Rollup merge of rust-lang#136634 - bjoernager:const-mut-cursor, r=m-ou-se

Stabilise `Cursor::{get_mut, set_position}` in `const` scenarios.

Closes: rust-lang#130801

This PR stabilises the `const_mut_cursor` feature gate.

Author: matthiaskrgr

std/src/io/cursor.rs

@@ -153,7 +153,7 @@ impl<T> Cursor<T> {
     /// let reference = buff.get_mut();
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_mut_cursor", issue = "130801")]
+    #[rustc_const_stable(feature = "const_mut_cursor", since = "CURRENT_RUSTC_VERSION")]
     pub const fn get_mut(&mut self) -> &mut T {
         &mut self.inner
     }
@@ -201,7 +201,7 @@ impl<T> Cursor<T> {
     /// assert_eq!(buff.position(), 4);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_mut_cursor", issue = "130801")]
+    #[rustc_const_stable(feature = "const_mut_cursor", since = "CURRENT_RUSTC_VERSION")]
     pub const fn set_position(&mut self, pos: u64) {
         self.pos = pos;
     }