Adjusted diagnostic output so that if there is no use in a item sequence,

then we just suggest the first legal position where you could inject a use.

To do this, I added `inject_use_span` field to `ModSpans`, and populate it in
parser (it is the span of the first token found after inner attributes, if any).
Then I rewrote the use-suggestion code to utilize it, and threw out some stuff
that is now unnecessary with this in place. (I think the result is easier to
understand.)

Then I added a test of issue 87613.

src/parse/parser.rs

@@ -113,7 +113,7 @@ pub(crate) fn parse_file_as_module(
         let result = catch_unwind(AssertUnwindSafe(|| {
             let mut parser = new_parser_from_file(sess.inner(), path, Some(span));
             match parser.parse_mod(&TokenKind::Eof) {
-                Ok((a, i, ast::ModSpans { inner_span })) => Some((a, i, inner_span)),
+                Ok((a, i, ast::ModSpans { inner_span, inject_use_span: _ })) => Some((a, i, inner_span)),
                 Err(mut e) => {
                     e.emit();
                     if sess.can_reset_errors() {

src/visitor.rs

@@ -916,7 +916,7 @@ fn format_mod(
         self.push_str(&ident_str);
 
         if let ast::ModKind::Loaded(ref items, ast::Inline::Yes, ref spans) = mod_kind {
-            let ast::ModSpans { inner_span } = *spans;
+            let ast::ModSpans{ inner_span, inject_use_span: _ } = *spans;
             match self.config.brace_style() {
                 BraceStyle::AlwaysNextLine => {
                     let indent_str = self.block_indent.to_string_with_newline(self.config);