Rollup merge of rust-lang#124941 - Skgland:stabilize-const-int-from-s…

…tr, r=dtolnay

Stabilize const `{integer}::from_str_radix` i.e. `const_int_from_str`

This PR stabilizes the feature `const_int_from_str`.

- ACP Issue: rust-lang/libs-team#74
- Implementation PR: rust-lang#99322
- Part of Tracking Issue: rust-lang#59133

API Change Diff:

```diff
impl {integer} {
- pub       fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
+ pub const fn from_str_radix(src: &str, radix: u32) -> Result<Self, ParseIntError>;
}

impl ParseIntError {
- pub       fn kind(&self) -> &IntErrorKind;
+ pub const fn kind(&self) -> &IntErrorKind;
}
```
This makes it easier to parse integers at compile-time, e.g.
the example from the Tracking Issue:

```rust
env!("SOMETHING").parse::<usize>().unwrap()
```

could now be achived  with

```rust
match usize::from_str_radix(env!("SOMETHING"), 10) {
  Ok(val) => val,
  Err(err) => panic!("Invalid value for SOMETHING environment variable."),
}
```

rather than having to depend on a library that implements or manually implement the parsing at compile-time.

---

Checklist based on [Libs Stabilization Guide - When there's const involved](https://std-dev-guide.rust-lang.org/development/stabilization.html#when-theres-const-involved)

I am treating this as a [partial stabilization](https://std-dev-guide.rust-lang.org/development/stabilization.html#partial-stabilizations) as it shares a tracking issue (and is rather small), so directly opening the partial stabilization PR for the subset (feature `const_int_from_str`) being stabilized.

- [x] ping Constant Evaluation WG
- [x] no unsafe involved
- [x] no `#[allow_internal_unstable]`
- [ ] usage of `intrinsic::const_eval_select` rust-lang#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang#124941 (comment))

core/src/lib.rs

@@ -127,7 +127,6 @@
 #![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_index_range_slice_index)]
-#![feature(const_int_from_str)]
 #![feature(const_intrinsic_copy)]
 #![feature(const_intrinsic_forget)]
 #![feature(const_ipv4)]

core/src/num/error.rs

@@ -113,7 +113,7 @@ pub enum IntErrorKind {
 impl ParseIntError {
     /// Outputs the detailed cause of parsing an integer failing.
     #[must_use]
-    #[rustc_const_unstable(feature = "const_int_from_str", issue = "59133")]
+    #[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
     #[stable(feature = "int_error_matching", since = "1.55.0")]
     pub const fn kind(&self) -> &IntErrorKind {
         &self.kind

core/src/num/mod.rs

@@ -1386,6 +1386,7 @@ from_str_radix_int_impl! { isize i8 i16 i32 i64 i128 usize u8 u16 u32 u64 u128 }
 #[doc(hidden)]
 #[inline(always)]
 #[unstable(issue = "none", feature = "std_internals")]
+#[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
 pub const fn can_not_overflow<T>(radix: u32, is_signed_ty: bool, digits: &[u8]) -> bool {
     radix <= 16 && digits.len() <= mem::size_of::<T>() * 2 - is_signed_ty as usize
 }
@@ -1435,7 +1436,7 @@ macro_rules! from_str_radix {
             #[doc = concat!("assert_eq!(", stringify!($int_ty), "::from_str_radix(\"A\", 16), Ok(10));")]
             /// ```
             #[stable(feature = "rust1", since = "1.0.0")]
-            #[rustc_const_unstable(feature = "const_int_from_str", issue = "59133")]
+            #[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
             pub const fn from_str_radix(src: &str, radix: u32) -> Result<$int_ty, ParseIntError> {
                 use self::IntErrorKind::*;
                 use self::ParseIntError as PIE;
@@ -1565,7 +1566,7 @@ macro_rules! from_str_radix_size_impl {
         #[doc = concat!("assert_eq!(", stringify!($size), "::from_str_radix(\"A\", 16), Ok(10));")]
         /// ```
         #[stable(feature = "rust1", since = "1.0.0")]
-        #[rustc_const_unstable(feature = "const_int_from_str", issue = "59133")]
+        #[rustc_const_stable(feature = "const_int_from_str", since = "CURRENT_RUSTC_VERSION")]
         pub const fn from_str_radix(src: &str, radix: u32) -> Result<$size, ParseIntError> {
             match <$t>::from_str_radix(src, radix) {
                 Ok(x) => Ok(x as $size),

core/tests/lib.rs

@@ -16,7 +16,6 @@
 #![feature(const_hash)]
 #![feature(const_heap)]
 #![feature(const_intrinsic_copy)]
-#![feature(const_int_from_str)]
 #![feature(const_maybe_uninit_as_mut_ptr)]
 #![feature(const_nonnull_new)]
 #![feature(const_pointer_is_aligned)]