Rollup merge of rust-lang#134016 - zachs18:stable-const-str-split_at, r=Amanieu

Stabilize `const_is_char_boundary` and `const_str_split_at`.

Tracking issues: rust-lang#131516, rust-lang#131518

Stabilized const API:

```rs
// in `core`
impl str {
    // const_is_char_boundary feature
    const fn is_char_boundary(&self, index: usize) -> bool;

    // const_str_split_at feature, depends on const_is_char_boundary
    const fn split_at(&self, mid: usize) -> (&str, &str);
    const fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str);
    const fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)>;
    const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)>;
}
```

This will allow safely splitting string slices during const-eval.

Closes rust-lang#131516, Closes rust-lang#131518

This will need FCP.
r? libs-api

IIUC these do not use any new const language features (i.e. they are implementable manually on stable 1.83.0 using `unsafe`: [playground link](https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=3679632cd1041084796241b7ac8edfbd)).

Cc ``@rust-lang/wg-const-eval`` (I don't know if I have the permissions for this ping; if not, someone else please ping wg-const-eval if it is necessary)

Author: workingjubilee

Diff for: core/src/str/mod.rs

@@ -361,7 +361,7 @@ impl str {
     /// ```
     #[must_use]
     #[stable(feature = "is_char_boundary", since = "1.9.0")]
-    #[rustc_const_unstable(feature = "const_is_char_boundary", issue = "131516")]
+    #[rustc_const_stable(feature = "const_is_char_boundary", since = "CURRENT_RUSTC_VERSION")]
     #[inline]
     pub const fn is_char_boundary(&self, index: usize) -> bool {
         // 0 is always ok.
@@ -818,7 +818,7 @@ impl str {
     #[inline]
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
-    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    #[rustc_const_stable(feature = "const_str_split_at", since = "CURRENT_RUSTC_VERSION")]
     pub const fn split_at(&self, mid: usize) -> (&str, &str) {
         match self.split_at_checked(mid) {
             None => slice_error_fail(self, 0, mid),
@@ -859,7 +859,7 @@ impl str {
     #[inline]
     #[must_use]
     #[stable(feature = "str_split_at", since = "1.4.0")]
-    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    #[rustc_const_stable(feature = "const_str_split_at", since = "CURRENT_RUSTC_VERSION")]
     pub const fn split_at_mut(&mut self, mid: usize) -> (&mut str, &mut str) {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
@@ -899,7 +899,7 @@ impl str {
     #[inline]
     #[must_use]
     #[stable(feature = "split_at_checked", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    #[rustc_const_stable(feature = "const_str_split_at", since = "CURRENT_RUSTC_VERSION")]
     pub const fn split_at_checked(&self, mid: usize) -> Option<(&str, &str)> {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {
@@ -940,7 +940,7 @@ impl str {
     #[inline]
     #[must_use]
     #[stable(feature = "split_at_checked", since = "1.80.0")]
-    #[rustc_const_unstable(feature = "const_str_split_at", issue = "131518")]
+    #[rustc_const_stable(feature = "const_str_split_at", since = "CURRENT_RUSTC_VERSION")]
     pub const fn split_at_mut_checked(&mut self, mid: usize) -> Option<(&mut str, &mut str)> {
         // is_char_boundary checks that the index is in [0, .len()]
         if self.is_char_boundary(mid) {