Auto merge of #51241 - glandium:globalalloc, r=sfackler,SimonSapin

Stabilize GlobalAlloc and #[global_allocator]

This PR implements the changes discussed in #49668 (comment)

Fixes #49668
Fixes #27389

This does not change the default global allocator: #36963

src/liballoc/alloc.rs

@@ -8,17 +8,15 @@
 // option. This file may not be copied, modified, or distributed
 // except according to those terms.
 
-#![unstable(feature = "allocator_api",
-            reason = "the precise API and guarantees it provides may be tweaked \
-                      slightly, especially to possibly take into account the \
-                      types being stored to make room for a future \
-                      tracing garbage collector",
-            issue = "32838")]
+//! Memory allocation APIs
+
+#![stable(feature = "alloc_module", since = "1.28.0")]
 
 use core::intrinsics::{min_align_of_val, size_of_val};
 use core::ptr::{NonNull, Unique};
 use core::usize;
 
+#[stable(feature = "alloc_module", since = "1.28.0")]
 #[doc(inline)]
 pub use core::alloc::*;
 
@@ -37,67 +35,112 @@ extern "Rust" {
     fn __rust_alloc_zeroed(size: usize, align: usize) -> *mut u8;
 }
 
+/// The global memory allocator.
+///
+/// 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.
+#[unstable(feature = "allocator_api", issue = "32838")]
 #[derive(Copy, Clone, Default, Debug)]
 pub struct Global;
 
-#[unstable(feature = "allocator_api", issue = "32838")]
-#[rustc_deprecated(since = "1.27.0", reason = "type renamed to `Global`")]
-pub type Heap = Global;
-
-#[unstable(feature = "allocator_api", issue = "32838")]
-#[rustc_deprecated(since = "1.27.0", reason = "type renamed to `Global`")]
-#[allow(non_upper_case_globals)]
-pub const Heap: Global = Global;
-
-unsafe impl GlobalAlloc for Global {
-    #[inline]
-    unsafe fn alloc(&self, layout: Layout) -> *mut Opaque {
-        let ptr = __rust_alloc(layout.size(), layout.align());
-        ptr as *mut Opaque
-    }
+/// Allocate memory with the global allocator.
+///
+/// This function forwards calls to the [`GlobalAlloc::alloc`] method
+/// of the allocator registered with the `#[global_allocator]` attribute
+/// if there is one, or the `std` crate’s default.
+///
+/// This function is expected to be deprecated in favor of the `alloc` method
+/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
+///
+/// # Safety
+///
+/// See [`GlobalAlloc::alloc`].
+#[stable(feature = "global_alloc", since = "1.28.0")]
+#[inline]
+pub unsafe fn alloc(layout: Layout) -> *mut u8 {
+    __rust_alloc(layout.size(), layout.align())
+}
 
-    #[inline]
-    unsafe fn dealloc(&self, ptr: *mut Opaque, layout: Layout) {
-        __rust_dealloc(ptr as *mut u8, layout.size(), layout.align())
-    }
+/// Deallocate memory with the global allocator.
+///
+/// This function forwards calls to the [`GlobalAlloc::dealloc`] method
+/// of the allocator registered with the `#[global_allocator]` attribute
+/// if there is one, or the `std` crate’s default.
+///
+/// This function is expected to be deprecated in favor of the `dealloc` method
+/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
+///
+/// # Safety
+///
+/// See [`GlobalAlloc::dealloc`].
+#[stable(feature = "global_alloc", since = "1.28.0")]
+#[inline]
+pub unsafe fn dealloc(ptr: *mut u8, layout: Layout) {
+    __rust_dealloc(ptr, layout.size(), layout.align())
+}
 
-    #[inline]
-    unsafe fn realloc(&self, ptr: *mut Opaque, layout: Layout, new_size: usize) -> *mut Opaque {
-        let ptr = __rust_realloc(ptr as *mut u8, layout.size(), layout.align(), new_size);
-        ptr as *mut Opaque
-    }
+/// Reallocate memory with the global allocator.
+///
+/// This function forwards calls to the [`GlobalAlloc::realloc`] method
+/// of the allocator registered with the `#[global_allocator]` attribute
+/// if there is one, or the `std` crate’s default.
+///
+/// This function is expected to be deprecated in favor of the `realloc` method
+/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
+///
+/// # Safety
+///
+/// See [`GlobalAlloc::realloc`].
+#[stable(feature = "global_alloc", since = "1.28.0")]
+#[inline]
+pub unsafe fn realloc(ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+    __rust_realloc(ptr, layout.size(), layout.align(), new_size)
+}
 
-    #[inline]
-    unsafe fn alloc_zeroed(&self, layout: Layout) -> *mut Opaque {
-        let ptr = __rust_alloc_zeroed(layout.size(), layout.align());
-        ptr as *mut Opaque
-    }
+/// Allocate zero-initialized memory with the global allocator.
+///
+/// This function forwards calls to the [`GlobalAlloc::alloc_zeroed`] method
+/// of the allocator registered with the `#[global_allocator]` attribute
+/// if there is one, or the `std` crate’s default.
+///
+/// This function is expected to be deprecated in favor of the `alloc_zeroed` method
+/// of the [`Global`] type when it and the [`Alloc`] trait become stable.
+///
+/// # Safety
+///
+/// See [`GlobalAlloc::alloc_zeroed`].
+#[stable(feature = "global_alloc", since = "1.28.0")]
+#[inline]
+pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
+    __rust_alloc_zeroed(layout.size(), layout.align())
 }
 
+#[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl Alloc for Global {
     #[inline]
-    unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<Opaque>, AllocErr> {
-        NonNull::new(GlobalAlloc::alloc(self, layout)).ok_or(AllocErr)
+    unsafe fn alloc(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
+        NonNull::new(alloc(layout)).ok_or(AllocErr)
     }
 
     #[inline]
-    unsafe fn dealloc(&mut self, ptr: NonNull<Opaque>, layout: Layout) {
-        GlobalAlloc::dealloc(self, ptr.as_ptr(), layout)
+    unsafe fn dealloc(&mut self, ptr: NonNull<u8>, layout: Layout) {
+        dealloc(ptr.as_ptr(), layout)
     }
 
     #[inline]
     unsafe fn realloc(&mut self,
-                      ptr: NonNull<Opaque>,
+                      ptr: NonNull<u8>,
                       layout: Layout,
                       new_size: usize)
-                      -> Result<NonNull<Opaque>, AllocErr>
+                      -> Result<NonNull<u8>, AllocErr>
     {
-        NonNull::new(GlobalAlloc::realloc(self, ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)
+        NonNull::new(realloc(ptr.as_ptr(), layout, new_size)).ok_or(AllocErr)
     }
 
     #[inline]
-    unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<Opaque>, AllocErr> {
-        NonNull::new(GlobalAlloc::alloc_zeroed(self, layout)).ok_or(AllocErr)
+    unsafe fn alloc_zeroed(&mut self, layout: Layout) -> Result<NonNull<u8>, AllocErr> {
+        NonNull::new(alloc_zeroed(layout)).ok_or(AllocErr)
     }
 }
 
@@ -111,9 +154,9 @@ 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 = Global.alloc(layout);
+        let ptr = alloc(layout);
         if !ptr.is_null() {
-            ptr as *mut u8
+            ptr
         } else {
             oom(layout)
         }
@@ -129,10 +172,23 @@ pub(crate) unsafe fn box_free<T: ?Sized>(ptr: Unique<T>) {
     // 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);
-        Global.dealloc(ptr as *mut Opaque, layout);
+        dealloc(ptr as *mut u8, layout);
     }
 }
 
+/// Abort on memory allocation error or failure.
+///
+/// Callers of memory allocation APIs wishing to abort computation
+/// in response to an allocation error are encouraged to call this function,
+/// rather than directly invoking `panic!` or similar.
+///
+/// The default behavior of this function is to print a message to standard error
+/// and abort the process.
+/// It can be replaced with [`set_oom_hook`] and [`take_oom_hook`].
+///
+/// [`set_oom_hook`]: ../../std/alloc/fn.set_oom_hook.html
+/// [`take_oom_hook`]: ../../std/alloc/fn.take_oom_hook.html
+#[stable(feature = "global_alloc", since = "1.28.0")]
 #[rustc_allocator_nounwind]
 pub fn oom(layout: Layout) -> ! {
     #[allow(improper_ctypes)]