Fix parsing of erroneously placed semicolons

[gurry]

Fixes #124935

The issue occurred with code like this where a semicolon occurs at the start before any items:

;
fn main() { }

The root cause lay in the parser wherein we weren't consuming the semicolon and getting it out of the way before trying to parse the item (i.e. main in this case). This meant we attempted to parse the semicolon as an item which failed and we stopped parsing further because of that failure. The end result was that no main was recorded in the AST which lead to the no main error later on in one of the lints.

This PR ensures that we consume all consecutive semicolons (as there could be more than one) before trying to parse an item.

[rustbot]

r? @lcnr

rustbot has assigned @lcnr.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[fmease]

Sorry about that, I didn't realize the author of the other PR was fixing a GH issue for precisely the same reason, I'll tell them to link issues in the future (I've already added a fixes keyword to their PR descr).

[gurry]

No worries @fmease. Thanks 😊

[dev-ardi]

What? I don't think I've fixed the issue, have I?

The PR I made existed because I'm very new and I wanted to document and make some improvements to the parser while I was reading the code in order to fix an unrelated issue :P

[fmease]

Oops, that's embarrassing 😅 I should've looked closer at the changes, I just saw both PRs swapping the order of maybe_consume_incorrect_semicolon and parse_item inside parse_mod and jumped to conclusions.

@gurry Please proceed

[dev-ardi]

I reimplemented this after rebasing master.

[gurry]

I reimplemented this after rebasing master.

Thanks @dev-ardi