Auto merge of #126958 - dtolnay:u32char, r=Mark-Simulacrum

Stabilize const unchecked conversion from u32 to char

Closes #89259.

The functions in this PR were left out of the initial set of `feature(const_char_convert)` stabilizations in #102470, but have since been unblocked by #118979.

If `unsafe { from_u32_unchecked(u) }` is called in const with a value for which `from_u32(u)` returns None, we get the following compile error.

```rust
fn main() {
    let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
}
```

```console
error[E0080]: it is undefined behavior to use this value
 --> src/main.rs:2:19
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |                   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered 0x0000d800, but expected a valid unicode scalar value (in `0..=0x10FFFF` but not in `0xD800..=0xDFFF`)
  |
  = note: The rules on what exactly is undefined behavior aren't clear, so this check might be overzealous. Please open an issue on the rustc repository if you believe it should not be considered undefined behavior.
  = note: the raw bytes of the constant (size: 4, align: 4) {
              00 d8 00 00                                     │ ....
          }

note: erroneous constant encountered
 --> src/main.rs:2:13
  |
2 |     let _ = const { unsafe { char::from_u32_unchecked(0xd800) } };
  |             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
```

Author: bors

library/core/src/char/methods.rs

@@ -223,7 +223,10 @@ impl char {
     /// assert_eq!('❤', c);
     /// ```
     #[stable(feature = "assoc_char_funcs", since = "1.52.0")]
-    #[rustc_const_unstable(feature = "const_char_from_u32_unchecked", issue = "89259")]
+    #[rustc_const_stable(
+        feature = "const_char_from_u32_unchecked",
+        since = "CURRENT_RUSTC_VERSION"
+    )]
     #[must_use]
     #[inline]
     pub const unsafe fn from_u32_unchecked(i: u32) -> char {

library/core/src/char/mod.rs

@@ -123,7 +123,7 @@ pub const fn from_u32(i: u32) -> Option<char> {
 /// Converts a `u32` to a `char`, ignoring validity. Use [`char::from_u32_unchecked`].
 /// instead.
 #[stable(feature = "char_from_unchecked", since = "1.5.0")]
-#[rustc_const_unstable(feature = "const_char_from_u32_unchecked", issue = "89259")]
+#[rustc_const_stable(feature = "const_char_from_u32_unchecked", since = "CURRENT_RUSTC_VERSION")]
 #[must_use]
 #[inline]
 pub const unsafe fn from_u32_unchecked(i: u32) -> char {

library/core/src/lib.rs

@@ -119,7 +119,6 @@
 #![feature(const_bigint_helper_methods)]
 #![feature(const_black_box)]
 #![feature(const_cell_into_inner)]
-#![feature(const_char_from_u32_unchecked)]
 #![feature(const_eval_select)]
 #![feature(const_exact_div)]
 #![feature(const_float_bits_conv)]