samples: add first Rust examples

The beginning of a set of Rust modules that showcase how Rust
modules look like and how to use the abstracted kernel features.

It also includes an example of a Rust host program with
several modules.

These samples also double as tests in the CI.

Reviewed-by: Kees Cook <[e-mail]>
Reviewed-by: Greg Kroah-Hartman <[e-mail]>
Co-developed-by: Alex Gaynor <[e-mail]>
Signed-off-by: Alex Gaynor <[e-mail]>
Co-developed-by: Finn Behrens <[e-mail]>
Signed-off-by: Finn Behrens <[e-mail]>
Co-developed-by: Wedson Almeida Filho <[e-mail]>
Signed-off-by: Wedson Almeida Filho <[e-mail]>
Co-developed-by: Milan Landaverde <[e-mail]>
Signed-off-by: Milan Landaverde <[e-mail]>
Signed-off-by: Miguel Ojeda <[e-mail]>

samples/Kconfig

···
 	  This demonstrates how a user may create their own CoreSight
 	  configurations and easily load them into the system at runtime.
 
+source "samples/rust/Kconfig"
+
 endif # SAMPLES
 
 config HAVE_SAMPLE_FTRACE_DIRECT

samples/Makefile

···
 obj-$(CONFIG_DEBUG_KMEMLEAK_TEST)	+= kmemleak/
 obj-$(CONFIG_SAMPLE_CORESIGHT_SYSCFG)	+= coresight/
 obj-$(CONFIG_SAMPLE_FPROBE)		+= fprobe/
+obj-$(CONFIG_SAMPLES_RUST)		+= rust/

samples/rust/Kconfig

···
+# SPDX-License-Identifier: GPL-2.0
+
+menuconfig SAMPLES_RUST
+	bool "Rust samples"
+	depends on RUST
+	help
+	  You can build sample Rust kernel code here.
+
+	  If unsure, say N.
+
+if SAMPLES_RUST
+
+config SAMPLE_RUST_MINIMAL
+	tristate "Minimal"
+	help
+	  This option builds the Rust minimal module sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_minimal.
+
+	  If unsure, say N.
+
+config SAMPLE_RUST_HOSTPROGS
+	bool "Host programs"
+	help
+	  This option builds the Rust host program samples.
+
+	  If unsure, say N.
+
+endif # SAMPLES_RUST

samples/rust/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+
+obj-$(CONFIG_SAMPLE_RUST_MINIMAL)		+= rust_minimal.o
+
+subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs

samples/rust/hostprogs/.gitignore

···
+# SPDX-License-Identifier: GPL-2.0
+
+single

samples/rust/hostprogs/Makefile

···
+# SPDX-License-Identifier: GPL-2.0
+
+hostprogs-always-y := single
+
+single-rust := y

samples/rust/hostprogs/a.rs

···
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust single host program sample: module `a`.
+
+pub(crate) fn f(x: i32) {
+    println!("The number is {}.", x);
+}

samples/rust/hostprogs/b.rs

···
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust single host program sample: module `b`.
+
+pub(crate) const CONSTANT: i32 = 42;

samples/rust/hostprogs/single.rs

···
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust single host program sample.
+
+mod a;
+mod b;
+
+fn main() {
+    println!("Hello world!");
+
+    a::f(b::CONSTANT);
+}

samples/rust/rust_minimal.rs

···
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust minimal sample.
+
+use kernel::prelude::*;
+
+module! {
+    type: RustMinimal,
+    name: b"rust_minimal",
+    author: b"Rust for Linux Contributors",
+    description: b"Rust minimal sample",
+    license: b"GPL",
+}
+
+struct RustMinimal {
+    numbers: Vec<i32>,
+}
+
+impl kernel::Module for RustMinimal {
+    fn init(_module: &'static ThisModule) -> Result<Self> {
+        pr_info!("Rust minimal sample (init)\n");
+        pr_info!("Am I built-in? {}\n", !cfg!(MODULE));
+
+        let mut numbers = Vec::new();
+        numbers.try_push(72)?;
+        numbers.try_push(108)?;
+        numbers.try_push(200)?;
+
+        Ok(RustMinimal { numbers })
+    }
+}
+
+impl Drop for RustMinimal {
+    fn drop(&mut self) {
+        pr_info!("My numbers are {:?}\n", self.numbers);
+        pr_info!("Rust minimal sample (exit)\n");
+    }
+}