macro expn detection in mut_mut.rs

[llogiq]

This relates to issue #67. We should add more tests, but I'm not too versed in macros, I'll have to learn them first.

1. &mut &mut by internal macro should warn
2. `&mut &mut' by external macro should not warn

[Manishearth]

Can we refactor this so that the expn-checking bit is more generic (and can be used by other lints)?

[Manishearth]

Oh wait it is, I see.

[Manishearth]

I'd like to have extensive tests for this macro detection thingy btw; it seems like something that could go wrong easily.

[llogiq]

Agreed. Are you OK with including regexp as a dev-dependency?

[Manishearth]

sure :)

[llogiq]

I removed the macro origin check because the regex! compiler plugin makes the callee something within the current code, which is pretty bad. The current version gets rid of the regex! warnings, but surprisingly still catches my mut_ptr! macro invocation.

So, this needs more tests. I'll see if I can create a copy of the mut_ptr! macro in another sub-crate and see if this still triggers mut_mut.

[llogiq]

For what it's worth, I have another uncommitted version that filters out everything that is not a simple macro_rules! definition and leaves mut_mut to check everything else. I'm now adding another run_pass test to check with a "normal" extra-crate macro.

[llogiq]

This was surprisingly easy, as extra-crate macros either don't have source at all or (in the case of regex! and builtins) have synthesized source that does not start with macro_rules.

@Manishearth barring any pain points from your site, I think this should be ready to merge and close issue #67.