Convert error line buffer to space indentation

The type of indentation is not important for error reporting. However,
keeping the indentation as tabs has two issues:

1. rustfmt keeps track of widths, not character offsets. For space
   indentation, these two are the same. For tab indentation, this leads
   to issues like [1].
2. annotate-snippet-rs always format leading tabs as four spaces. Thus,
   formatter error reporting is not faithful and ranges will not be
   marked correctly (or need another transformation).

Replacing tabs with spaces for error reporting as early as possible
solves these issues.

[1] rust-lang#6442

Author: Naseschwarz

src/formatting.rs

@@ -593,13 +593,22 @@ impl<'a> FormatLines<'a> {
         }
     }
 
+    fn line_buffer_with_leading_spaces(&self) -> String {
+        if self.config.hard_tabs() {
+            let leading_tabs = self.line_buffer.chars().take_while(|&c| c == '\t').count();
+            " ".repeat(self.config.tab_spaces() * leading_tabs) + &self.line_buffer[leading_tabs..]
+        } else {
+            self.line_buffer.clone()
+        }
+    }
+
     fn push_err(&mut self, kind: ErrorKind, is_comment: bool, is_string: bool) {
         self.errors.push(FormattingError {
             line: self.cur_line,
             kind,
             is_comment,
             is_string,
-            line_buffer: self.line_buffer.clone(),
+            line_buffer: self.line_buffer_with_leading_spaces(),
         });
     }