Partial implementation of uniform paths 2.0 to land before beta

[petrochenkov]

Reimplementation of uniform paths using in-scope resolution rather than canaries is a minor breaking change due to stricter future-proofing, so it needs to be landed before beta or backported later.

I hope to implement at least something until beta so we have less to backport.
r? @Mark-Simulacrum

[Mark-Simulacrum]

Beta/nightly promotion is what I think matters here and that'll happen early next week so I don't think there's a super big rush here?

[Mark-Simulacrum]

Changes look good modulo some tests though I'm not super familiar with this area if code - r=me if you don't think we need them

[petrochenkov]

Beta/nightly promotion is what I think matters here and that'll happen early next week

Oh, that's great, I thought it's supposed to be Oct 26 (same day as stable release).
Then I'll perhaps manage to do everything during the weekend.

[Mark-Simulacrum]

We're delaying beta/nightly promotion due to RC2.

[pnkfelix]

@petrochenkov are you planning on doing further revision to this PR, e.g. adding tests? Or do you just want to copy over the r+ marker from @Mark-Simulacrum ?

[petrochenkov]

@pnkfelix
Yes, during remainder of the week.
(Right now this is only refactoring, not even tests needed.)

[bors]

☔ The latest upstream changes (presumably #54145) made this pull request unmergeable. Please resolve the merge conflicts.

[rust-highfive]

The job x86_64-gnu-llvm-5.0 of your PR failed on Travis (raw log). Through arcane magic we have determined that the following fragments from the build log may contain information about the problem.

Click to expand the log.

travis_time:end:0602586a:start=1540684739814040976,finish=1540684740999024716,duration=1184983740
$ git checkout -qf FETCH_HEAD
travis_fold:end:git.checkout

Encrypted environment variables have been removed for security reasons.
See https://docs.travis-ci.com/user/pull-requests/#Pull-Requests-and-Security-Restrictions
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
$ export SCCACHE_REGION=us-west-1
Setting environment variables from .travis.yml
$ export SCCACHE_BUCKET=rust-lang-ci-sccache2
---
[00:14:21]    Compiling rustc_traits v0.0.0 (/checkout/src/librustc_traits)
[00:16:20]    Compiling rustc_privacy v0.0.0 (/checkout/src/librustc_privacy)
[00:16:29]    Compiling rustc_resolve v0.0.0 (/checkout/src/librustc_resolve)
[00:16:29]    Compiling rustc_plugin v0.0.0 (/checkout/src/librustc_plugin)
[00:16:31] error[E0599]: no method named `gensym` found for type `syntax::ast::Symbol` in the current scope
[00:16:31]    --> librustc_resolve/build_reduced_graph.rs:414:73
[00:16:31]     |
[00:16:31] 414 |                             Some(Ident::new(keywords::Underscore.name().gensym(), use_tree.span)),
[00:16:31]     |                                             |                           |
[00:16:31]     |                                             |                           this is an associated function, not a method
[00:16:31]     |                                             |                           this is an associated function, not a method
[00:16:31]     |                                             help: use associated function syntax instead: `syntax::ast::Symbol::gensym`
[00:16:31]     |
[00:16:31]     = note: found the following associated functions; to be used as methods, functions must have a `self` parameter
[00:16:31]     = note: the candidate is defined in an impl for the type `syntax::ast::Symbol`
[00:16:31]     = help: did you mean `gensymed`?
[00:16:32] error: aborting due to previous error
[00:16:32] 
[00:16:32] For more information about this error, try `rustc --explain E0599`.
[00:16:32] error: Could not compile `rustc_resolve`.

I'm a bot! I can only do what humans tell me to, so if this was not helpful or you have suggestions for improvements, please ping or otherwise contact @TimNN. (Feature Requests)

[petrochenkov]

@Mark-Simulacrum
Ok, no more delays. This is ready for landing and the rest I'll implement later.

All the commits are refactorings except for b45b4da (bug-fix), acdbd06 (drive-by diagnostics improvement) and c57f0a7 (which is somewhat essential and fixes imports with empty groups (a::b::{}) on 2018 edition).

[Mark-Simulacrum]

@bors r+ p=5

Beta promotion has already happened so I'll prepare a beta backport as well.

[bors]

📌 Commit c57f0a7 has been approved by Mark-Simulacrum

[bors]

⌛ Testing commit c57f0a7 with merge 1982f18...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: Mark-Simulacrum
Pushing 1982f18 to master...