Fix panic in annotated_snippet dependency (#4968).

* Internally, rustfmt preserves tabs and counts them as multiple
  characters (based on configuration).
* The annotated_snippet dependency always counts tabs as 1 character.
* If rustfmt tries to display an error on a line containing tabs,
  the indicies are mismatched.
* In the extreme case, annotated_snippet may try to access out-of-range
  indices, and panic.
* This change is based on the code review by camsteffen on PR #5039 by
  karyon: have rustfmt internally replace tabs with the corresponding
  number of spaces, so that columns/indices in the buffer passed to
  annotated_snippet are counted (unambiguously and) the same.

src/formatting.rs

@@ -589,12 +589,14 @@ impl<'a> FormatLines<'a> {
     fn char(&mut self, c: char, kind: FullCodeCharKind) {
         self.newline_count = 0;
         self.line_len += if c == '\t' {
+            self.line_buffer
+                .push_str(&" ".repeat(self.config.tab_spaces()));
             self.config.tab_spaces()
         } else {
-            1
+            self.line_buffer.push(c);
+            c.len_utf8()
         };
         self.last_was_space = c.is_whitespace();
-        self.line_buffer.push(c);
         if kind.is_string() {
             self.current_line_contains_string_literal = true;
         }

tests/target/issue-4968.rs

@@ -0,0 +1,12 @@
+// rustfmt-hard_tabs: true
+// rustfmt-max_width: 40
+// rustfmt-error_on_unformatted: true
+// rustfmt-error_on_line_overflow: true
+
+fn foo(x: u32) {
+	if x > 10 {
+		if x > 20 {
+			println!("0123456789abcdefghijklmnopqrstuvwxyz");
+		}
+	}
+}