samples: rust: add initial ro file system sample

The sample only registers the file system and doesn't really provide any
functionality beyond having it listed in `/proc/filesystems`. It is the
starting point though where new functionality will be used as it is
added to the abstractions.

Signed-off-by: Wedson Almeida Filho <walmeida@microsoft.com>

Author: wedsonaf

samples/rust/Kconfig

@@ -41,6 +41,16 @@ config SAMPLE_RUST_PRINT
 
 	  If unsure, say N.
 
+config SAMPLE_RUST_ROFS
+	tristate "Read-only file system"
+	help
+	  This option builds the Rust read-only file system sample.
+
+	  To compile this as a module, choose M here:
+	  the module will be called rust_rofs.
+
+	  If unsure, say N.
+
 config SAMPLE_RUST_HOSTPROGS
 	bool "Host programs"
 	help

samples/rust/Makefile

@@ -3,5 +3,6 @@
 obj-$(CONFIG_SAMPLE_RUST_MINIMAL)		+= rust_minimal.o
 obj-$(CONFIG_SAMPLE_RUST_INPLACE)		+= rust_inplace.o
 obj-$(CONFIG_SAMPLE_RUST_PRINT)			+= rust_print.o
+obj-$(CONFIG_SAMPLE_RUST_ROFS)			+= rust_rofs.o
 
 subdir-$(CONFIG_SAMPLE_RUST_HOSTPROGS)		+= hostprogs

samples/rust/rust_rofs.rs

@@ -0,0 +1,33 @@
+// SPDX-License-Identifier: GPL-2.0
+
+//! Rust read-only file system sample.
+
+use kernel::prelude::*;
+use kernel::{c_str, fs, macros::pin_data};
+
+module! {
+    type: RoFsModule,
+    name: "rust_rofs",
+    author: "Rust for Linux Contributors",
+    description: "Rust read-only file system sample",
+    license: "GPL",
+}
+
+struct RoFs;
+impl fs::FileSystem for RoFs {
+    const NAME: &'static CStr = c_str!("rust-fs");
+}
+
+#[pin_data]
+struct RoFsModule {
+    #[pin]
+    fs_reg: fs::Registration,
+}
+
+impl kernel::InPlaceModule for RoFsModule {
+    fn init(module: &'static ThisModule) -> impl PinInit<Self, Error> {
+        try_pin_init!(Self {
+            fs_reg <- fs::Registration::new::<RoFs>(module),
+        })
+    }
+}