Avoid reporting a capacity of zero when there is still space in the map.

src/map.rs

@@ -619,6 +619,15 @@ impl<K, V, S, A: Allocator + Clone> HashMap<K, V, S, A> {
         self.table.capacity()
     }
 
+    /// Returns the maximum number of elements the map could potentially hold
+    /// without reallocating.
+    ///
+    /// This number is an upper bound.
+    #[cfg_attr(feature = "inline-more", inline)]
+    pub fn max_capacity(&self) -> usize {
+        self.table.max_capacity()
+    }
+
     /// An iterator visiting all keys in arbitrary order.
     /// The iterator element type is `&'a K`.
     ///

src/raw/mod.rs

@@ -1389,6 +1389,15 @@ impl<T, A: Allocator + Clone> RawTable<T, A> {
         self.table.items + self.table.growth_left
     }
 
+    /// Returns the maximum number of elements the map could potentially hold
+    /// without reallocating.
+    ///
+    /// This number is an upper bound.
+    #[inline]
+    pub fn max_capacity(&self) -> usize {
+        bucket_mask_to_capacity(self.table.bucket_mask)
+    }
+
     /// Returns the number of elements in the table.
     #[inline]
     pub fn len(&self) -> usize {
@@ -2485,6 +2494,15 @@ impl<A: Allocator + Clone> RawTableInner<A> {
         // SAFETY: the caller must uphold the safety contract for `erase` method.
         self.set_ctrl(index, ctrl);
         self.items -= 1;
+
+        // If the last item was removed we can do some cleanup
+        if unlikely(self.items == 0) {
+            let desired_capacity = bucket_mask_to_capacity(self.bucket_mask);
+            if self.growth_left <= desired_capacity / 2 {
+                // Restore an accurate view of the capacity
+                self.clear_no_drop()
+            }
+        }
     }
 }