Warn about semantics predicate at the beginning of lexer rule because of it degradates performance a lot

[KvanTTT]

From the article: https://tomassetti.me/improving-the-performance-of-an-antlr-parser/ :

Mike Lischke, pointed out is that you should not put one at the beginning of a lexer rule. That is because rules with semantic predicates at the beginning are not cached. And in lexer rules the effect of no caching, is really heavy. All lexer rules are handled as if they were alternatives in a single big outer rule. So, even if only one lexer rule contains a predicate on the left hand side, no lexer rule is cached at all! When you consider that all your keywords also are lexer rules, you can easily imagine how thousands of lexer rules behave, performance-wise, if they are not cached.

I've also encountered the degradation several times.

TOKEN_1: {code}? 'TOKEN_1; // Warning
TOKEN_2: 'TOKEN_2' {code}?; // Ok

@mike-lischke what do you think about introducing of such warning?

[mike-lischke]

You mean having the ANTLR4 tool spit out a warning like that?

[KvanTTT]

Yes, something like Semantics predicate at the beginning of the lexer rule significantly degradates performance

[mike-lischke]

Sounds good, sure!

[KvanTTT]

Also, I suggest copying this info from the article to the ANTLR documentation.