Consider regions that lead to very small back paddings as unsuitable

If the resulting back padding is too small to be stored as a hole, we treat the hole as unsuitable for the allocation. This way, we avoid unrecoverable leaks of that back padding, which can add up over time and fragment the heap.

Author: phil-opp

src/hole.rs

@@ -120,10 +120,7 @@ impl Cursor {
             alloc_ptr = aligned_addr;
             alloc_size = required_size;
 
-            // Okay, time to move onto the back padding. Here, we are opportunistic -
-            // if it fits, we sits. Otherwise we just skip adding the back padding, and
-            // sort of assume that the allocation is actually a bit larger than it
-            // actually needs to be.
+            // Okay, time to move onto the back padding.
             //
             // NOTE: Because we always use `HoleList::align_layout`, the size of
             // the new allocation is always "rounded up" to cover any partial gaps that
@@ -141,10 +138,9 @@ impl Cursor {
                     size: (hole_end as usize) - (back_padding_start as usize),
                 })
             } else {
-                // No, it does not. We are now pretending the allocation now
-                // holds the extra 0..size_of::<Hole>() bytes that are not
-                // big enough to hold what SHOULD be back_padding
-                None
+                // No, it does not. We don't want to leak any heap bytes, so we
+                // consider this hole unsuitable for the requested allocation.
+                return Err(self);
             };
         }