Compile_fail for regular tests

[hkBst]

Would be great to be able to annotate regular tests as failing to compile.

 Is it going to come for regular tests? Such that I can write `#[compile_fail]` above my test functions?

Originally posted by @faulesocke in rust-lang/rust#12335 (comment)

[weihanglo]

Cargo doesn't know how to parse source code. Without the support from rustc, Cargo can do nothing on attributes unfortunately.

Apart from that, could you describe your expectation and scenario about #[compile_fail]? Is there any existing crate doing similar things a good candidate to integration into Cargo (compiletest-rs maybe)?

I would appreciate if you share more contexts. Thank you.

[epage]

A common crate for testing proc macros is trybuild

[hkBst]

@weihanglo My direct interest was in using this to assert that certain code does not pass type checking due to variance, but I think that only really makes sense for invariant types. For those one could write tests to assert that they are neither covariant nor contravariant.

[ehuss]

Another alternative is to use a doctest, which is compiled as a separate file and supports the compile_fail attribute to indicate that the example should fail to compile.

I think it is unlikely that this could be supported with an attribute, as I suspect it would be nearly impossible for rustc to "allow" a subsection of code to fail to compile.

Can you provide an example of the kind of test that you would want to assert a specific kind of variance?

[hkBst]

There are several invariant types in std that could have tests written for them, like so:

#[test]
// should fail to compile because Cell is invariant
fn cell_is_covariant_over_inner_type() {
    fn is_covariant<'a>(_c: &'a std::cell::Cell<&'a str>) {}
    let cell_of_static = std::cell::Cell::<&'static str>::new("static-string");
    let _ = is_covariant(&cell_of_static);
}