Change comment to // Safety: ...

This changes a comment to be explicit on how it's safe we can avoid
validating UTF-8.

serde/src/ser/impls.rs

@@ -736,8 +736,9 @@ impl Serialize for net::Ipv4Addr {
  // Skip over delimiters that we initialized buf with
  written += format_u8(*oct, &mut buf[written + 1..]) + 1;
  }
- // We've only written ASCII bytes to the buffer, so it is valid UTF-8
- serializer.serialize_str(unsafe { str::from_utf8_unchecked(&buf[..written]) })
+ // Safety: We've only written ASCII bytes to the buffer, so it is valid UTF-8
+ let buf = unsafe { str::from_utf8_unchecked(&buf[..written]) };
+ serializer.serialize_str(buf)
  } else {
  self.octets().serialize(serializer)
  }