zephyr: docgen

Add a small docgen crate.  To help facilitate checking the docs, this
project can be built, and then `cargo doc` run there.  This includes the
symlink for the config file, because that is needed in order for this to
be able to build.

Signed-off-by: David Brown <[email protected]>

docgen/.cargo/config.toml

@@ -0,0 +1 @@
+../../../../../zephyr/build/rust/sample-cargo-config.toml

docgen/.gitignore

@@ -0,0 +1,7 @@
+# In this case, we do want the symlink checked in.  We'll assume we have the module in the standard
+# module place.
+#
+# On Windows, this symlink will just get checked out as a regular file and will have to be replaced
+# with a copy (or real symlinks enabled).  But, this shouldn't affect CI runs of the documentation,
+# which are done on Linux.
+!.cargo/

docgen/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+
+project(hello_rust_world)
+rust_cargo_application()

docgen/Cargo.toml

@@ -0,0 +1,16 @@
+# Copyright (c) 2024 Linaro LTD
+# SPDX-License-Identifier: Apache-2.0
+
+[package]
+# This must be rustapp for now.
+name = "rustapp"
+version = "3.7.0"
+edition = "2021"
+description = "A small application to generate documentation"
+license = "Apache-2.0 or MIT"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+zephyr = "3.7.0"

docgen/prj.conf

@@ -0,0 +1,5 @@
+# Copyright (c) 2024 Linaro LTD
+# SPDX-License-Identifier: Apache-2.0
+
+CONFIG_RUST=y
+CONFIG_RUST_ALLOC=y

docgen/src/lib.rs

@@ -0,0 +1,16 @@
+// Copyright (c) 2024 Linaro LTD
+// SPDX-License-Identifier: Apache-2.0
+
+#![no_std]
+
+use zephyr::printkln;
+
+// Reference the Zephyr crate so that the panic handler gets used.  This is only needed if no
+// symbols from the crate are directly used.
+extern crate zephyr;
+
+#[no_mangle]
+extern "C" fn rust_main() {
+    printkln!("Hello world from Rust on {}",
+              zephyr::kconfig::CONFIG_BOARD);
+}