Rollup merge of #124941 - Skgland:stabilize-const-int-from-str, 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/rust#99322
- Part of Tracking Issue: rust-lang/rust#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/rust#124625 in `from_str_radix_assert` to change the error message between compile-time and run-time
- [ ] [rust-labg/libs-api FCP](rust-lang/rust#124941 (comment))

Author: tgross35

tests/ui/from_str_radix_10.fixed

@@ -1,4 +1,3 @@
-#![feature(const_int_from_str)]
 #![warn(clippy::from_str_radix_10)]
 
 mod some_mod {
@@ -61,7 +60,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 
-fn issue_12732() {
+// https://github.com/rust-lang/rust-clippy/issues/12731
+fn issue_12731() {
     const A: Result<u32, std::num::ParseIntError> = u32::from_str_radix("123", 10);
     const B: () = {
         let _ = u32::from_str_radix("123", 10);

tests/ui/from_str_radix_10.rs

@@ -1,4 +1,3 @@
-#![feature(const_int_from_str)]
 #![warn(clippy::from_str_radix_10)]
 
 mod some_mod {
@@ -61,7 +60,8 @@ fn main() -> Result<(), Box<dyn std::error::Error>> {
     Ok(())
 }
 
-fn issue_12732() {
+// https://github.com/rust-lang/rust-clippy/issues/12731
+fn issue_12731() {
     const A: Result<u32, std::num::ParseIntError> = u32::from_str_radix("123", 10);
     const B: () = {
         let _ = u32::from_str_radix("123", 10);

tests/ui/from_str_radix_10.stderr

@@ -1,5 +1,5 @@
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:29:5
+  --> tests/ui/from_str_radix_10.rs:28:5
    |
 LL |     u32::from_str_radix("30", 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `"30".parse::<u32>()`
@@ -8,43 +8,43 @@ LL |     u32::from_str_radix("30", 10)?;
    = help: to override `-D warnings` add `#[allow(clippy::from_str_radix_10)]`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:32:5
+  --> tests/ui/from_str_radix_10.rs:31:5
    |
 LL |     i64::from_str_radix("24", 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `"24".parse::<i64>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:34:5
+  --> tests/ui/from_str_radix_10.rs:33:5
    |
 LL |     isize::from_str_radix("100", 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `"100".parse::<isize>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:36:5
+  --> tests/ui/from_str_radix_10.rs:35:5
    |
 LL |     u8::from_str_radix("7", 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `"7".parse::<u8>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:38:5
+  --> tests/ui/from_str_radix_10.rs:37:5
    |
 LL |     u16::from_str_radix(&("10".to_owned() + "5"), 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `("10".to_owned() + "5").parse::<u16>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:40:5
+  --> tests/ui/from_str_radix_10.rs:39:5
    |
 LL |     i128::from_str_radix(Test + Test, 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `(Test + Test).parse::<i128>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:44:5
+  --> tests/ui/from_str_radix_10.rs:43:5
    |
 LL |     i32::from_str_radix(string, 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `string.parse::<i32>()`
 
 error: this call to `from_str_radix` can be replaced with a call to `str::parse`
-  --> tests/ui/from_str_radix_10.rs:48:5
+  --> tests/ui/from_str_radix_10.rs:47:5
    |
 LL |     i32::from_str_radix(&stringier, 10)?;
    |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: try: `stringier.parse::<i32>()`