Recover from struct nested in struct

[Rageking8]

Fixes #101540

[rust-highfive]

r? @TaKO8Ki

(rust-highfive has picked a reviewer for you, use r? to override)

[Rageking8]

First time contributing to an open source project, please guide me step by step if there are any mistakes made in this PR. Thanks.

[TaKO8Ki]

Sorry for the late response. I left some comments.

[TaKO8Ki]

Could you use Parser::parse_item_struct instead?

[TaKO8Ki]

The implementation would eventually look something like the following:

// create a snapshot by using `Parser::ccreate_snapshot_for_diagnostic` at line 1707
// ..
} else if self.eat_keyword(kw::Struct) {
    match self.parse_item_struct() {
        Ok((ident, _)) => {
            let mut err = self.struct_span_err(
                lo.with_hi(ident.span.hi()),
                &format!("structs are not allowed in {adt_ty} definitions"),
            );
            err.help("consider creating a new `struct` definition instead of nesting");
            err
        }
        Err(err) => {
            err.cancel();
            self.restore_snapshot(snapshot);
            self.expected_ident_found()
        }
    }
}

[TaKO8Ki]

Could you just use &str?

Suggested change

	&format!("consider creating a new `struct` definition instead of nesting"),
	"consider creating a new `struct` definition instead of nesting",

[TaKO8Ki]

I think this suggestion should be like the following:

Suggested change

	err.span_suggestion(
	item.unwrap().span,
	&format!("consider creating a new `struct` definition instead of nesting"),
	"",
	Applicability::MaybeIncorrect,
	);
	err.span_help("consider creating a new `struct` definition instead of nesting");

[TaKO8Ki]

@rustbot author

[Rageking8]

The recent miri to subtree thing kinda messed up my git stuff (I am new to git). Will do this in a separate PR.