Add FixedSizeBlockAllocator::fallback_alloc method

src/allocator/fixed_size_block.rs

@@ -1,3 +1,6 @@
+use alloc::alloc::Layout;
+use core::ptr;
+
 /// The block sizes to use.
 ///
 /// The sizes must each be power of 2 because they are also used as
@@ -30,4 +33,12 @@ impl FixedSizeBlockAllocator {
     pub unsafe fn init(&mut self, heap_start: usize, heap_size: usize) {
         self.fallback_allocator.init(heap_start, heap_size);
     }
+
+    /// Allocates using the fallback allocator.
+    fn fallback_alloc(&mut self, layout: Layout) -> *mut u8 {
+        match self.fallback_allocator.allocate_first_fit(layout) {
+            Ok(ptr) => ptr.as_ptr(),
+            Err(_) => ptr::null_mut(),
+        }
+    }
 }