Explicitly annotate unsafe blocks in unsafe fns

This is a warning now that rust-lang/rfcs#2585 has landed.

library/alloc/src/boehm.rs

@@ -13,15 +13,15 @@ pub(crate) struct BoehmGcAllocator;
 #[unstable(feature = "allocator_api", issue = "32838")]
 unsafe impl GlobalAlloc for BoehmAllocator {
     unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
-        GC_malloc_uncollectable(layout.size()) as *mut u8
+        unsafe { GC_malloc_uncollectable(layout.size()) as *mut u8 }
     }
 
     unsafe fn dealloc(&self, ptr: *mut u8, _: Layout) {
-        GC_free(ptr as *mut c_void);
+        unsafe { GC_free(ptr as *mut c_void) };
     }
 
     unsafe fn realloc(&self, ptr: *mut u8, _: Layout, new_size: usize) -> *mut u8 {
-        GC_realloc(ptr as *mut c_void, new_size) as *mut u8
+        unsafe { GC_realloc(ptr as *mut c_void, new_size) as *mut u8 }
     }
 }
 

library/alloc/src/gc.rs

@@ -99,7 +99,7 @@ impl<T> Gc<MaybeUninit<T>> {
     /// behaviour.
     pub unsafe fn assume_init(self) -> Gc<T> {
         let ptr = self.ptr.as_ptr() as *mut GcBox<MaybeUninit<T>>;
-        Gc::from_inner((&mut *ptr).assume_init())
+        unsafe { Gc::from_inner((&mut *ptr).assume_init()) }
     }
 }
 
@@ -139,7 +139,9 @@ impl<T> GcBox<T> {
 
     fn register_finalizer(&mut self) {
         unsafe extern "C" fn fshim<T>(obj: *mut c_void, _meta: *mut c_void) {
-            ManuallyDrop::drop(&mut *(obj as *mut ManuallyDrop<T>));
+            unsafe {
+                ManuallyDrop::drop(&mut *(obj as *mut ManuallyDrop<T>));
+            }
         }
 
         unsafe {
@@ -192,7 +194,7 @@ impl<T> GcBox<MaybeUninit<T>> {
         // is reclaimed, T will be dropped. We need to find its vptr and replace the
         // GcDummyDrop vptr in the block header with it.
         self.register_finalizer();
-        NonNull::new_unchecked(self as *mut _ as *mut GcBox<T>)
+        unsafe { NonNull::new_unchecked(self as *mut _ as *mut GcBox<T>) }
     }
 }