Rollup merge of rust-lang#76135 - CDirkx:const-option, r=dtolnay

Stabilize some Option methods as const

Stabilize the following methods of `Option` as const:
 - `is_some`
 - `is_none`
 - `as_ref`

These methods are currently const under the unstable feature `const_option` (tracking issue: rust-lang#67441).
I believe these methods to be eligible for stabilization because of the stabilization of rust-lang#49146 (Allow if and match in constants) and the trivial implementations, see also:  [PR#75463](rust-lang#75463).

Related: rust-lang#76225

Author: RalfJung

library/core/src/option.rs

@@ -175,7 +175,7 @@ impl<T> Option<T> {
     /// ```
     #[must_use = "if you intended to assert that this has a value, consider `.unwrap()` instead"]
     #[inline]
-    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn is_some(&self) -> bool {
         matches!(*self, Some(_))
@@ -195,7 +195,7 @@ impl<T> Option<T> {
     #[must_use = "if you intended to assert that this doesn't have a value, consider \
                   `.and_then(|| panic!(\"`Option` had a value when expected `None`\"))` instead"]
     #[inline]
-    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn is_none(&self) -> bool {
         !self.is_some()
@@ -254,7 +254,7 @@ impl<T> Option<T> {
     /// println!("still can print text: {:?}", text);
     /// ```
     #[inline]
-    #[rustc_const_unstable(feature = "const_option", issue = "67441")]
+    #[rustc_const_stable(feature = "const_option", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn as_ref(&self) -> Option<&T> {
         match *self {

library/core/tests/option.rs

@@ -356,3 +356,19 @@ fn test_replace() {
     assert_eq!(x, Some(3));
     assert_eq!(old, None);
 }
+
+#[test]
+fn option_const() {
+    // test that the methods of `Option` are usable in a const context
+
+    const OPTION: Option<usize> = Some(32);
+
+    const REF: Option<&usize> = OPTION.as_ref();
+    assert_eq!(REF, Some(&32));
+
+    const IS_SOME: bool = OPTION.is_some();
+    assert!(IS_SOME);
+
+    const IS_NONE: bool = OPTION.is_none();
+    assert!(!IS_NONE);
+}