Fix FileMap::line_begin_pos().

The method relied on the FileMap still being under construction in
order for it to do what the name promises. It's now independent of
the current state.

src/libsyntax/parse/lexer/comments.rs

@@ -240,9 +240,11 @@ fn read_block_comment(rdr: &mut StringReader,
     let mut lines: Vec<String> = Vec::new();
 
     // 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 mut src_index = rdr.src_index(rdr.filemap.line_begin_pos(rdr.pos));
     let end_src_index = rdr.src_index(rdr.pos);
-    assert!(src_index <= end_src_index);
+    assert!(src_index <= end_src_index,
+        "src_index={}, end_src_index={}, line_begin_pos={}",
+        src_index, end_src_index, rdr.filemap.line_begin_pos(rdr.pos).to_u32());
     let mut n = 0;
     while src_index < end_src_index {
         let c = char_at(&rdr.src, src_index);

src/libsyntax_pos/lib.rs

@@ -976,11 +976,9 @@ impl FileMap {
     }
 
     /// Return the BytePos of the beginning of the current line.
-    pub fn line_begin_pos(&self) -> BytePos {
-        match self.lines.last() {
-            Some(&line_pos) => line_pos,
-            None => self.start_pos,
-        }
+    pub fn line_begin_pos(&self, pos: BytePos) -> BytePos {
+        let line_index = self.lookup_line(pos).unwrap();
+        self.lines[line_index]
     }
 
     /// Add externally loaded source.