How do you run doc tests in feature gated implementations? - testing

Question

I noticed that if you have a feature gate like #[feature(cfg = "nightly")] around a trait implementation, the doctest is skipped by a call to cargo test, even on a nightly rustc. I tried cargo test --all-features, but the results were the same. (Commenting out the gate results in the tests being run, of course.) I didn't see anything in the Rust Reference about this, either.
How do you ensure tests on feature gated implementations run?
For reference, here's my working Rust version.
rustc 1.17.0-nightly (c0b7112ba 2017-03-02)
binary: rustc
commit-hash: c0b7112ba246d96f253ba845d91f36c0b7398e42
commit-date: 2017-03-02

Answer 1

Features that are not part of the default set must always be explicitly opted into. You do that by passing an argument at the command line or by listing them in the [dependencies] section if they are dependencies.
src/lib.rs
/// ```
/// assert!(false);
/// ```
#[cfg(feature = "nightly")]
trait Foo {}
Cargo.toml
[package]
name = "wuzzy"
version = "0.1.0"
authors = ["An Devloper <an.devloper#example.com>"]
[features]
nightly = []
Running cargo test --features=nightly causes the assertion to fire, and cargo test ignores the test. This all works on stable Rust.
$ cargo test --features=nightly
Doc-tests wuzzy
running 1 test
test src/lib.rs - Foo (line 1) ... FAILED
test result: FAILED. 0 passed; 1 failed; 0 ignored; 0 measured
$ cargo test
Doc-tests wuzzy
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured