Indent code past the widest line number

Fixes rust-lang#11715

Author: tbelaire

src/libsyntax/diagnostic.rs

@@ -588,12 +588,18 @@ fn highlight_lines(err: &mut EmitterWriter,
     let display_line_infos = &lines.lines[..display_lines];
     let display_line_strings = &line_strings[..display_lines];
 
+    // Calculate the widest number to format evenly and fix #11715
+    assert!(display_line_infos.len > 0);
+    let mut max_line_num = display_line_infos[display_line_infos.len() - 1].line_index + 1;
+    let mut digits = 0;
+    while max_line_num > 0 { max_line_num /= 10; digits += 1; }
     // Print the offending lines
     for (line_info, line) in display_line_infos.iter().zip(display_line_strings.iter()) {
-        try!(write!(&mut err.dst, "{}:{} {}\n",
+        try!(write!(&mut err.dst, "{}:{:>width$} {}\n",
                     fm.name,
                     line_info.line_index + 1,
-                    line));
+                    line,
+                    width=digits));
     }
 
     // If we elided something, put an ellipsis.