    mod private {
        pub trait Sealed {}

        // Implement for those same types, but no others.
        impl<'a, Num: super::ArgNumber> Sealed for super::ArgReg<'a, Num> {}
        impl<Num: super::RetNumber> Sealed for super::RetReg<Num> {}
        impl<'a> Sealed for super::SyscallNumber<'a> {}
        impl Sealed for super::A0 {}
        impl Sealed for super::A1 {}
        impl Sealed for super::A2 {}
        impl Sealed for super::A3 {}
        impl Sealed for super::A4 {}
        impl Sealed for super::A5 {}
        #[cfg(target_arch = "mips")]
        impl Sealed for super::A6 {}
        #[cfg(target_arch = "x86")]
        impl Sealed for super::SocketArg {}
        impl Sealed for super::R0 {}
    }