Add comments to explain new unsafe code.

Author: SimonSapin

src/lib.rs

@@ -174,7 +174,13 @@ macro_rules! _cssparser_internal__max_len {
         struct Dummy;
 
         // MAX_LENGTH is generated by cssparser__match_ignore_ascii_case__max_len
-        let mut buffer: [u8; MAX_LENGTH] = unsafe { ::std::mem::uninitialized() };
+        let mut buffer: [u8; MAX_LENGTH] =
+            // `buffer` is only used in `_match_ignore_ascii_case__to_lowercase`,
+            // which initializes with `copy_from_slice` the part of the buffer it uses,
+            // before it uses it.
+            unsafe {
+                ::std::mem::uninitialized()
+            };
         let $output = $crate::_match_ignore_ascii_case__to_lowercase(&mut buffer, $input);
     }
 }
@@ -188,6 +194,8 @@ pub fn _match_ignore_ascii_case__to_lowercase<'a>(buffer: &'a mut [u8], input: &
         if let Some(first_uppercase) = input.bytes().position(|byte| matches!(byte, b'A'...b'Z')) {
             buffer.copy_from_slice(input.as_bytes());
             std::ascii::AsciiExt::make_ascii_lowercase(&mut buffer[first_uppercase..]);
+            // `buffer` was initialized to a copy of `input` (which is &str so well-formed UTF-8)
+            // then lowercased (which preserves UTF-8 well-formedness)
             unsafe {
                 Some(::std::str::from_utf8_unchecked(buffer))
             }