Panic when trying to extend an unitialized Heap

Author: phil-opp

src/hole.rs

@@ -454,6 +454,8 @@ impl HoleList {
     }
 
     pub(crate) unsafe fn extend(&mut self, by: usize) {
+        assert!(!self.top.is_null(), "tried to extend an empty heap");
+
         let top = self.top;
 
         let dead_space = top.align_offset(align_of::<Hole>());
@@ -810,4 +812,10 @@ pub mod test {
         // available size is too small to store a hole -> it should panic
         unsafe { HoleList::new(heap_start.sub(1), 2 * core::mem::size_of::<usize>()) };
     }
+
+    #[test]
+    #[should_panic]
+    fn extend_empty() {
+        unsafe { HoleList::empty().extend(16) };
+    }
 }

src/lib.rs

@@ -246,6 +246,8 @@ impl Heap {
     /// at least `2 * size_of::<usize>`, keeping the amount a multiple of
     /// `size_of::<usize>`.
     ///
+    /// Calling this method on an uninitialized Heap will panic.
+    ///
     /// # Safety
     ///
     /// The amount of data given in `by` MUST exist directly after the original
@@ -255,8 +257,6 @@ impl Heap {
     /// Even if this operation doesn't increase the [usable size][`Self::size`]
     /// by exactly `by` bytes, those bytes are still owned by the Heap for
     /// later use.
-    ///
-    /// Calling this method on an uninitialized Heap is undefined behavior.
     pub unsafe fn extend(&mut self, by: usize) {
         self.holes.extend(by);
     }