ulib: Add a Rust example

The U-Boot library can be used from Rust fairly easily. Add an example
for this, following along the lines of the existing ulib example.

Note that the new way of representing C strings is not used for now,
since it is not available in v1.75 of cargo, as shipped by Ubuntu 24.04

Co-developed-by: Claude <noreply@anthropic.com>
Co-developed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>

examples/rust/README.md

@@ -0,0 +1,120 @@
+# Technical Notes - Rust U-Boot Integration
+
+This directory contains a simple Rust implementation for using U-Boot's
+library. Both static and dynamic linking (of libu-boot) are available.
+
+This directory is intended to be used separately from U-Boot's build system,
+e.g. by copying it somewhere else.
+
+For comprehensive documentation, see :doc:`/doc/develop/ulib`.
+
+## Build System Success
+
+The following components are included:
+
+### 1. u-boot-sys Crate
+
+The `lib/rust/u-boot-sys` crate provides FFI bindings organized into modules:
+- `uboot_lib`: Library management functions (`ulib_*`)
+- `uboot_api`: U-Boot API functions (`ub_*`)
+- `os`: OS abstraction functions (`os_*`)
+
+Functions are re-exported at the crate root for convenience.
+
+### 2. Example Program Structure
+
+- `demo.rs`: Main program using the u-boot-sys crate
+- `rust_helper.rs`: Helper functions similar to C demo_helper.c
+- Modular design with proper separation of concerns
+
+### 3. Build Configuration
+- `build.rs` configures linking with both static and dynamic libraries
+- Uses `--whole-archive` for static linking to preserve U-Boot linker lists
+- Links required system libraries (pthread, dl, rt, SDL2) with proper ordering
+- Uses U-Boot's existing stack-protection implementation from stackprot.c
+
+### 4. Integration
+- Makefile can be use standalone or from U-Boot build system
+- Cargo project depends on `u-boot-sys` crate from `../../lib/rust`
+- Examples are built automatically as part of U-Boot's build system
+
+### 5. Runtime Execution ✅
+- `demo` (dynamic) and `demo-static` executables work the same
+- Calls library init with `ulib_init()`
+- File operations using U-Boot's OS abstraction layer
+- Uses renamed U-Boot library functions (`ub_printf`)
+- Clean shutdown with `ulib_uninit()`
+
+## Architecture Overview
+
+### u-boot-sys Crate Design
+- Located in `lib/rust/` for reuse across U-Boot Rust projects
+- Follows Rust `*-sys` naming conventions for FFI binding crates
+- Intended to provide a safe, well-documented interfaces to U-Boot functions
+- Organized into logical modules with re-exports for convenience
+
+### Dependency Structure
+```
+examples/rust/demo → u-boot-sys → U-Boot C library
+                                      ↓
+                              libu-boot.so/.a
+```
+
+### Build System Integration
+- Uses standard Cargo dependency resolution
+- Makefile provides U-Boot-specific environment setup
+- Compatible with both U-Boot's build system and standalone Cargo builds
+
+## Features Demonstrated
+
+- **FFI Integration**: Shows how to call U-Boot C functions from Rust
+- **Library Initialization**: Proper `ulib_init()` and `ulib_uninit()` usage
+- **File Operations**: Using U-Boot's `os_open()`, `os_fgets()`, `os_close()`
+- **Print Functions**: Using U-Boot's `ub_printf()` vs Rust's `println!()`
+- **Mixed Languages**: Combining Rust and C functionality seamlessly
+
+## Prerequisites
+
+1. **Rust toolchain**: Install from https://rustup.rs/
+2. **U-Boot library**: Build U-Boot sandbox with library support:
+   ```bash
+   make sandbox_defconfig
+   make
+   ```
+
+## Usage Examples
+
+```bash
+# Ensure U-Boot is built first
+# Note this builds rust examples at /tmp/b/sandbox/example/rust
+make O=/tmp/b/sandbox sandbox_defconfig all
+
+# Build both versions using Makefile
+cd examples/rust
+make UBOOT_BUILD=/tmp/b/sandbox srctree=../..
+
+# Or build directly with Cargo
+env UBOOT_BUILD=/tmp/b/sandbox cargo build --release
+
+# Test dynamic version
+LD_LIBRARY_PATH=/tmp/b/sandbox ./demo
+
+# Test static version
+./demo-static
+
+# Run tests
+make test
+```
+
+## License
+
+GPL-2.0+ (same as U-Boot)
+
+Note: Linking with U-Boot's GPL-2.0+ library makes your program subject to
+GPL licensing terms.
+
+## Documentation and Further Reading
+
+- See `doc/develop/ulib.rst`
+- **C examples**: Located in `examples/ulib/` for comparison
+- **u-boot-sys crate**: API documentation via `cargo doc` in `lib/rust/`