Auto merge of rust-lang#77187 - TimDiekmann:box-alloc, r=Amanieu

Support custom allocators in `Box`

r? `@Amanieu`

This pull request requires a crater run.

### Prior work:
- rust-lang#71873
- rust-lang#58457
- [`alloc-wg`](https://github.com/TimDiekmann/alloc-wg)-crate

Currently blocked on:
- ~rust-lang#77118~
- ~rust-lang/chalk#615 (rust-lang#77515)~

Author: bors

library/alloc/src/alloc.rs

@@ -2,8 +2,13 @@
 
 #![stable(feature = "alloc_module", since = "1.28.0")]
 
-use core::intrinsics::{self, min_align_of_val, size_of_val};
-use core::ptr::{self, NonNull, Unique};
+#[cfg(not(test))]
+use core::intrinsics;
+use core::intrinsics::{min_align_of_val, size_of_val};
+
+use core::ptr::Unique;
+#[cfg(not(test))]
+use core::ptr::{self, NonNull};
 
 #[stable(feature = "alloc_module", since = "1.28.0")]
 #[doc(inline)]
@@ -39,8 +44,12 @@ extern "Rust" {
 /// accessed through the [free functions in `alloc`](self#functions).
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
+#[cfg(not(test))]
 pub struct Global;
 
+#[cfg(test)]
+pub use std::alloc::Global;
+
 /// Allocate memory with the global allocator.
 ///
 /// This function forwards calls to the [`GlobalAlloc::alloc`] method
@@ -144,6 +153,7 @@ pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
     unsafe { __rust_alloc_zeroed(layout.size(), layout.align()) }
 }
 
+#[cfg(not(test))]
 impl Global {
     #[inline]
     fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocError> {
@@ -207,6 +217,7 @@ impl Global {
 }
 
 #[unstable(feature = "allocator_api", issue = "32838")]
+#[cfg(not(test))]
 unsafe impl AllocRef for Global {
     #[inline]
     fn alloc(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
@@ -313,12 +324,12 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
 // well.
 // For example if `Box` is changed to  `struct Box<T: ?Sized, A: AllocRef>(Unique<T>, A)`,
 // this function has to be changed to `fn box_free<T: ?Sized, A: AllocRef>(Unique<T>, A)` as well.
-pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
+pub(crate) unsafe fn box_free<T: ?Sized, A: AllocRef>(ptr: Unique<T>, alloc: A) {
     unsafe {
         let size = size_of_val(ptr.as_ref());
         let align = min_align_of_val(ptr.as_ref());
         let layout = Layout::from_size_align_unchecked(size, align);
-        Global.dealloc(ptr.cast().into(), layout)
+        alloc.dealloc(ptr.cast().into(), layout)
     }
 }