Stabilize doctest-xcompile

[ehuss]

This stabilizes the doctest-xcompile feature by unconditionally enabling it.

Closes #7040
Closes #12118

What is being stabilized?

This changes it so that cargo will run doctests when using the --target flag for a target that is not the host. Previously, cargo would ignore doctests (and show a note if passing --verbose).

A wrapper for running the doctest can be specified with the target.*.runner configuration option (which is powered by the --test-runtool rustdoc flag). This would typically be something like qemu to run under emulation. It is my understanding that this should work just like running other kinds of tests.

Additionally, the target.*.linker config option is honored for using a custom linker.

Already stabilized in rustdoc is the ability to ignore tests per-target.

Motivation

The lack of doctest cross-compile support has always been simply due to the lack of functionality in rustdoc to support this. Rustdoc gained the ability to cross-compile doctests some time ago, but there were additional flags like the test runner that were not stabilized until just recently.

This is intended to ensure that projects have full test coverage even when doing cross-compilation. It can be surprising to some that this was not happening, particularly since cargo is silent about it.

Risks

The cargo team had several conversations about how to roll out this feature. Ultimately we decided to enable it unconditionally with the understanding that most projects will probably want to have their doctests covered, and that any breakage will be a local concern that can be resolved by either fixing the test or ignoring the target.

Tests in rust-lang/rust run into this issue, particularly on android, and those will need to be fixed before this reaches beta. This is something I am looking into.

Some cross-compiling scenarios may need codegen flags that are not supported. It's not clear how common this will be, or if ignoring will be a solution, or how difficult it would be to update rustdoc and cargo to support these. Additionally, the split between RUSTFLAGS and RUSTDOCFLAGS can be cumbersome.

Implementation history

• Allow cross-compiling doctests rust#60387 -- Support added to rustdoc to support the --target flag and runtool and per-target-ignores.
• Added ability to crosscompile doctests #6892 -- Initial support in cargo.
• Add documentation for the -Zdoctest-xcompile feature #7391 -- Added unstable documentation.
• Fix target for doc test cross compilation #8094 -- Fix target for doc test cross compilation
• Fix doctests not running with --target=HOST. #8358 -- Fixed regression with --target=HOST not working on stable.
• Add a note about doctest xcompile. #10132 -- Added note about doctests not running (under --verbose).
• rustdoc: Fixes with --test-run-directory and relative paths. rust#112751 -- Fixed --test-run-directory interaction with --test-runtool.
• Stabilize flags for doctest cross compilation rust#137096 -- Stabilization (and rename) of the rustdoc --test-runtool and --test-runtool-arg CLI args, and drops --enable-per-target-ignores unconditionally enabling it.

Test coverage

Cargo tests:

• artifact_dep::cross_doctests_works_with_artifacts -- Checks that doctest has access to the artifact dependencies.
• build_script::duplicate_script_with_extra_env -- Checks that build-script env and cfg values are correctly handled on host versus target when cross running doctests.
• cross_compile::cross_tests -- Basic test that cross-compiled tests work.
• cross_compile::doctest_xcompile_linker -- Checks that the linker config argument works.
• custom_target::custom_target_minimal -- Checks that .json targets work with rustdoc cross tests.
• test::cargo_test_doctest_xcompile_ignores -- Checks the ignore-* syntax works.
• test::cargo_test_doctest_xcompile_runner -- Checks runner with cross doctests.
• test::cargo_test_doctest_xcompile_no_runner -- Checks cross doctests without a runner.

Rustdoc tests:

• run-make/doctest-runtool -- Tests behavior of --test-run-directory with relative paths of the runner.
• rustdoc/doctest/doctest-runtool -- Tests for --test-runtool and --test-runtool-arg.

Future concerns

There have been some discussions (rust-lang/testing-devex-team#5) about changing how doctests are driven. My understanding is that stabilizing this should not affect those plans, since if cargo becomes the driver, it will simply need to build things with --target and use the appropriate runner.

Change notes

This PR changed tests a little:

• artifact_dep::no_cross_doctests_works_with_artifacts was changed now that doctests actually work.
• cross_compile::cross_tests was changed to properly check doctests.
• cross_compile::no_cross_doctests dropped since it is no longer relevant.
• standard_lib::doctest didn't need -Zdoctest-xcompile since -Zbuild-std no longer uses a target.
• test::cargo_test_doctest_xcompile was removed since it is a duplicate of cross_compile::cross_tests

I think this should probably wait until the next release cutoff, moving this to 1.89 (will update the PR accordingly if that happens).

[rustbot]

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[ehuss]

@rust-lang/cargo I'm proposing to stabilize this, though I think it should wait until 1.89. I figured I'd get this process started in case anyone had any concerns or needs time to look at it.

@rfcbot fcp merge

[rfcbot]

Team member @ehuss has proposed to merge this. The next step is review by the rest of the tagged team members:

• @Eh2406
• @Muscraft
• @Rustin170506
• @arlosi
• @ehuss
• @epage
• @joshtriplett
• @weihanglo

No concerns currently listed.

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[rfcbot]

🔔 This is now entering its final comment period, as per the review above. 🔔