Add `DerefMut` for `Lazy[Cell/Lock]` that delegates to the unstable `force_mut()`

2 files changed, 18 insertions, 2 deletions

diff --git a/library/core/src/cell/lazy.rs b/library/core/src/cell/lazy.rs
index 5ac33516684..c9c4a6d3111 100644
--- a/library/core/src/cell/lazy.rs
+++ b/library/core/src/cell/lazy.rs
@@ -1,6 +1,6 @@
 use super::UnsafeCell;
 use crate::hint::unreachable_unchecked;
-use crate::ops::Deref;
+use crate::ops::{Deref, DerefMut};
 use crate::{fmt, mem};
 
 enum State<T, F> {
@@ -284,6 +284,14 @@ impl<T, F: FnOnce() -> T> Deref for LazyCell<T, F> {
     }
 }
 
+#[stable(feature = "lazy_deref_mut", since = "CURRENT_RUSTC_VERSION")]
+impl<T, F: FnOnce() -> T> DerefMut for LazyCell<T, F> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        LazyCell::force_mut(self)
+    }
+}
+
 #[stable(feature = "lazy_cell", since = "1.80.0")]
 impl<T: Default> Default for LazyCell<T> {
     /// Creates a new lazy value using `Default` as the initializing function.
diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 40510f56134..4379a1519c6 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -1,7 +1,7 @@
 use super::once::ExclusiveState;
 use crate::cell::UnsafeCell;
 use crate::mem::ManuallyDrop;
-use crate::ops::Deref;
+use crate::ops::{Deref, DerefMut};
 use crate::panic::{RefUnwindSafe, UnwindSafe};
 use crate::sync::Once;
 use crate::{fmt, ptr};
@@ -312,6 +312,14 @@ impl<T, F: FnOnce() -> T> Deref for LazyLock<T, F> {
     }
 }
 
+#[stable(feature = "lazy_deref_mut", since = "CURRENT_RUSTC_VERSION")]
+impl<T, F: FnOnce() -> T> DerefMut for LazyLock<T, F> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        LazyLock::force_mut(self)
+    }
+}
+
 #[stable(feature = "lazy_cell", since = "1.80.0")]
 impl<T: Default> Default for LazyLock<T> {
     /// Creates a new lazy value using `Default` as the initializing function.