add test confirming that ArcIntern<str> is the size of usize

benches/get_container.rs

@@ -1,6 +1,6 @@
 //! This is a benchmark to demonstrate that cached types (`String`, `str` as of now)
 //! are faster than non-cached types because the lack of getting container from a dashmap.
-//! 
+//!
 //! The results show a whopping 26% performance gain for short `ArcIntern<String>`.
 
 use criterion::*;

src/arc.rs

@@ -511,6 +511,15 @@ fn arc_has_niche() {
         std::mem::size_of::<Option<ArcIntern<String>>>(),
         std::mem::size_of::<usize>(),
     );
+
+    assert_eq!(
+        std::mem::size_of::<ArcIntern<str>>(),
+        std::mem::size_of::<usize>(),
+    );
+    assert_eq!(
+        std::mem::size_of::<Option<ArcIntern<str>>>(),
+        std::mem::size_of::<usize>(),
+    );
 }
 
 #[test]

src/arena.rs

@@ -323,10 +323,10 @@ impl<'a, T: ?Sized> ArenaIntern<'a, T> {
     /// struct Bar {
     ///     baz: String,
     /// }
-    /// 
+    ///
     /// struct Foo<'a>(ArenaIntern<'a, Bar>);
     /// ```
-    /// 
+    ///
     /// The following code does not compile.
     /// ```compile_fail
     /// # use internment::ArenaIntern;