Fix shrink_in_place implementation

src/lib.rs

@@ -200,10 +200,30 @@ unsafe impl Alloc for Jemalloc {
         layout: Layout,
         new_size: usize,
     ) -> Result<(), CannotReallocInPlace> {
+        if new_size == layout.size() { return Ok(()); }
         let flags = layout_to_flags(layout.align(), new_size);
-        let shrunk_size = ffi::xallocx(ptr.cast().as_ptr(), new_size, 0, flags);
-        debug_assert!(shrunk_size >= new_size);
-        Ok(())
+        let usable_size = ffi::xallocx(ptr.cast().as_ptr(), new_size, 0, flags);
+        if usable_size < layout.size() {
+            // `shrink_in_place` succeeds if the usable size of the shrunk
+            // allocation is smaller than that of the current allocation (and
+            // therefore, than the originally requested size). This means that:
+            //
+            // * the size-class of the allocation was changed to the size-class
+            // of `new_size`, therefore the allocation can be deallocated with
+            // `new_size`
+            //
+            // * if `new_size` lies in the same size class as `layout.size()`,
+            // and `new_size != layout.size()` then `shrink_in_place` will error
+            // even though technically the allocation can be deallocated
+            // properly with `new_size` - the problem is that `xallocx` will
+            // return `layout.size()` if shrinking failed, and we cannot detect
+            // whether that means that shrinking failed or that `new_size` is
+            // part of the same size-class efficiently.
+            //
+            Ok(())
+        } else {
+            Err(CannotReallocInPlace)
+        }
     }
 }
 

tests/shrink_in_place.rs

@@ -27,7 +27,6 @@ fn shrink_in_place() {
             let new_l = Layout::from_size_align(new_sz, 1).unwrap();
             Jemalloc.dealloc(ptr, new_l);
         } else {
-            assert!(false);
             // if shrink in place failed - deallocate with the old layout
             Jemalloc.dealloc(ptr, orig_l);
         }