Associate an allocator to boxes

This turns `Box<T>` into `Box<T, A: Alloc = Global>`. This is a
minimalist change to achieve this, not touching anything that could have
backwards incompatible consequences like requiring type annotations in
places where they currently aren't required,
per #50822 (comment)

src/liballoc/alloc.rs

@@ -40,10 +40,14 @@ extern "Rust" {
 /// This type implements the [`Alloc`] trait by forwarding calls
 /// to the allocator registered with the `#[global_allocator]` attribute
 /// if there is one, or the `std` crate’s default.
+#[cfg(not(test))]
 #[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 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
@@ -116,6 +120,7 @@ pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
     __rust_alloc_zeroed(layout.size(), layout.align())
 }
 
+#[cfg(not(test))]
 #[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl Alloc for Global {
     #[inline]
@@ -154,25 +159,23 @@ unsafe fn exchange_malloc(size: usize, align: usize) -> *mut u8 {
         align as *mut u8
     } else {
         let layout = Layout::from_size_align_unchecked(size, align);
-        let ptr = alloc(layout);
-        if !ptr.is_null() {
-            ptr
-        } else {
-            handle_alloc_error(layout)
+        match Global.alloc(layout) {
+            Ok(ptr) => ptr.as_ptr(),
+            Err(_) => handle_alloc_error(layout),
         }
     }
 }
 
 #[cfg_attr(not(test), lang = "box_free")]
 #[inline]
-pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
+pub(crate) unsafe fn box_free<T: ?Sized, A: Alloc>(ptr: Unique<T>, mut a: A) {
     let ptr = ptr.as_ptr();
     let size = size_of_val(&*ptr);
     let align = min_align_of_val(&*ptr);
     // We do not allocate for Box<T> when T is ZST, so deallocation is also not necessary.
     if size != 0 {
         let layout = Layout::from_size_align_unchecked(size, align);
-        dealloc(ptr as *mut u8, layout);
+        a.dealloc(NonNull::new_unchecked(ptr).cast(), layout);
     }
 }