Resolve clippy::single_char_add_str

    error: calling `push_str()` using a single-character string literal
       --> src/diagnostics.rs:299:9
        |
    299 |         helps.last_mut().unwrap().1.push_str("\n");
        |         ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `push` with a character literal: `helps.last_mut().unwrap().1.push('\n')`
        |
        = note: `-D clippy::single-char-add-str` implied by `-D clippy::all`
        = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#single_char_add_str

Author: dtolnay

src/diagnostics.rs

@@ -296,7 +296,7 @@ fn report_msg<'mir, 'tcx>(
     // Show help messages.
     if !helps.is_empty() {
         // Add visual separator before backtrace.
-        helps.last_mut().unwrap().1.push_str("\n");
+        helps.last_mut().unwrap().1.push('\n');
         for (span_data, help) in helps {
             if let Some(span_data) = span_data {
                 err.span_help(span_data.span(), &help);

src/lib.rs

@@ -20,7 +20,6 @@
     clippy::manual_map,
     clippy::needless_lifetimes,
     clippy::new_without_default,
-    clippy::single_char_add_str,
     clippy::single_char_pattern,
     clippy::single_match,
     clippy::unnecessary_mut_passed,