Ergonomic ref counting

[spastorino]

This is an experimental first version of ergonomic ref counting.

This first version implements most of the RFC but doesn't implement any of the optimizations. This was left for following iterations.

RFC: rust-lang/rfcs#3680
Tracking issue: #132290
Project goal: rust-lang/rust-project-goals#107

r? @nikomatsakis

[nikomatsakis]

Very close! First round of feedback.

[spastorino]

I've pushed some tests and I've found some parsing errors, a diagnostic ICE and another issue that needs handling during borrowck. CI is going to be red but is fine as this is not yet done.

[rustbot]

Some changes occurred in src/tools/rustfmt

cc @rust-lang/rustfmt

Some changes occurred in rustc_ty_utils::consts.rs

cc @BoxyUwU

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred to MIR optimizations

cc @rust-lang/wg-mir-opt

The rustc-dev-guide subtree was changed. If this PR only touches the dev guide consider submitting a PR directly to rust-lang/rustc-dev-guide otherwise thank you for updating the dev guide with your changes.

cc @BoxyUwU, @jieyouxu, @Kobzol

Some changes occurred in src/tools/clippy

cc @rust-lang/clippy

[spastorino]

@nikomatsakis I'm going to do some further reviews, probably add some more tests, add comments, etc, etc but would leave this up for review as it is "almost" done.

[nikomatsakis]

I looked over the UseCloned trait definition to double check the stability attributes -- seems good. Maybe add this slight modification to the test.

[spastorino]

@bors r=nikomatsakis

[bors]

📌 Commit d2bde63 has been approved by nikomatsakis

It is now in the queue for this repository.