libsyntax: forbid visibility modifiers for enum variants

[matklad]

fixes #28433

This just removes visibility parsing from the parser. No custom error message is provided: I guess default "expected identifier, found keyword pub" is already good.

So the ast::Variant_ vis is always Inherited. I guess it would be possible to get rid of this field now, but I'm not sure that it is in scope of this PR.

The tests are changed as follows:

• issue-3993-2.rs is removed (it is not relevant any more)
• issue-28433.rs is added (it's basically simplified issue-3993-2.rs)
• nothing is done to issue-3993. It does not seem relevant to pub/priv qualifiers on individual variants #3993 git hub issue
• useless-priv.rs is renamed to useless-pub.rs and a case about enum is dropped

[rust-highfive]

r? @huonw

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[nagisa]

This test will not pass because of #28439 I think.

[matklad]

How can I run this this test fater than make -j8 check?

[nagisa]

I’d suggest waiting for travis build to finish for now.

Otherwise the fastest way without editing makefiles is to do make -j8 check-stage2-cfail TESTNAME=issue-28433 I think.

[sfackler]

cc @alexcrichton not sure if we'd want to warn for a bit before adjusting the parser.

[alexcrichton]

I've started a crater build to help evaluate the impact here. I personally be fine just turning this into a hard error immediately and sending PRs to any affected crates to update the syntax.

cc @rust-lang/lang, a semi-language-change but mostly just a bugfix

[alexcrichton]

Closing in favor of #28442