TAMER: Display for Symbol

One of the benefits of storing a reference to the interned string on the
symbol itself is that we get to get its underlying value essentially for
free.

tamer/src/sym.rs

@@ -263,6 +263,7 @@ use bumpalo::Bump;
 use fxhash::FxBuildHasher;
 use std::cell::{Cell, RefCell};
 use std::collections::HashMap;
+use std::fmt;
 use std::hash::BuildHasher;
 use std::num::NonZeroU32;
 use std::ops::Deref;
@@ -372,6 +373,16 @@ impl<'i> Deref for Symbol<'i> {
     }
 }
 
+impl<'i> fmt::Display for Symbol<'i> {
+    /// Display name of underlying string.
+    ///
+    /// Since symbols contain pointers to their interned slices,
+    ///   we effectively get this for free.
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        write!(f, "{}", self.str)
+    }
+}
+
 /// Create, store, compare, and retrieve [`Symbol`] values.
 ///
 /// Interners accept string slices and produce values of type [`Symbol`].
@@ -654,6 +665,13 @@ mod test {
 
             assert_eq!(index, Symbol::new(index, "").index());
         }
+
+        #[test]
+        fn displays_as_interned_value() {
+            let sym = Symbol::new(SymbolIndex::from_u32(1), "foo");
+
+            assert_eq!(format!("{}", sym), sym.str);
+        }
     }
 
     mod interner {