Add test for issue 4350

Closes 4350

Its unclear which commit resolved this, but the original issue is no
longer reproducible.

tests/target/issue_4350.rs

@@ -0,0 +1,13 @@
+//rustfmt-format_macro_bodies: true
+
+macro_rules! mto_text_left {
+    ($buf:ident, $n:ident, $pos:ident, $state:ident) => {{
+        let cursor = loop {
+            state = match iter.next() {
+                None if $pos == DP::Start => break last_char_idx($buf),
+                None /*some comment */ => break 0,
+            };
+        };
+        Ok(saturate_cursor($buf, cursor))
+    }};
+}