Don't debug-print Interned or PrivateZst

compiler/rustc_data_structures/src/intern.rs

@@ -1,5 +1,6 @@
 use crate::stable_hasher::{HashStable, StableHasher};
 use std::cmp::Ordering;
+use std::fmt::{self, Debug};
 use std::hash::{Hash, Hasher};
 use std::ops::Deref;
 use std::ptr;
@@ -20,7 +21,6 @@ mod private {
 /// The `PrivateZst` field means you can pattern match with `Interned(v, _)`
 /// but you can only construct a `Interned` with `new_unchecked`, and not
 /// directly.
-#[derive(Debug)]
 #[rustc_pass_by_value]
 pub struct Interned<'a, T>(pub &'a T, pub private::PrivateZst);
 
@@ -108,5 +108,11 @@ where
     }
 }
 
+impl<T: Debug> Debug for Interned<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        self.0.fmt(f)
+    }
+}
+
 #[cfg(test)]
 mod tests;