Add experimental unnamed_enum_variants feature gate

[kupiakos]

This parses the basic syntax and defines a feature gate for the unnamed enum variants lang experiment, tracking issue #156628.

cc @joshtriplett, @scottmcm, @tmandry

[rustbot]

The parser was modified, potentially altering the grammar of (stable) Rust
which would be a breaking change.

cc @fmease

rust-analyzer is developed in its own repository. If possible, consider making this change to rust-lang/rust-analyzer instead.

cc @rust-lang/rust-analyzer

[rustbot]

r? @Kivooeo

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[ChayimFriedman2]

Please separate the rust-analyzer changes into a PR for rust-lang/rust-analyzer.

View changes since this review

[rustbot]

Reminder, once the PR becomes ready for a review, use @rustbot ready.

[kupiakos]

I've opened a PR for the rust-analyzer bits and will rebase this PR once those changes are pushed over to rust-lang/rust.

In the meantime, the rest of the PR should be able to receive feedback as-is.

@rustbot ready

[ChayimFriedman2]

You can remove the r-a changes from this PR. The other changes are not dependent on them. And also, I don't want to merge the r-a PR until the rustc PR is merged.

[kupiakos]

@ChayimFriedman2 Ah, I'd mistakenly thought that the r-a parser was actually being utilized by rustc here based on the rustc dev guide. But indeed this change isn't dependent on r-a. I've removed the r-a bits from this PR entirely as requested.