Remove StringReader::col.

It only has a single use, within code handling indented block comments.
We can replace that with the new `FileMap::col_pos()`, which computes
the col position (BytePos instead of CharPos) based on the record of the
last newline char (which we already record).

This is actually an improvement, because
`trim_whitespace_prefix_and_push_line()` was using `col`, which is a
`CharPos`, as a slice index, which is a byte/char confusion.

src/libsyntax/parse/lexer/comments.rs

@@ -238,7 +238,19 @@ fn read_block_comment(rdr: &mut StringReader,
     debug!(">>> block comment");
     let p = rdr.pos;
     let mut lines: Vec<String> = Vec::new();
-    let col = rdr.col;
+
+    // Count the number of chars since the start of the line by rescanning.
+    let mut src_index = rdr.src_index(rdr.filemap.line_begin_pos());
+    let end_src_index = rdr.src_index(rdr.pos);
+    assert!(src_index <= end_src_index);
+    let mut n = 0;
+    while src_index < end_src_index {
+        let c = char_at(&rdr.src, src_index);
+        src_index += c.len_utf8();
+        n += 1;
+    }
+    let col = CharPos(n);
+
     rdr.bump();
     rdr.bump();
 

src/libsyntax/parse/lexer/mod.rs

@@ -44,8 +44,6 @@ pub struct StringReader<'a> {
     pub next_pos: BytePos,
     /// The absolute offset within the codemap of the current character
     pub pos: BytePos,
-    /// The column of the next character to read
-    pub col: CharPos,
     /// The current character (which has been read from self.pos)
     pub ch: Option<char>,
     pub filemap: Lrc<syntax_pos::FileMap>,
@@ -175,7 +173,6 @@ fn new_raw_internal(sess: &'a ParseSess, filemap: Lrc<syntax_pos::FileMap>) -> S
             sess,
             next_pos: filemap.start_pos,
             pos: filemap.start_pos,
-            col: CharPos(0),
             ch: Some('\n'),
             filemap,
             end_src_index: src.len(),
@@ -442,9 +439,6 @@ pub fn bump(&mut self) {
                 if self.save_new_lines_and_multibyte {
                     self.filemap.next_line(self.next_pos);
                 }
-                self.col = CharPos(0);
-            } else {
-                self.col = self.col + CharPos(1);
             }
             if next_ch_len > 1 {
                 if self.save_new_lines_and_multibyte {

src/libsyntax_pos/lib.rs

@@ -971,6 +971,15 @@ pub fn next_line(&self, pos: BytePos) {
         lines.push(pos);
     }
 
+    /// Return the BytePos of the beginning of the current line.
+    pub fn line_begin_pos(&self) -> BytePos {
+        let lines = self.lines.borrow();
+        match lines.last() {
+            Some(&line_pos) => line_pos,
+            None => self.start_pos,
+        }
+    }
+
     /// Add externally loaded source.
     /// If the hash of the input doesn't match or no input is supplied via None,
     /// it is interpreted as an error and the corresponding enum variant is set.