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()
     }
 }
 

library/core/src/fmt/mod.rs

@@ -2521,22 +2521,12 @@ impl<T: Copy + Debug> Debug for Cell<T> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T: ?Sized + Debug> Debug for RefCell<T> {
     fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+        let mut d = f.debug_struct("RefCell");
         match self.try_borrow() {
-            Ok(borrow) => f.debug_struct("RefCell").field("value", &borrow).finish(),
-            Err(_) => {
-                // The RefCell is mutably borrowed so we can't look at its value
-                // here. Show a placeholder instead.
-                struct BorrowedPlaceholder;
-
-                impl Debug for BorrowedPlaceholder {
-                    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
-                        f.write_str("<borrowed>")
-                    }
-                }
-
-                f.debug_struct("RefCell").field("value", &BorrowedPlaceholder).finish()
-            }
-        }
+            Ok(borrow) => d.field("value", &borrow),
+            Err(_) => d.field("value", &format_args!("<borrowed>")),
+        };
+        d.finish()
     }
 }
 

library/std/src/sync/lazy_lock.rs

@@ -222,10 +222,12 @@ 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()
     }
 }
 

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());

library/std/src/sync/once_lock.rs

@@ -365,10 +365,12 @@ impl<T> Default for OnceLock<T> {
 #[stable(feature = "once_cell", since = "1.70.0")]
 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()
     }
 }
 

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());