Rollup merge of #109318 - joboet:better_fmt_placeholder, r=dtolnay

Make `Debug` representations of `[Lazy, Once]*[Cell, Lock]` consistent with `Mutex` and `RwLock`

`Mutex` prints `<locked>` as a field value when its inner value cannot be accessed, but the lazy types print a fixed string like "`OnceCell(Uninit)`". This could cause confusion if the inner type is a unit type named `Uninit` and does not respect the pretty-printing flag. With this change, the format message is now "`OnceCell(<uninit>)`", consistent with `Mutex`.

library/core/src/cell/once.rs

@@ -250,10 +250,12 @@ impl<T> Default for OnceCell<T> {
 #[stable(feature = "once_cell", since = "1.70.0")]
 impl<T: fmt::Debug> fmt::Debug for OnceCell<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_tuple("OnceCell");
         match self.get() {
-            Some(v) => f.debug_tuple("OnceCell").field(v).finish(),
-            None => f.write_str("OnceCell(Uninit)"),
-        }
+            Some(v) => d.field(v),
+            None => d.field(&format_args!("<uninit>")),
+        };
+        d.finish()
     }
 }