No placeholders when using `[]` and `maybe_placeholders=True`

[zimri-leisher]

Describe the bug

If you define a rule which includes an alias inside of an optional [], e.g.:

rule: "something" [_alias_rule]

And parse it with maybe_placeholders=True, there won't be a placeholder None value. After inspection of the code, this is due to a problem with the _will_not_get_removed func in FindRuleSize, which doesn't count aliases. This leads to too few _EMPTY symbols produced in EBNF_BNF maybe handler. Replacing _alias_rule with a non-alias fixes the issue.

[MegaIng]

What do you mean with "Alias rule"?

The issue here is that _alias_rule starts with an underscore, meaning it gets inlined. The grammar compiler doesn't know how many children it will contain - it might be exactly one, it might be 0 or it might be 10.

Don't use an underscore rule inside of [] if you want placeholders.

[zimri-leisher]

Ah I see, for some reason I thought _ prefixed rules were called aliases.

Don't use an underscore rule inside of [] if you want placeholders.

Yes, I realized that. Perhaps worth a mention that in the docs? They just say:

[item item ..] - Maybe. Same as (item item ..)?, but when maybe_placeholders=True, generates None if there is no match.