Document compile_fail for doc tests

[clarfonthey]

You can mark doctests as compile_fail but this isn't actually documented anywhere in the Rust book, even though it should be.

[frewsxcv]

should_panic is documented, compile_fail is not

https://doc.rust-lang.org/book/documentation.html#running-documentation-tests

[clarfonthey]

Updated the OP.

[steveklabnik]

probably part of #42322

[kennytm]

There is also ```test_harness, currently only used in The Book second edition.

[clarfonthey]

Is there actually a way for people to make compile_fail tests outside of documentation for non-rustc crates?

[Mark-Simulacrum]

The compiletest crate will allow writing them.

[clarfonthey]

@Mark-Simulacrum makes sense! To me it seems reasonable that you'd be able to create a test using an attribute like this:

#[compile_fail]
fn bad() {
    fn my_clone<T>(v: Vec<T>) -> Vec<T> {
        v.clone()
    }
}

Which would cover most use cases. To me it seems like this inconsistency in doc / other tests should be either addressed or more properly documented.

[frewsxcv]

There is a PR open for this: #43949.