rustdoc: passing --nocapture to the test runner still captures (and hides) test output

[RalfJung]

I originally reported this against cargo at rust-lang/cargo#10792, but was told this is more of a rustdoc problem.

The goal is that when I have a diverging doctest, like the following silly one:

/// ```rust
/// eprintln!("stderr");
/// println!("stdout");
/// loop {}
/// ```
pub fn uwu() {}

Then I want to be able to use cargo test --doc -- --nocapture to debug what is going on by seeing the output of the test. But that doesn't happen, output is still captured. --nocapture seems to only control whether the output of rustc as it builds the test is captured or printed; it has no effect on the stdout/stderr of the test itself. That is quite inconsistent with regular unit tests where it controls the output of the test itself.

Instead I have to do RUSTDOCFLAGS="-Zunstable-options --nocapture" cargo test --doc, which is rather hard to find.

To make things even more surprising, RUSTDOCFLAGS="-Zunstable-options --nocapture" cargo test --doc -- --nocapture fails with an error: "Option 'nocapture' given more than once".

Cc @rust-lang/rustdoc

[RalfJung]

See rust-lang/cargo#10792 (comment) for a proposed patch by @ehuss

[camelid]

IIRC (though I may be incorrect) rustdoc's --nocapture also doesn't print the output until it's finished executing, so it wouldn't work to debug hangs.

[RalfJung]

I tried it, and it does print output immediately for me.

[camelid]

Hmm, I seem to recall that with --nocapture it captured the output and printed it at the end, but I might be confusing it with something else.

[dan-da]

Instead I have to do RUSTDOCFLAGS="-Zunstable-options --nocapture" cargo test --doc, which is rather hard to find.

Trying this with 1.74.1 (stable) toolchain gives this error:

error: the option `Z` is only accepted on the nightly compiler

error: the option `nocapture` is only accepted on the nightly compiler

help: consider switching to a nightly toolchain: `rustup default nightly`

So it would seem there is no way to use --nocapture for doc tests in stable rust.

The only workaround I know of is to temporarily place a panic!() call in the test, so it will fail and print the output.

This workaround is of course tedious and not satisfactory.

[cprice404]

My experience is the same as what @dan-da described. It would be extremely helpful to be able to put printlns in the doctests during development.