Cache projections in trans
This introduces a cache for the results of projection and normalization in trans. This is in addition to the existing cache that is per-inference-context. Trans is an easy place to put the cache because we are guaranteed not to have type parameters and also we don't expect any failures or inference variables, so there is no need to cache or follow-up on obligations that come along with. (As evidenced by the fact that this particular code would panic if any error occurred.)
That said, I am not sure this is 100% the best place for it; I sort of wanted a cache like we have in the fulfillment context for global names; but that cache only triggers when all subsequent obligations are satisfied, and since projections don't have an entry in the obligation jungle there is no easy place to put it. I considered caching both the result and obligations globally, but haven't really tried implementing it. It might be a good next step.
Regardless, this cache seems to have no real effect on bootstrap time (maybe a slight improvement), but on [the futures.rs test case I was looking at](https://github.com/rust-lang-nursery/rustc-benchmarks/pull/6), it improves performance quite a bit:
| phase | before | after |
| ----- | ------ | ----- |
| collection | 0.79s | 0.46s |
| translation | 6.8s | 3.2s |
| total | 11.92s | 7.15s |
r? @arielb1
Refactor `PathListItem`s
This refactors away variant `Mod` of `ast::PathListItemKind` and refactors the remaining variant `Ident` to a struct `ast::PathListItem_`.
Implement the `!` type
This implements the never type (`!`) and hides it behind the feature gate `#[feature(never_type)]`. With the feature gate off, things should build as normal (although some error messages may be different). With the gate on, `!` is usable as a type and diverging type variables (ie. types that are unconstrained by anything in the code) will default to `!` instead of `()`.
Whenever a node whould be reported as deprecated:
- check if the parent item is also deprecated
- if it is and both were deprecated by the same attribute
- skip the deprecation warning
fixes#35128closes#16490
Move variant_size_differences out of trans
Also enhances the error message a bit, fixes#30505 on the way, and adds
a test (which was missing).
Closes#34018
This is a spiritual succesor to #34268/8531d581, in which we replaced a
number of matches of None to the unit value with `if let` conditionals
where it was judged that this made for clearer/simpler code (as would be
recommended by Manishearth/rust-clippy's `single_match` lint). The same
rationale applies to matches of None to the empty block.
This PR refactors the 'errors' part of libsyntax into its own crate (librustc_errors). This is the first part of a few refactorings to simplify error reporting and potentially support more output formats (like a standardized JSON output and possibly an --explain mode that can work with the user's code), though this PR stands on its own and doesn't assume further changes.
As part of separating out the errors crate, I have also refactored the code position portion of codemap into its own crate (libsyntax_pos). While it's helpful to have the common code positions in a separate crate for the new errors crate, this may also enable further simplifications in the future.
