Cleanup and consolidate operator/method dispatch code

[nikomatsakis]

Once upon a time, both operator dispatch and method lookup went through the same tangled, twisty paths. It was a mess. But lo, they were severed and made two. Each could follow its own path. Overall, this is progress. But there is still room for improvement:

1. The second half of "operator dispatch" is basically the same as confirm::confirm, but with some slight differences.
2. The autoderef loop for [] is basically the same as probe, but with some slight differences (e.g., at each step it consists builtin [] as well).
3. The probe loop, which uses check::autoderef, isn't able to be part of an inference transaction between check::autoderef uses operator dispatch which adds things into the main fulfillment context, thus leaking inference types etc outside the transaction.

It feels like things could still be cleaned up a bit further, allowing for more code reuse and happiness all around.

Some FIXMEs are scattered about at relevant points of the code.

[nikomatsakis]

I am happy to mentor.

[steveklabnik]

@nikomatsakis is this ticket still valid?

[nikomatsakis]

On Thu, Dec 31, 2015 at 10:26:12AM -0800, Steve Klabnik wrote:

@nikomatsakis is this ticket still valid?

yes

[Phyllostachys]

Inspired by this and having been a long time fan of Rust and wanting to contribute, I'm interested in solving this issue. With that said, is there anything I need to know to get started? Also, where can I start looking?

[nikomatsakis]

@Phyllostachys to be honest it's a bit out of cache for me. I'd have to go read into the code some to see where things stand. Anyway, usually a good place to start is to try and touch base at some point on IRC or over e-mail.

[Phyllostachys]

I'll have to jump on IRC tonight. Who would I email if I emailed someone?

[nikomatsakis]

@Phyllostachys me, my e-mail is on my github page

[ahmedcharles]

@nikomatsakis Are you still willing to mentor this?

[Mark-Simulacrum]

@ahmedcharles Are you still interested? We can try to connect you with niko if so

[nikomatsakis]

Well, it's quite out of cache, but let me start by giving a few very basic pointers. The relevant code is now found at src/librustc_typeck/check/method/mod.rs -- the fns in question are lookup_method() and lookup_method_in_trait_adjusted(). You may find the README.md in that same directory useful.