Enable 'const_char_encode_utf8' feature; Refactor 'encode_utf8_raw'; Add safety comments;

Author: bjoernager

library/core/src/char/methods.rs

@@ -1,7 +1,6 @@
 //! impl char {}
 
 use super::*;
-use crate::hint::unreachable_unchecked;
 use crate::slice;
 use crate::str::from_utf8_unchecked_mut;
 use crate::unicode::printable::is_printable;
@@ -1767,32 +1766,29 @@ const fn len_utf8(code: u32) -> usize {
 #[inline]
 pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
     let len = len_utf8(code);
-    // Note that the original message is not const-compatible due to formatting.
-    assert!(
-        len <= dst.len(),
-        "encode_utf8: buffer does not have enough bytes to encode code point",
-    );
-    match len {
-        1 => {
-            dst[0x0] = code as u8;
+    match (len, &mut *dst) {
+        (1, [a, ..]) => {
+            *a = code as u8;
         }
-        2 => {
-            dst[0x0] = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
-            dst[0x1] = (code & 0x3F) as u8 | TAG_CONT;
+        (2, [a, b, ..]) => {
+            *a = (code >> 6 & 0x1F) as u8 | TAG_TWO_B;
+            *b = (code & 0x3F) as u8 | TAG_CONT;
         }
-        3 => {
-            dst[0x0] = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
-            dst[0x1] = (code >> 6 & 0x3F) as u8 | TAG_CONT;
-            dst[0x2] = (code & 0x3F) as u8 | TAG_CONT;
+        (3, [a, b, c, ..]) => {
+            *a = (code >> 12 & 0x0F) as u8 | TAG_THREE_B;
+            *b = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+            *c = (code & 0x3F) as u8 | TAG_CONT;
         }
-        4 => {
-            dst[0x0] = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
-            dst[0x1] = (code >> 12 & 0x3F) as u8 | TAG_CONT;
-            dst[0x2] = (code >> 6 & 0x3F) as u8 | TAG_CONT;
-            dst[0x3] = (code & 0x3F) as u8 | TAG_CONT;
+        (4, [a, b, c, d, ..]) => {
+            *a = (code >> 18 & 0x07) as u8 | TAG_FOUR_B;
+            *b = (code >> 12 & 0x3F) as u8 | TAG_CONT;
+            *c = (code >> 6 & 0x3F) as u8 | TAG_CONT;
+            *d = (code & 0x3F) as u8 | TAG_CONT;
         }
-        _ => unsafe { unreachable_unchecked() },
+        // Note that the original message is not const-compatible due to formatting.
+        _ => panic!("encode_utf8: buffer does not have enough bytes to encode code point"),
     };
+    // SAFETY: `<&mut [u8]>::as_mut_ptr` returns a valid pointer and `len` has been tested to be valid.
     unsafe { slice::from_raw_parts_mut(dst.as_mut_ptr(), len) }
 }
 

library/core/src/lib.rs

@@ -118,6 +118,7 @@
 #![feature(const_array_into_iter_constructors)]
 #![feature(const_bigint_helper_methods)]
 #![feature(const_black_box)]
+#![feature(const_char_encode_utf8)]
 #![feature(const_cell_into_inner)]
 #![feature(const_eval_select)]
 #![feature(const_exact_div)]