Rollup merge of #82078 - lopopolo:char-u8-const-fn, r=m-ou-se

Make char and u8 methods const

char methods `len_utf8`, `len_utf16`, `to_ascii_lowercase`, `eq_ignore_ascii_case` can be made const.

`u8` methods `to_ascii_lowercase`, `to_ascii_uppercase` are required to be const as well.

`u8::eq_ignore_ascii_case` was additionally made const.

Rebase of https://github.com/rust-lang/rust/pull/79549 originally authored by ``@YenForYang.`` Changes from that PR:

- Squashed all commits from #79549.
- rebased to latest upstream master.
- Removed const attributes for `char::escape_unicode` and `char::escape_default`.
- Updated `since` attributes for `const` stabilization to 1.52.0.

cc ``@m-ou-se.``

library/core/src/char/methods.rs

@@ -569,8 +569,9 @@ impl char {
     /// assert_eq!(len, tokyo.len());
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_char_len_utf", since = "1.52.0")]
     #[inline]
-    pub fn len_utf8(self) -> usize {
+    pub const fn len_utf8(self) -> usize {
         len_utf8(self as u32)
     }
 
@@ -594,8 +595,9 @@ impl char {
     /// assert_eq!(len, 2);
     /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
+    #[rustc_const_stable(feature = "const_char_len_utf", since = "1.52.0")]
     #[inline]
-    pub fn len_utf16(self) -> usize {
+    pub const fn len_utf16(self) -> usize {
         let ch = self as u32;
         if (ch & 0xFFFF) == ch { 1 } else { 2 }
     }
@@ -1086,8 +1088,9 @@ impl char {
     /// [`make_ascii_uppercase()`]: #method.make_ascii_uppercase
     /// [`to_uppercase()`]: #method.to_uppercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn to_ascii_uppercase(&self) -> char {
+    pub const fn to_ascii_uppercase(&self) -> char {
         if self.is_ascii_lowercase() {
             (*self as u8).ascii_change_case_unchecked() as char
         } else {
@@ -1118,8 +1121,9 @@ impl char {
     /// [`make_ascii_lowercase()`]: #method.make_ascii_lowercase
     /// [`to_lowercase()`]: #method.to_lowercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn to_ascii_lowercase(&self) -> char {
+    pub const fn to_ascii_lowercase(&self) -> char {
         if self.is_ascii_uppercase() {
             (*self as u8).ascii_change_case_unchecked() as char
         } else {
@@ -1143,8 +1147,9 @@ impl char {
     /// assert!(!upper_a.eq_ignore_ascii_case(&lower_z));
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &char) -> bool {
+    pub const fn eq_ignore_ascii_case(&self, other: &char) -> bool {
         self.to_ascii_lowercase() == other.to_ascii_lowercase()
     }
 
@@ -1561,7 +1566,7 @@ impl char {
 }
 
 #[inline]
-fn len_utf8(code: u32) -> usize {
+const fn len_utf8(code: u32) -> usize {
     if code < MAX_ONE_B {
         1
     } else if code < MAX_TWO_B {

library/core/src/num/mod.rs

@@ -195,8 +195,9 @@ impl u8 {
     ///
     /// [`make_ascii_uppercase`]: Self::make_ascii_uppercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn to_ascii_uppercase(&self) -> u8 {
+    pub const fn to_ascii_uppercase(&self) -> u8 {
         // Unset the fifth bit if this is a lowercase letter
         *self & !((self.is_ascii_lowercase() as u8) * ASCII_CASE_MASK)
     }
@@ -218,15 +219,16 @@ impl u8 {
     ///
     /// [`make_ascii_lowercase`]: Self::make_ascii_lowercase
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn to_ascii_lowercase(&self) -> u8 {
+    pub const fn to_ascii_lowercase(&self) -> u8 {
         // Set the fifth bit if this is an uppercase letter
         *self | (self.is_ascii_uppercase() as u8 * ASCII_CASE_MASK)
     }
 
     /// Assumes self is ascii
     #[inline]
-    pub(crate) fn ascii_change_case_unchecked(&self) -> u8 {
+    pub(crate) const fn ascii_change_case_unchecked(&self) -> u8 {
         *self ^ ASCII_CASE_MASK
     }
 
@@ -243,8 +245,9 @@ impl u8 {
     /// assert!(lowercase_a.eq_ignore_ascii_case(&uppercase_a));
     /// ```
     #[stable(feature = "ascii_methods_on_intrinsics", since = "1.23.0")]
+    #[rustc_const_stable(feature = "const_ascii_methods_on_intrinsics", since = "1.52.0")]
     #[inline]
-    pub fn eq_ignore_ascii_case(&self, other: &u8) -> bool {
+    pub const fn eq_ignore_ascii_case(&self, other: &u8) -> bool {
         self.to_ascii_lowercase() == other.to_ascii_lowercase()
     }