std: 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`.

4 files changed, 12 insertions, 20 deletions

diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 8e9ea293ce4..1d0ed4bb452 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -157,10 +157,12 @@ impl<T: Default> Default for LazyLock<T> {
 #[unstable(feature = "lazy_cell", issue = "109736")]
 impl<T: fmt::Debug, F> fmt::Debug for LazyLock<T, F> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_tuple("LazyLock");
         match self.get() {
-            Some(v) => f.debug_tuple("LazyLock").field(v).finish(),
-            None => f.write_str("LazyLock(Uninit)"),
-        }
+            Some(v) => d.field(v),
+            None => d.field(&format_args!("<uninit>")),
+        };
+        d.finish()
     }
 }
 
diff --git a/library/std/src/sync/mutex.rs b/library/std/src/sync/mutex.rs
index b8fec6902a0..b4ae6b7e07e 100644
--- a/library/std/src/sync/mutex.rs
+++ b/library/std/src/sync/mutex.rs
@@ -490,13 +490,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for Mutex<T> {
                 d.field("data", &&**err.get_ref());
             }
             Err(TryLockError::WouldBlock) => {
-                struct LockedPlaceholder;
-                impl fmt::Debug for LockedPlaceholder {
-                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                        f.write_str("<locked>")
-                    }
-                }
-                d.field("data", &LockedPlaceholder);
+                d.field("data", &format_args!("<locked>"));
             }
         }
         d.field("poisoned", &self.poison.get());
diff --git a/library/std/src/sync/once_lock.rs b/library/std/src/sync/once_lock.rs
index 8c34375ea07..cd8a62d55c5 100644
--- a/library/std/src/sync/once_lock.rs
+++ b/library/std/src/sync/once_lock.rs
@@ -366,10 +366,12 @@ impl<T> const Default for OnceLock<T> {
 #[stable(feature = "once_cell", since = "CURRENT_RUSTC_VERSION")]
 impl<T: fmt::Debug> fmt::Debug for OnceLock<T> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        let mut d = f.debug_tuple("OnceLock");
         match self.get() {
-            Some(v) => f.debug_tuple("Once").field(v).finish(),
-            None => f.write_str("Once(Uninit)"),
-        }
+            Some(v) => d.field(v),
+            None => d.field(&format_args!("<uninit>")),
+        };
+        d.finish()
     }
 }
 
diff --git a/library/std/src/sync/rwlock.rs b/library/std/src/sync/rwlock.rs
index 7c409cb3e97..26aaa2414c9 100644
--- a/library/std/src/sync/rwlock.rs
+++ b/library/std/src/sync/rwlock.rs
@@ -485,13 +485,7 @@ impl<T: ?Sized + fmt::Debug> fmt::Debug for RwLock<T> {
                 d.field("data", &&**err.get_ref());
             }
             Err(TryLockError::WouldBlock) => {
-                struct LockedPlaceholder;
-                impl fmt::Debug for LockedPlaceholder {
-                    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-                        f.write_str("<locked>")
-                    }
-                }
-                d.field("data", &LockedPlaceholder);
+                d.field("data", &format_args!("<locked>"));
             }
         }
         d.field("poisoned", &self.poison.get());