Rollup merge of #102470 - est31:stabilize_const_char_convert, r=joshtriplett

Stabilize const char convert

Split out `const_char_from_u32_unchecked` from `const_char_convert` and stabilize the rest, i.e. stabilize the following functions:

```Rust
impl char {
    pub const fn from_u32(self, i: u32) -> Option<char>;
    pub const fn from_digit(self, num: u32, radix: u32) -> Option<char>;
    pub const fn to_digit(self, radix: u32) -> Option<u32>;
}

// Available through core::char and std::char
mod char {
    pub const fn from_u32(i: u32) -> Option<char>;
    pub const fn from_digit(num: u32, radix: u32) -> Option<char>;
}
```

And put the following under the `from_u32_unchecked` const stability gate as it needs `Option::unwrap` which isn't const-stable (yet):

```Rust
impl char {
    pub const unsafe fn from_u32_unchecked(i: u32) -> char;
}

// Available through core::char and std::char
mod char {
    pub const unsafe fn from_u32_unchecked(i: u32) -> char;
}
```

cc the tracking issue #89259 (which I'd like to keep open for `const_char_from_u32_unchecked`).

library/core/src/char/convert.rs

@@ -18,7 +18,6 @@ pub(super) const fn from_u32(i: u32) -> Option<char> {
 }
 
 /// Converts a `u32` to a `char`, ignoring validity. See [`char::from_u32_unchecked`].
-#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
 #[inline]
 #[must_use]
 pub(super) const unsafe fn from_u32_unchecked(i: u32) -> char {

library/core/src/char/methods.rs

@@ -140,7 +140,7 @@ impl char {
     /// assert_eq!(None, c);
     /// ```
     #[stable(feature = "assoc_char_funcs", since = "1.52.0")]
-    #[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+    #[rustc_const_stable(feature = "const_char_convert", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     #[inline]
     pub const fn from_u32(i: u32) -> Option<char> {
@@ -183,7 +183,7 @@ impl char {
     /// assert_eq!('❤', c);
     /// ```
     #[stable(feature = "assoc_char_funcs", since = "1.52.0")]
-    #[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+    #[rustc_const_unstable(feature = "const_char_from_u32_unchecked", issue = "89259")]
     #[must_use]
     #[inline]
     pub const unsafe fn from_u32_unchecked(i: u32) -> char {
@@ -241,7 +241,7 @@ impl char {
     /// let _c = char::from_digit(1, 37);
     /// ```
     #[stable(feature = "assoc_char_funcs", since = "1.52.0")]
-    #[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+    #[rustc_const_stable(feature = "const_char_convert", since = "CURRENT_RUSTC_VERSION")]
     #[must_use]
     #[inline]
     pub const fn from_digit(num: u32, radix: u32) -> Option<char> {
@@ -338,7 +338,7 @@ impl char {
     /// let _ = '1'.to_digit(37);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
-    #[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+    #[rustc_const_stable(feature = "const_char_convert", since = "CURRENT_RUSTC_VERSION")]
     #[must_use = "this returns the result of the operation, \
                   without modifying the original"]
     #[inline]

library/core/src/char/mod.rs

@@ -110,7 +110,7 @@ pub fn decode_utf16<I: IntoIterator<Item = u16>>(iter: I) -> DecodeUtf16<I::Into
 
 /// Converts a `u32` to a `char`. Use [`char::from_u32`] instead.
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+#[rustc_const_stable(feature = "const_char_convert", since = "CURRENT_RUSTC_VERSION")]
 #[must_use]
 #[inline]
 pub const fn from_u32(i: u32) -> Option<char> {
@@ -120,7 +120,7 @@ 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_convert", issue = "89259")]
+#[rustc_const_unstable(feature = "const_char_from_u32_unchecked", issue = "89259")]
 #[must_use]
 #[inline]
 pub const unsafe fn from_u32_unchecked(i: u32) -> char {
@@ -130,7 +130,7 @@ pub const unsafe fn from_u32_unchecked(i: u32) -> char {
 
 /// Converts a digit in the given radix to a `char`. Use [`char::from_digit`] instead.
 #[stable(feature = "rust1", since = "1.0.0")]
-#[rustc_const_unstable(feature = "const_char_convert", issue = "89259")]
+#[rustc_const_stable(feature = "const_char_convert", since = "CURRENT_RUSTC_VERSION")]
 #[must_use]
 #[inline]
 pub const fn from_digit(num: u32, radix: u32) -> Option<char> {

library/core/src/lib.rs

@@ -104,7 +104,7 @@
 #![feature(const_black_box)]
 #![feature(const_caller_location)]
 #![feature(const_cell_into_inner)]
-#![feature(const_char_convert)]
+#![feature(const_char_from_u32_unchecked)]
 #![feature(const_clone)]
 #![feature(const_cmp)]
 #![feature(const_discriminant)]