--cfg accepts invalid cfgs

[brson]

List meta items are not allowed in cfg specs because the compiler reserves them for built-in predicates like any.

So for example you can't write

foo.rs:1:7: 1:16 error: invalid predicate `foo`
foo.rs:1 #[cfg(foo(bar))]
               ^~~~~~~~~

You can though write `rustc --cfg "foo(bar))". This should be similarly rejected.

cc #31278

[brson]

This is probably pretty easy. Comparing the parsing in libsyntax with the parsing in librustc_driver and fix the latter to be like the former. Add a test to src/tests/compile-fail with the compile-flags: directive set to --cfg whatever.

[kamalmarhubi]

Should rustc --cfg any(foo) be accepted?

[kamalmarhubi]

I'm assuming it should not for now.

[brson]

@kamalmarhubi No it should not.

[kamalmarhubi]

Excellent, less fixup work for me :-)