rust: support running documentation tests in-kernel

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
    }

So far, we were compiling and running them in the host as any other
Rust documentation test. However, that meant we could not run tests
that use kernel APIs (though we were compile-testing them, which was
already useful to keep the documentation in sync with the code).

Now, the documentation tests for the `kernel` crate are transformed
into a KUnit test suite and run within the kernel at boot time,
if enabled. This means now we can run tests that use kernel APIs.

This has also other side effects, such as now using the in-kernel
`alloc`, which does not have infallible allocations (previously,
we did use the in-tree `alloc` in the built sysroot, but it was
built via `build-std` without the same `cfg` set). This requires
fixing a few tests.

Another side effect is that Clippy is now run for doctests too,
which also requires a few changes in tests.

The CI is also changed to enable KUnit, our generated test suite
and to ensure they pass.

Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: ojeda

.github/workflows/ci.yaml

@@ -266,6 +266,9 @@ jobs:
           ! grep -v 'at mm/debug_vm_pgtable.c:' qemu-stdout.log | grep '] WARNING:'
 
       # Check
+      - run: |
+          grep '] ok 1 - rust_kernel_doctests$' qemu-stdout.log
+
       - run: |
           grep '] rust_minimal: Rust minimal sample (init)$' qemu-stdout.log
           grep '] rust_minimal: Am I built-in? false$'       qemu-stdout.log

.github/workflows/kernel-arm-debug.config

@@ -1802,7 +1802,7 @@ CONFIG_UNCOMPRESS_INCLUDE="debug/uncompress.h"
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 # CONFIG_FAULT_INJECTION is not set
 CONFIG_ARCH_HAS_KCOV=y
@@ -1827,6 +1827,7 @@ CONFIG_RUST_OPT_LEVEL_2=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-arm-release.config

@@ -1725,7 +1725,7 @@ CONFIG_UNCOMPRESS_INCLUDE="debug/uncompress.h"
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 # CONFIG_FAULT_INJECTION is not set
 CONFIG_ARCH_HAS_KCOV=y
@@ -1785,6 +1785,7 @@ CONFIG_RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-arm64-debug-thinlto.config

@@ -1450,7 +1450,7 @@ CONFIG_PID_IN_CONTEXTIDR=y
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 CONFIG_FUNCTION_ERROR_INJECTION=y
 # CONFIG_FAULT_INJECTION is not set
@@ -1476,6 +1476,7 @@ CONFIG_RUST_OPT_LEVEL_1=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-arm64-debug.config

@@ -1445,7 +1445,7 @@ CONFIG_PID_IN_CONTEXTIDR=y
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 CONFIG_FUNCTION_ERROR_INJECTION=y
 # CONFIG_FAULT_INJECTION is not set
@@ -1471,6 +1471,7 @@ CONFIG_RUST_OPT_LEVEL_1=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-arm64-release-thinlto.config

@@ -1368,7 +1368,7 @@ CONFIG_SAMPLE_RUST_HOSTPROGS=y
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 # CONFIG_FAULT_INJECTION is not set
 CONFIG_ARCH_HAS_KCOV=y
@@ -1393,6 +1393,7 @@ CONFIG_RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-arm64-release.config

@@ -1363,7 +1363,7 @@ CONFIG_SAMPLE_RUST_HOSTPROGS=y
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 # CONFIG_FAULT_INJECTION is not set
 CONFIG_ARCH_HAS_KCOV=y
@@ -1388,6 +1388,7 @@ CONFIG_RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-ppc64le-debug.config

@@ -1592,6 +1592,7 @@ CONFIG_RUST_OPT_LEVEL_0=y
 # CONFIG_RUST_OPT_LEVEL_Z is not set
 CONFIG_RUST_BUILD_ASSERT_ALLOW=y
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-ppc64le-release.config

@@ -1473,7 +1473,7 @@ CONFIG_PRINT_STACK_DEPTH=64
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 CONFIG_ARCH_HAS_KCOV=y
 CONFIG_CC_HAS_SANCOV_TRACE_PC=y
 # CONFIG_KCOV is not set
@@ -1496,6 +1496,7 @@ CONFIG_RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C=y
 # CONFIG_RUST_BUILD_ASSERT_ALLOW is not set
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
 CONFIG_RUST_BUILD_ASSERT_DENY=y
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking
 

.github/workflows/kernel-riscv64-debug.config

@@ -1296,7 +1296,7 @@ CONFIG_SAMPLE_RUST_HOSTPROGS=y
 #
 # Kernel Testing and Coverage
 #
-# CONFIG_KUNIT is not set
+CONFIG_KUNIT=y
 # CONFIG_NOTIFIER_ERROR_INJECTION is not set
 # CONFIG_FAULT_INJECTION is not set
 CONFIG_ARCH_HAS_KCOV=y
@@ -1320,6 +1320,7 @@ CONFIG_RUST_OPT_LEVEL_0=y
 # CONFIG_RUST_OPT_LEVEL_Z is not set
 CONFIG_RUST_BUILD_ASSERT_ALLOW=y
 # CONFIG_RUST_BUILD_ASSERT_WARN is not set
+CONFIG_RUST_KERNEL_KUNIT_TEST=y
 # end of Rust hacking
 # end of Kernel hacking