Update comments; Refactor 'len_utf8';

Author: bjoernager

library/core/src/char/methods.rs

@@ -1736,14 +1736,11 @@ impl EscapeDebugExtArgs {
 
 #[inline]
 const fn len_utf8(code: u32) -> usize {
-    if code < MAX_ONE_B {
-        1
-    } else if code < MAX_TWO_B {
-        2
-    } else if code < MAX_THREE_B {
-        3
-    } else {
-        4
+    match code {
+        ..MAX_ONE_B   => 1,
+        ..MAX_TWO_B   => 2,
+        ..MAX_THREE_B => 3,
+        _             => 4
     }
 }
 
@@ -1785,7 +1782,7 @@ pub const fn encode_utf8_raw(code: u32, dst: &mut [u8]) -> &mut [u8] {
             *c = (code >> 6 & 0x3F) as u8 | TAG_CONT;
             *d = (code & 0x3F) as u8 | TAG_CONT;
         }
-        // Note that the original message is not const-compatible due to formatting.
+        // Note that we cannot format in constant expressions.
         _ => panic!("encode_utf8: buffer does not have enough bytes to encode code point"),
     };
     // SAFETY: `<&mut [u8]>::as_mut_ptr` is guaranteed to return a valid pointer and `len` has been tested to be within bounds.