How use extern crate in a doctest?

[FranklinChen]

I tried the following in a doctest and it failed. I do not know a workaround.

/// ```rust{.example}
/// extern crate core;
/// // stuff needing core...
/// ```

[steveklabnik]

I think Rustdoc doesn't allow this :/

[alexcrichton]

Can you paste the full error log from what you were trying to do? Rustdoc tests definitely allow for extern crate directives.

[FranklinChen]

$ cargo test
   Doc-tests tau

running 1 test
test TAU_0 ... FAILED

failures:

---- TAU_0 stdout ----
    <anon>:5:5: 5:23 error: "extern crate" declarations are not allowed here
    <anon>:5     extern crate core;
                 ^~~~~~~~~~~~~~~~~~
    error: aborting due to previous error
    task 'TAU_0' panicked at 'Box<Any>', /Users/rustbuild/src/rust-buildbot/slave/nightly-mac/build/src/libsyntax/diagnostic.rs:142

[alexcrichton]

Ah that's just part of how rustdoc wraps all tests in a fn main wrapper. You can get around that like so:

# extern crate core;
# fn main() {
// code to test
# }

[FranklinChen]

But I want to have multiple doctest examples in multiple places. It would be nice to be able to specify a "prelude" of declarations for use in multiple doctests. In any case, it would be nice to have an explicit semantics of how doctest is supposed to work. I confess that all I could do was look at random Rust source code and http://doc.rust-lang.org/rustdoc.html#testing-the-documentation and guess what to do. I really like the doctest concept.

[alexcrichton]

Yes unfortunately you'll have to duplicate the logic across doc tests, and we also do not have a formal specification for this as-is, but you can see the function here

[felixc]

Related to the request to specify a "prelude": rust-lang/rfcs#919