fix(expand): prevent infinity loop in macro containing only "///"

[bvanjoi]

Fixes #112342

Issue #112342 was caused by an infinity loop in parse_tt_inner, and the state of it is as follows:

• matcher: [Sequence, Token(Doc), SequenceKleeneOpNoSep(op: ZeroOrMore), Eof]

• loop:

Iteration	Action
0	enter Sequence
1	enter Token(Doc) and mp.idx += 1 had been executed
2	enter SequenceKleeneOpNoSep and reset mp.idx to 1
3	enter Token(Doc) again

To prevent the infinite loop, a check for whether it only contains DocComment in check_lhs_no_empty_seq had been added.

[rustbot]

r? @eholk

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[est31]

r=me except two small nits, very good PR edit: see comment below

[est31]

Hmm yeah so what is happening here is that the compiler attempts to try another repetition, infinitely many times. This would also happen for code like:

macro_rules! foo {
    ($()*) => {};
}
foo!();

Similarly:

macro_rules! foo {
    ($($(foo:tt)?)*) => {};
}

But thankfully the compiler catches that before it gets into the matching code and errors with "repetition matches empty token tree".

I think it would be best to instead edit check_lhs_no_empty_seqa to check if there is a sequence that has nothing but a doc comment. Make sure that it also errors for multiple doc comments :)

cc also #5067, #42755, #57597

[bvanjoi]

it would be best to instead edit check_lhs_no_empty_seqa to check if there is a sequence that has nothing but a doc comment.

nice solution!

[bvanjoi]

btw, what do you think emit warning such as "doc comment is Illegal syntax, it may throw error in future"?

[Nilstrieb]

doing it like @est31 suggested is what I just wanted to suggest too, so that sounds great

emitting a future compatibility warning for doc comments in matcher position doesn't sound too bad to me but is a separate issue that should be discussed with the language team.

[bvanjoi]

Already revised, thank you both for your help, I learned a lot

[est31]

r=me if you add a test for /// together with another /// doc comment, as well as one test with /// and $(foo:tt)?.

[est31]

Some final nits. I promised approval before, but given you are learning I thought this would be useful to know.

[est31]

@bors r=nilstrieb,est31

as per #112345 (comment)

[bors]

📌 Commit 5eafab3 has been approved by nilstrieb,est31

It is now in the queue for this repository.