Changing the alloc() to accept &self instead of &mut self

library/alloc/src/alloc.rs

@@ -145,7 +145,7 @@ pub unsafe fn alloc_zeroed(layout: Layout) -> *mut u8 {
 
 impl Global {
     #[inline]
-    fn alloc_impl(&mut self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocErr> {
+    fn alloc_impl(&self, layout: Layout, zeroed: bool) -> Result<NonNull<[u8]>, AllocErr> {
         match layout.size() {
             0 => Ok(NonNull::slice_from_raw_parts(layout.dangling(), 0)),
             // SAFETY: `layout` is non-zero in size,
@@ -208,7 +208,7 @@ impl Global {
 #[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl AllocRef for Global {
     #[inline]
-    fn alloc(&mut self, layout: Layout) -> Result<NonNull<[u8]>, AllocErr> {
+    fn alloc(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocErr> {
         self.alloc_impl(layout, false)
     }
 

library/alloc/src/raw_vec/tests.rs

@@ -1,4 +1,5 @@
 use super::*;
+use std::cell::Cell;
 
 #[test]
 fn allocator_param() {
@@ -17,17 +18,17 @@ fn allocator_param() {
     // A dumb allocator that consumes a fixed amount of fuel
     // before allocation attempts start failing.
     struct BoundedAlloc {
-        fuel: usize,
+        fuel: Cell<usize>,
     }
     unsafe impl AllocRef for BoundedAlloc {
-        fn alloc(&mut self, layout: Layout) -> Result<NonNull<[u8]>, AllocErr> {
+        fn alloc(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocErr> {
             let size = layout.size();
-            if size > self.fuel {
+            if size > self.fuel.get() {
                 return Err(AllocErr);
             }
             match Global.alloc(layout) {
                 ok @ Ok(_) => {
-                    self.fuel -= size;
+                    self.fuel.update(|old| old - size);
                     ok
                 }
                 err @ Err(_) => err,
@@ -38,11 +39,11 @@ fn allocator_param() {
         }
     }
 
-    let a = BoundedAlloc { fuel: 500 };
+    let a = BoundedAlloc { fuel: Cell::new(500) };
     let mut v: RawVec<u8, _> = RawVec::with_capacity_in(50, a);
-    assert_eq!(v.alloc.fuel, 450);
+    assert_eq!(v.alloc.fuel.get(), 450);
     v.reserve(50, 150); // (causes a realloc, thus using 50 + 150 = 200 units of fuel)
-    assert_eq!(v.alloc.fuel, 250);
+    assert_eq!(v.alloc.fuel.get(), 250);
 }
 
 #[test]