From 35c65a8c0cf018d472afec5adf9da8242adc8c23 Mon Sep 17 00:00:00 2001
From: George Bateman <george.bateman16@gmail.com>
Date: Wed, 19 Jun 2024 21:44:47 +0100
Subject: [PATCH] Make Option::as_[mut_]slice const

---
 library/core/src/option.rs   | 10 ++++++----
 library/core/tests/option.rs |  9 +++++++++
 2 files changed, 15 insertions(+), 4 deletions(-)

diff --git a/library/core/src/option.rs b/library/core/src/option.rs
index 4d6ba2fa3b3..8ec7716012f 100644
--- a/library/core/src/option.rs
+++ b/library/core/src/option.rs
@@ -797,7 +797,8 @@ pub const fn as_pin_mut(self: Pin<&mut Self>) -> Option<Pin<&mut T>> {
     #[inline]
     #[must_use]
     #[stable(feature = "option_as_slice", since = "1.75.0")]
-    pub fn as_slice(&self) -> &[T] {
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_slice(&self) -> &[T] {
         // SAFETY: When the `Option` is `Some`, we're using the actual pointer
         // to the payload, with a length of 1, so this is equivalent to
         // `slice::from_ref`, and thus is safe.
@@ -811,7 +812,7 @@ pub fn as_slice(&self) -> &[T] {
         unsafe {
             slice::from_raw_parts(
                 (self as *const Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
-                usize::from(self.is_some()),
+                self.is_some() as usize,
             )
         }
     }
@@ -851,7 +852,8 @@ pub fn as_slice(&self) -> &[T] {
     #[inline]
     #[must_use]
     #[stable(feature = "option_as_slice", since = "1.75.0")]
-    pub fn as_mut_slice(&mut self) -> &mut [T] {
+    #[rustc_const_unstable(feature = "const_option_ext", issue = "91930")]
+    pub const fn as_mut_slice(&mut self) -> &mut [T] {
         // SAFETY: When the `Option` is `Some`, we're using the actual pointer
         // to the payload, with a length of 1, so this is equivalent to
         // `slice::from_mut`, and thus is safe.
@@ -867,7 +869,7 @@ pub fn as_mut_slice(&mut self) -> &mut [T] {
         unsafe {
             slice::from_raw_parts_mut(
                 (self as *mut Self).byte_add(core::mem::offset_of!(Self, Some.0)).cast(),
-                usize::from(self.is_some()),
+                self.is_some() as usize,
             )
         }
     }
diff --git a/library/core/tests/option.rs b/library/core/tests/option.rs
index b1b9492f182..336a79a02ce 100644
--- a/library/core/tests/option.rs
+++ b/library/core/tests/option.rs
@@ -574,4 +574,13 @@ fn as_slice() {
     assert_eq!(Some(43).as_mut_slice(), &[43]);
     assert_eq!(None::<i32>.as_slice(), &[]);
     assert_eq!(None::<i32>.as_mut_slice(), &[]);
+
+    const A: &[u32] = Some(44).as_slice();
+    const B: &[u32] = None.as_slice();
+    const _: () = {
+        let [45] = Some(45).as_mut_slice() else { panic!() };
+        let []: &[u32] = None.as_mut_slice() else { panic!() };
+    };
+    assert_eq!(A, &[44]);
+    assert_eq!(B, &[]);
 }