Regex: Inform user about slow regex! macro

[killercup]

As discussed in #587, the regex! compiler plugin is currently slower than the runtime based Regex::new. We should inform the user about that.

[llogiq]

I have built the lint, but I still have to stop it from matching all parts of the macro expansion. @BurntSushi is there an expression that all regex! expansions have in common?

[BurntSushi]

@llogiq Hmm, good question! That's actually pretty tricky to answer. The code is here. I think for the forseeable future, if you find a type with the letters nfa in them (ignoring case), then that should be sufficient.

[llogiq]

Oh, I can already check if an expression is part of the macro expansion. Unfortunately, the lint matches multiple times per regex!(_).

[BurntSushi]

@llogiq Ah, so you need a uniquely identifying expression? Can you use a type definition? e.g.,

struct Nfa<'t>

[llogiq]

Hmmm... I think I'll try to use the outer block then (and a Visitor to skip walking when found).

[llogiq]

I have a regex_macro branch with my current code. Even using blocks doesn't keep it from being run multiple times. Either I'll have to go crate-(or at least module-) wide instead of function wide and keep the matched positions in some kind of set to remove the duplicate warnings, or someone finds a better solution. cc @Manishearth