exact-reader 0.1.5

use std::{fmt, marker::PhantomData, ptr::NonNull};

// stoleen from https://gitlab.com/fee1-dead/unique
#[repr(transparent)]
pub struct Unique<T: ?Sized>(NonNull<T>, PhantomData<T>);

/// `Unique` pointers are `Send` if `T` is `Send` because the data they
/// reference is unaliased. Note that this aliasing invariant is
/// unenforced by the type system; the abstraction using the
/// `Unique` must enforce it.
unsafe impl<T: Send + ?Sized> Send for Unique<T> {}

/// `Unique` pointers are `Sync` if `T` is `Sync` because the data they
/// reference is unaliased. Note that this aliasing invariant is
/// unenforced by the type system; the abstraction using the
/// `Unique` must enforce it.
unsafe impl<T: Sync + ?Sized> Sync for Unique<T> {}

impl<T: Sized> Unique<T> {
    /// Creates a new `Unique` that is dangling, but well-aligned.
    ///
    /// This is useful for initializing types which lazily allocate, like
    /// `Vec::new` does.
    ///
    /// Note that the pointer value may potentially represent a valid pointer to
    /// a `T`, which means this must not be used as a "not yet initialized"
    /// sentinel value. Types that lazily allocate must track initialization by
    /// some other means.
    #[inline]
    pub const fn dangling() -> Self {
        // SAFETY: mem::align_of() returns a valid, non-null pointer. The
        // conditions to call new_unchecked() are thus respected.
        unsafe { Unique::new_unchecked(std::mem::align_of::<T>() as *mut T) }
    }
}

impl<T: ?Sized> Unique<T> {
    /// Creates a new `Unique`.
    ///
    /// # Safety
    ///
    /// `ptr` must be non-null.
    #[inline]
    pub const unsafe fn new_unchecked(ptr: *mut T) -> Self {
        // SAFETY: the caller must guarantee that `ptr` is non-null.
        Unique(NonNull::new_unchecked(ptr), PhantomData)
    }

    /// Creates a new `Unique` if `ptr` is non-null.
    #[inline]
    pub fn new(ptr: *mut T) -> Option<Self> {
        if !ptr.is_null() {
            // SAFETY: The pointer has already been checked and is not null.
            Some(unsafe { Self::new_unchecked(ptr) })
        } else {
            None
        }
    }

    /// Acquires the underlying `*mut` pointer.
    #[inline]
    pub const fn as_ptr(self) -> *mut T {
        self.0.as_ptr()
    }

    /// Dereferences the content.
    ///
    /// The resulting lifetime is bound to self so this behaves "as if"
    /// it were actually an instance of T that is getting borrowed. If a longer
    /// (unbound) lifetime is needed, use `&*my_ptr.as_ptr()`.
    #[inline]
    pub unsafe fn as_ref(&self) -> &T {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a reference.
        &*self.as_ptr()
    }

    /// Mutably dereferences the content.
    ///
    /// The resulting lifetime is bound to self so this behaves "as if"
    /// it were actually an instance of T that is getting borrowed. If a longer
    /// (unbound) lifetime is needed, use `&mut *my_ptr.as_ptr()`.
    #[inline]
    pub unsafe fn as_mut(&mut self) -> &mut T {
        // SAFETY: the caller must guarantee that `self` meets all the
        // requirements for a mutable reference.
        &mut *self.as_ptr()
    }

    /// Casts to a pointer of another type.
    #[inline]
    pub const fn cast<U>(self) -> Unique<U> {
        // SAFETY: Unique::new_unchecked() creates a new unique and needs
        // the given pointer to not be null.
        // Since we are passing self as a pointer, it cannot be null.
        unsafe { Unique::new_unchecked(self.as_ptr() as *mut U) }
    }
}

impl<T: ?Sized> Clone for Unique<T> {
    #[inline]
    fn clone(&self) -> Self {
        *self
    }
}

impl<T: ?Sized> Copy for Unique<T> {}

impl<T: ?Sized> fmt::Debug for Unique<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Pointer::fmt(&self.as_ptr(), f)
    }
}

impl<T: ?Sized> fmt::Pointer for Unique<T> {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        fmt::Pointer::fmt(&self.as_ptr(), f)
    }
}

impl<T: ?Sized> From<&mut T> for Unique<T> {
    #[inline]
    fn from(reference: &mut T) -> Self {
        // SAFETY: A mutable reference cannot be null
        unsafe { Unique::new_unchecked(reference as _) }
    }
}

impl<T: ?Sized> From<Unique<T>> for NonNull<T> {
    #[inline]
    fn from(unique: Unique<T>) -> Self {
        unique.0
    }
}

impl<T: ?Sized> From<NonNull<T>> for Unique<T> {
    #[inline]
    fn from(reference: NonNull<T>) -> Self {
        Unique(reference, PhantomData)
    }
}