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 `()`.
Turn on new errors and json mode
This PR is a big-switch, but on a well-worn path:
* Turns on new errors by default (and removes old skool)
* Moves json output from behind a flag
The RFC for new errors [landed](https://github.com/rust-lang/rfcs/pull/1644) and as part of that we wanted some bake time. It's now had a few weeks + all the time leading up to the RFC of people banging on it. We've also had [editors updating to the new format](https://github.com/saviorisdead/RustyCode/pull/159) and expect more to follow.
We also have an [issue on old skool](https://github.com/rust-lang/rust/issues/35330) that needs to be fixed as more errors are switched to the new style, but it seems silly to fix old skool errors when we fully intend to throw the switch in the near future.
This makes it lean towards "why not just throw the switch now, rather than waiting a couple more weeks?" I only know of vim that wanted to try to parse the new format but were not sure how, and I think we can reach out to them and work out something in the 8 weeks before this would appear in a stable release.
We've [hashed out](https://github.com/rust-lang/rust/issues/35330) stabilizing JSON output, and it seems like people are relatively happy making what we have v1 and then likely adding to it in the future. The idea is that we'd maintain backward compatibility and just add new fields as needed. We'll also work on a separate output format that'd be better suited for interactive tools like IDES (since JSON message can get a little long depending on the error).
This PR stabilizes JSON mode, allowing its use without `-Z unstable-options`
Combined, this gives editors two ways to support errors going forward: parsing the new error format or using the JSON mode. By moving JSON to stable, we can also add support to Cargo, which plugin authors tell us does help simplify their support story.
r? @nikomatsakis
cc @rust-lang/tools
Closes https://github.com/rust-lang/rust/issues/34826
Address ICEs running w/ incremental compilation and building glium
Fixes for various ICEs I encountered trying to build glium with incremental compilation enabled. Building glium now works. Of the 4 ICEs, I have test cases for 3 of them -- I didn't isolate a test for the last commit and kind of want to go do other things -- most notably, figuring out why incremental isn't saving much *effort*.
But if it seems worthwhile and I can come back and try to narrow down the problem.
r? @michaelwoerister
Fixes#34991Fixes#32015
Centralize and clean type error reporting
Refactors the code that handles type errors to be cleaner and fixes various edge cases.
This made the already-bad "type mismatch resolving" error message somewhat uglier. I want to fix that in another commit before this PR is merged.
Fixes#31173
r? @jonathandturner, cc @nikomatsakis
Refactor constant evaluation to use a single error reporting function
that reports a type-error-like message.
Also, unify all error codes with the "constant evaluation error" message
to just E0080, and similarly for a few other duplicate codes. The old
situation was a total mess, and now that we have *something* we can
further iterate on the UX.
