From b7e68dfc945c4cf8d1753cc3a8be48a71bb39d5a Mon Sep 17 00:00:00 2001
From: joboet <jonasboettiger@icloud.com>
Date: Sat, 18 Mar 2023 20:06:12 +0100
Subject: [PATCH] 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`.
---
 library/core/src/cell/once.rs     |  8 +++++---
 library/core/src/fmt/mod.rs       | 20 +++++---------------
 library/std/src/sync/lazy_lock.rs |  8 +++++---
 library/std/src/sync/mutex.rs     |  8 +-------
 library/std/src/sync/once_lock.rs |  8 +++++---
 library/std/src/sync/rwlock.rs    |  8 +-------
 6 files changed, 22 insertions(+), 38 deletions(-)

diff --git a/library/core/src/cell/once.rs b/library/core/src/cell/once.rs
index 5dc2d523198..c9655c180f9 100644
--- a/library/core/src/cell/once.rs
+++ b/library/core/src/cell/once.rs
@@ -250,10 +250,12 @@ fn default() -> Self {
 #[stable(feature = "once_cell", since = "CURRENT_RUSTC_VERSION")]
 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()
     }
 }
 
diff --git a/library/core/src/fmt/mod.rs b/library/core/src/fmt/mod.rs
index fcda097f01f..0c7b2c5df99 100644
--- a/library/core/src/fmt/mod.rs
+++ b/library/core/src/fmt/mod.rs
@@ -2669,22 +2669,12 @@ fn fmt(&self, f: &mut Formatter<'_>) -> Result {
 #[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()
     }
 }
 
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 @@ fn default() -> 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 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 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 @@ fn default() -> 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 @@ fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                 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());