Rollup merge of #42822 - ChrisMacNaughton:guard-traits, r=alexcrichton

Ensure Guard types impl Display & Debug

Fixes #24372

Author: Mark-Simulacrum

src/libcore/cell.rs

@@ -942,6 +942,13 @@ impl<'b, T: ?Sized> Ref<'b, T> {
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<Ref<'b, U>> for Ref<'b, T> {}
 
+#[stable(feature = "std_guard_impls", since = "1.20")]
+impl<'a, T: ?Sized + fmt::Display> fmt::Display for Ref<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(f)
+    }
+}
+
 impl<'b, T: ?Sized> RefMut<'b, T> {
     /// Make a new `RefMut` for a component of the borrowed data, e.g. an enum
     /// variant.
@@ -1034,6 +1041,13 @@ impl<'b, T: ?Sized> DerefMut for RefMut<'b, T> {
 #[unstable(feature = "coerce_unsized", issue = "27732")]
 impl<'b, T: ?Sized + Unsize<U>, U: ?Sized> CoerceUnsized<RefMut<'b, U>> for RefMut<'b, T> {}
 
+#[stable(feature = "std_guard_impls", since = "1.20")]
+impl<'a, T: ?Sized + fmt::Display> fmt::Display for RefMut<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        self.value.fmt(f)
+    }
+}
+
 /// The core primitive for interior mutability in Rust.
 ///
 /// `UnsafeCell<T>` is a type that wraps some `T` and indicates unsafe interior operations on the

src/libstd/sync/mutex.rs

@@ -440,6 +440,13 @@ impl<'a, T: ?Sized + fmt::Debug> fmt::Debug for MutexGuard<'a, T> {
     }
 }
 
+#[stable(feature = "std_guard_impls", since = "1.20")]
+impl<'a, T: ?Sized + fmt::Display> fmt::Display for MutexGuard<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
 pub fn guard_lock<'a, T: ?Sized>(guard: &MutexGuard<'a, T>) -> &'a sys::Mutex {
     &guard.__lock.inner
 }

src/libstd/sync/rwlock.rs

@@ -370,6 +370,13 @@ impl<'a, T: fmt::Debug> fmt::Debug for RwLockReadGuard<'a, T> {
     }
 }
 
+#[stable(feature = "std_guard_impls", since = "1.20")]
+impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockReadGuard<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
 #[stable(feature = "std_debug", since = "1.16.0")]
 impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
     fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
@@ -379,6 +386,13 @@ impl<'a, T: fmt::Debug> fmt::Debug for RwLockWriteGuard<'a, T> {
     }
 }
 
+#[stable(feature = "std_guard_impls", since = "1.20")]
+impl<'a, T: ?Sized + fmt::Display> fmt::Display for RwLockWriteGuard<'a, T> {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        (**self).fmt(f)
+    }
+}
+
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<'rwlock, T: ?Sized> Deref for RwLockReadGuard<'rwlock, T> {
     type Target = T;