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use crate::Owns;
use core::{
    alloc::{AllocErr, AllocInit, AllocRef, Layout, MemoryBlock, ReallocPlacement},
    ptr::NonNull,
};

/// An emphatically empty implementation of `AllocRef`.
///
/// Although it has no direct use, it is useful as a "terminator" in composite allocators.
///
/// # Examples
///
/// The `NullAlloc` will always return `Err`:
///
/// ```rust
/// #![feature(allocator_api)]
///
/// use alloc_compose::NullAlloc;
/// use std::alloc::{AllocInit, AllocRef, Global, Layout};
///
/// let memory = NullAlloc.alloc(Layout::new::<u32>(), AllocInit::Uninitialized);
/// assert!(memory.is_err())
/// ```
///
/// Even if a zero-sized allocation is requested:
///
/// ```rust
/// #![feature(allocator_api)]
///
/// use alloc_compose::NullAlloc;
/// use std::alloc::{AllocInit, AllocRef, Global, Layout};
///
/// let memory = NullAlloc.alloc(Layout::new::<()>(), AllocInit::Uninitialized);
/// assert!(memory.is_err())
/// ```
#[derive(Debug, Copy, Clone)]
pub struct NullAlloc;

unsafe impl AllocRef for NullAlloc {
    /// Will always return `Err(AllocErr)`.
    fn alloc(&mut self, _layout: Layout, _init: AllocInit) -> Result<MemoryBlock, AllocErr> {
        Err(AllocErr)
    }

    /// Must not be called, as `alloc` always fails.
    unsafe fn dealloc(&mut self, _ptr: NonNull<u8>, _layout: Layout) {
        unreachable!("NullAlloc::dealloc must never be called as `alloc` always fails")
    }

    /// Must not be called, as `alloc` always fails.
    unsafe fn grow(
        &mut self,
        _ptr: NonNull<u8>,
        _layout: Layout,
        _new_size: usize,
        _placement: ReallocPlacement,
        _init: AllocInit,
    ) -> Result<MemoryBlock, AllocErr> {
        unreachable!("NullAlloc::grow must never be called as `alloc` always fails")
    }

    /// Must not be called, as `alloc` always fails.
    unsafe fn shrink(
        &mut self,
        _ptr: NonNull<u8>,
        _layout: Layout,
        _new_size: usize,
        _placement: ReallocPlacement,
    ) -> Result<MemoryBlock, AllocErr> {
        unreachable!("NullAlloc::shrink must never be called as `alloc` always fails")
    }
}

impl Owns for NullAlloc {
    /// Will always return `false.
    fn owns(&self, _memory: MemoryBlock) -> bool {
        false
    }
}