diff --git a/src/lib.rs b/src/lib.rs
index d277547..1fe5032 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1253,10 +1253,14 @@ where
     T: fmt::Debug,
 {
     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
-        fmt.debug_struct("Slab")
-            .field("len", &self.len)
-            .field("cap", &self.capacity())
-            .finish()
+        if fmt.alternate() {
+            fmt.debug_map().entries(self.iter()).finish()
+        } else {
+            fmt.debug_struct("Slab")
+                .field("len", &self.len)
+                .field("cap", &self.capacity())
+                .finish()
+        }
     }
 }