docs: rust: document unit testing

Explain how to run unit tests and documentation tests.

Note that the documentation uses "--arch=x86_64" to run KUnit tests
because UML is not working at the moment [1].

[1] Rust-for-Linux/linux#881
Signed-off-by: José Expósito <[email protected]>

Author: JoseExposito

Documentation/rust/index.rst

@@ -13,6 +13,7 @@ in the kernel, please read the quick-start.rst guide.
     general-information
     coding-guidelines
     arch-support
+    unit-testing
 
 .. only::  subproject and html
 

Documentation/rust/unit-testing.rst

@@ -0,0 +1,49 @@
+.. SPDX-License-Identifier: GPL-2.0
+
+Unit Testing
+============
+
+Like in any other Rust project it is possible to write and run unit tests and
+documentation tests in the kernel.
+
+Running Unit Tests
+------------------
+
+Unit tests in the kernel are identical to user-space Rust tests:
+
+.. code-block:: rust
+
+	#[cfg(test)]
+	mod tests {
+	    #[test]
+	    fn it_works() {
+	        let result = 2 + 2;
+	        assert_eq!(result, 4);
+	    }
+	}
+
+And can be run using the ``rusttest`` Make target:
+
+.. code-block:: bash
+
+	$ make LLVM=1 rusttest
+
+Running Documentation Tests
+---------------------------
+
+Like in user-space, it is possible to write documentation tests:
+
+.. code-block:: rust
+
+	/// ```
+	/// let result = 2 + 2;
+	/// assert_eq!(result, 4);
+	/// ```
+
+Documentation tests use KUnit and, thanks to the configuration file present in
+``rust/.kunitconfig``, it is possible to run them using the ``kunit.py`` tool:
+
+.. code-block:: bash
+
+	$ ./tools/testing/kunit/kunit.py run --kunitconfig=rust \
+	  --make_options LLVM=1 --arch=x86_64