Date:   Wed, 14 Jun 2023 20:19:37 -0400
From:   Matt Gilbride <mattgilbride@...gle.com>
To:     Miguel Ojeda <ojeda@...nel.org>
Cc:     David Gow <davidgow@...gle.com>,
        Brendan Higgins <brendan.higgins@...ux.dev>,
        Wedson Almeida Filho <wedsonaf@...il.com>,
        Alex Gaynor <alex.gaynor@...il.com>,
        Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
        Björn Roy Baron <bjorn3_gh@...tonmail.com>,
        Benno Lossin <benno.lossin@...ton.me>,
        Alice Ryhl <aliceryhl@...gle.com>,
        Andreas Hindborg <nmi@...aspace.dk>,
        Philip Li <philip.li@...el.com>, kunit-dev@...glegroups.com,
        linux-kselftest@...r.kernel.org, rust-for-linux@...r.kernel.org,
        linux-kernel@...r.kernel.org, patches@...ts.linux.dev
Subject: Re: [PATCH 5/6] rust: support running Rust documentation tests as
 KUnit ones

> Rust has documentation tests: these are typically examples of
> usage of any item (e.g. function, struct, module...).
>
> They are very convenient because they are just written
> alongside the documentation. For instance:
>
>     /// Sums two numbers.
>     ///
>     /// ```
>     /// assert_eq!(mymod::f(10, 20), 30);
>     /// ```
>     pub fn f(a: i32, b: i32) -> i32 {
>         a + b
>     }
>
> In userspace, the tests are collected and run via `rustdoc`.
> Using the tool as-is would be useful already, since it allows
> to compile-test most tests (thus enforcing they are kept
> in sync with the code they document) and run those that do not
> depend on in-kernel APIs.
>
> However, by transforming the tests into a KUnit test suite,
> they can also be run inside the kernel. Moreover, the tests
> get to be compiled as other Rust kernel objects instead of
> targeting userspace.
>
> On top of that, the integration with KUnit means the Rust
> support gets to reuse the existing testing facilities. For
> instance, the kernel log would look like:
>
>     KTAP version 1
>     1..1
>         KTAP version 1
>         # Subtest: rust_doctests_kernel
>         1..59
>         # Doctest from line 13
>         ok 1 rust_doctest_kernel_build_assert_rs_0
>         # Doctest from line 56
>         ok 2 rust_doctest_kernel_build_assert_rs_1
>         # Doctest from line 122
>         ok 3 rust_doctest_kernel_init_rs_0
>         ...
>         # Doctest from line 150
>         ok 59 rust_doctest_kernel_types_rs_2
>     # rust_doctests_kernel: pass:59 fail:0 skip:0 total:59
>     # Totals: pass:59 fail:0 skip:0 total:59
>     ok 1 rust_doctests_kernel
>
> Therefore, add support for running Rust documentation tests
> in KUnit. Some other notes about the current implementation
> and support follow.
>
> The transformation is performed by a couple scripts written
> as Rust hostprogs.
>
> Tests using the `?` operator are also supported as usual, e.g.:
>
>     /// ```
>     /// # use kernel::{spawn_work_item, workqueue};
>     /// spawn_work_item!(workqueue::system(), || pr_info!("x"))?;
>     /// # Ok::<(), Error>(())
>     /// ```
>
> The tests are also compiled with Clippy under `CLIPPY=1`, just like
> normal code, thus also benefitting from extra linting.
>
> The names of the tests are currently automatically generated.
> This allows to reduce the burden for documentation writers,
> while keeping them fairly stable for bisection. This is an
> improvement over the `rustdoc`-generated names, which include
> the line number; but ideally we would like to get `rustdoc` to
> provide the Rust item path and a number (for multiple examples
> in a single documented Rust item).
>
> In order for developers to easily see from which original line
> a failed doctests came from, a KTAP diagnostic line is printed
> to the log. In the future, we may be able to use a proper KUnit
> facility to append this sort of information instead.
>
> A notable difference from KUnit C tests is that the Rust tests
> appear to assert using the usual `assert!` and `assert_eq!`
> macros from the Rust standard library (`core`). We provide
> a custom version that forwards the call to KUnit instead.
> Importantly, these macros do not require passing context,
> unlike the KUnit C ones (i.e. `struct kunit *`). This makes
> them easier to use, and readers of the documentation do not need
> to care about which testing framework is used. In addition, it
> may allow us to test third-party code more easily in the future.
>
> However, a current limitation is that KUnit does not support
> assertions in other tasks. Thus we presently simply print an
> error to the kernel log if an assertion actually failed. This
> should be revisited to properly fail the test, perhaps saving
> the context somewhere else, or letting KUnit handle it.
>
> Signed-off-by: Miguel Ojeda <ojeda@...nel.org>

Tested-by: Matt Gilbride <mattgilbride@...gle.com>

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