Rollup merge of #129334 - ChayimFriedman2:more-lazy-methods, r=Amanieu

Implement (part of) ACP 429: add `DerefMut` to `Lazy[Cell/Lock]`

`DerefMut` is instantly stable, as a trait impl. That means this needs an FCP.

``@rustbot`` label +needs-fcp

https://github.com/rust-lang/libs-team/issues/429

1 files changed, 9 insertions, 1 deletions

diff --git a/library/std/src/sync/lazy_lock.rs b/library/std/src/sync/lazy_lock.rs
index 78cf8841efe..82e5fe05db5 100644
--- a/library/std/src/sync/lazy_lock.rs
+++ b/library/std/src/sync/lazy_lock.rs
@@ -1,7 +1,7 @@
 use super::poison::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};
@@ -313,6 +313,14 @@ 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.