Auto merge of #105096 - LegionMammal978:copied-allocators, r=Amanieu

Clarify that copied allocators must behave the same

Currently, the safety documentation for `Allocator` says that a cloned or moved allocator must behave the same as the original. However, it does not specify that a copied allocator must behave the same, and it's possible to construct an allocator that permits being moved or cloned, but sometimes produces a new allocator when copied.

<details>
<summary>Contrived example which results in a Miri error</summary>

```rust
#![feature(allocator_api, once_cell, strict_provenance)]
use std::{
    alloc::{AllocError, Allocator, Global, Layout},
    collections::HashMap,
    hint,
    marker::PhantomPinned,
    num::NonZeroUsize,
    pin::Pin,
    ptr::{addr_of, NonNull},
    sync::{LazyLock, Mutex},
};

mod source_allocator {
    use super::*;

    // `SourceAllocator` has 3 states:
    // - invalid value: is_cloned == false, source != self.addr()
    // - source value:  is_cloned == false, source == self.addr()
    // - cloned value:  is_cloned == true
    pub struct SourceAllocator {
        is_cloned: bool,
        source: usize,
        _pin: PhantomPinned,
    }

    impl SourceAllocator {
        // Returns a pinned source value (pointing to itself).
        pub fn new_source() -> Pin<Box<Self>> {
            let mut b = Box::new(Self {
                is_cloned: false,
                source: 0,
                _pin: PhantomPinned,
            });
            b.source = b.addr();
            Box::into_pin(b)
        }

        fn addr(&self) -> usize {
            addr_of!(*self).addr()
        }

        // Invalid values point to source 0.
        // Source values point to themselves.
        // Cloned values point to their corresponding source.
        fn source(&self) -> usize {
            if self.is_cloned || self.addr() == self.source {
                self.source
            } else {
                0
            }
        }
    }

    // Copying an invalid value produces an invalid value.
    // Copying a source value produces an invalid value.
    // Copying a cloned value produces a cloned value with the same source.
    impl Copy for SourceAllocator {}

    // Cloning an invalid value produces an invalid value.
    // Cloning a source value produces a cloned value with that source.
    // Cloning a cloned value produces a cloned value with the same source.
    impl Clone for SourceAllocator {
        fn clone(&self) -> Self {
            if self.is_cloned || self.addr() != self.source {
                *self
            } else {
                Self {
                    is_cloned: true,
                    source: self.source,
                    _pin: PhantomPinned,
                }
            }
        }
    }

    static SOURCE_MAP: LazyLock<Mutex<HashMap<NonZeroUsize, usize>>> =
        LazyLock::new(Default::default);

    // SAFETY: Wraps `Global`'s methods with additional tracking.
    // All invalid values share blocks with each other.
    // Each source value shares blocks with all cloned values pointing to it.
    // Cloning an allocator always produces a compatible allocator:
    // - Cloning an invalid value produces another invalid value.
    // - Cloning a source value produces a cloned value pointing to it.
    // - Cloning a cloned value produces another cloned value with the same source.
    // Moving an allocator always produces a compatible allocator:
    // - Invalid values remain invalid when moved.
    // - Source values cannot be moved, since they are always pinned to the heap.
    // - Cloned values keep the same source when moved.
    unsafe impl Allocator for SourceAllocator {
        fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
            let mut map = SOURCE_MAP.lock().unwrap();
            let block = Global.allocate(layout)?;
            let block_addr = block.cast::<u8>().addr();
            map.insert(block_addr, self.source());
            Ok(block)
        }

        unsafe fn deallocate(&self, block: NonNull<u8>, layout: Layout) {
            let mut map = SOURCE_MAP.lock().unwrap();
            let block_addr = block.addr();
            // SAFETY: `block` came from an allocator that shares blocks with this allocator.
            if map.remove(&block_addr) != Some(self.source()) {
                hint::unreachable_unchecked()
            }
            Global.deallocate(block, layout)
        }
    }
}
use source_allocator::SourceAllocator;

// SAFETY: `alloc1` and `alloc2` must share blocks.
unsafe fn test_same(alloc1: &SourceAllocator, alloc2: &SourceAllocator) {
    let ptr = alloc1.allocate(Layout::new::<i32>()).unwrap();
    alloc2.deallocate(ptr.cast(), Layout::new::<i32>());
}

fn main() {
    let orig = &*SourceAllocator::new_source();
    let orig_cloned1 = &orig.clone();
    let orig_cloned2 = &orig.clone();
    let copied = &{ *orig };
    let copied_cloned1 = &copied.clone();
    let copied_cloned2 = &copied.clone();
    unsafe {
        test_same(orig, orig_cloned1);
        test_same(orig_cloned1, orig_cloned2);
        test_same(copied, copied_cloned1);
        test_same(copied_cloned1, copied_cloned2);
        test_same(orig, copied); // error
    }
}
```
</details>

This could result in issues in the future for algorithms that specialize on `Copy` types. Right now, nothing in the standard library that depends on `Allocator + Clone` is susceptible to this issue, but I still think it would make sense to specify that copying an allocator is always as valid as cloning it.

Author: bors