Rollup merge of rust-lang#36921 - nnethercote:two-lexer-tweaks, r=nrc

Two lexer tweaks

19 days later, I haven't received a review of my commits in rust-lang#36470. In an attempt to make some progress, I'm going to split up the changes. Here are the ones that don't relate to renaming things.

src/libsyntax/parse/lexer/mod.rs

@@ -417,11 +417,9 @@ impl<'a> StringReader<'a> {
         self.last_pos = self.pos;
         let current_byte_offset = self.byte_offset(self.pos).to_usize();
         if current_byte_offset < self.source_text.len() {
-            assert!(self.curr.is_some());
             let last_char = self.curr.unwrap();
             let ch = char_at(&self.source_text, current_byte_offset);
-            let next = current_byte_offset + ch.len_utf8();
-            let byte_offset_diff = next - current_byte_offset;
+            let byte_offset_diff = ch.len_utf8();
             self.pos = self.pos + Pos::from_usize(byte_offset_diff);
             self.curr = Some(ch);
             self.col = self.col + CharPos(1);
@@ -509,11 +507,7 @@ impl<'a> StringReader<'a> {
 
                     // line comments starting with "///" or "//!" are doc-comments
                     let doc_comment = self.curr_is('/') || self.curr_is('!');
-                    let start_bpos = if doc_comment {
-                        self.pos - BytePos(3)
-                    } else {
-                        self.last_pos - BytePos(2)
-                    };
+                    let start_bpos = self.last_pos - BytePos(2);
 
                     while !self.is_eof() {
                         match self.curr.unwrap() {