Auto merge of rust-lang#76136 - CDirkx:const-result, r=dtolnay

Stabilize some Result methods as const

Stabilize the following methods of Result as const:
 - `is_ok`
 - `is_err`
 - `as_ref`

A test is also included, analogous to the test for `const_option`.

These methods are currently const under the unstable feature `const_result` (tracking issue: rust-lang#67520).
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) and [PR#76135](rust-lang#76135).

Note: these methods are the only methods currently under the `const_result` feature, thus this PR results in the removal of the feature.

Related: rust-lang#76225

Author: bors

core/src/lib.rs

@@ -87,7 +87,6 @@
 #![feature(const_ptr_offset)]
 #![feature(const_ptr_offset_from)]
 #![feature(const_raw_ptr_comparison)]
-#![feature(const_result)]
 #![feature(const_slice_from_raw_parts)]
 #![feature(const_slice_ptr_len)]
 #![feature(const_size_of_val)]

core/src/result.rs

@@ -273,7 +273,7 @@ impl<T, E> Result<T, E> {
     /// assert_eq!(x.is_ok(), false);
     /// ```
     #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
-    #[rustc_const_unstable(feature = "const_result", issue = "67520")]
+    #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn is_ok(&self) -> bool {
@@ -294,7 +294,7 @@ impl<T, E> Result<T, E> {
     /// assert_eq!(x.is_err(), true);
     /// ```
     #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
-    #[rustc_const_unstable(feature = "const_result", issue = "67520")]
+    #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
     #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn is_err(&self) -> bool {
@@ -438,7 +438,7 @@ impl<T, E> Result<T, E> {
     /// assert_eq!(x.as_ref(), Err(&"Error"));
     /// ```
     #[inline]
-    #[rustc_const_unstable(feature = "const_result", issue = "67520")]
+    #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub const fn as_ref(&self) -> Result<&T, &E> {
         match *self {

core/tests/result.rs

@@ -304,3 +304,19 @@ fn test_result_as_deref_mut() {
     let expected_result = Result::Err::<&mut u32, &mut Vec<i32>>(&mut expected_vec);
     assert_eq!(mut_err.as_deref_mut(), expected_result);
 }
+
+#[test]
+fn result_const() {
+    // test that the methods of `Result` are usable in a const context
+
+    const RESULT: Result<usize, bool> = Ok(32);
+
+    const REF: Result<&usize, &bool> = RESULT.as_ref();
+    assert_eq!(REF, Ok(&32));
+
+    const IS_OK: bool = RESULT.is_ok();
+    assert!(IS_OK);
+
+    const IS_ERR: bool = RESULT.is_err();
+    assert!(!IS_ERR)
+}