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/Makefile

@@ -0,0 +1,95 @@
+# SPDX-License-Identifier: GPL-2.0+
+#
+# Makefile for U-Boot Rust example
+#
+# Copyright 2025 Canonical Ltd.
+# Written by Simon Glass <simon.glass@canonical.com>
+
+# This Makefile integrates with the U-Boot build system, similar to
+# examples/ulib
+#
+# Usage: cd examples/rust; make UBOOT_BUILD=/tmp/b/sandbox srctree=../..
+#
+
+# Default paths - can be overridden
+UBOOT_BUILD ?= /tmp/b/sandbox
+srctree ?= ../..
+OUTDIR ?= .
+
+# Export for build.rs
+export UBOOT_BUILD
+
+# Common source dependencies
+RUST_SOURCES = Cargo.toml src/demo.rs build.rs
+
+# Absolute output directory for Cargo builds
+OUTDIR_ABS = $(abspath $(OUTDIR))
+
+# Ensure 'cargo' is available
+CARGO := $(shell command -v cargo 2> /dev/null)
+ifeq ($(CARGO),)
+$(error "Cargo not found. Please install Rust toolchain from \
+https://rustup.rs/")
+endif
+
+# Default target - build both static and dynamic versions like examples_ulib
+all: $(OUTDIR)/demo $(OUTDIR)/demo_static
+
+# Create output directory
+$(OUTDIR):
+	@mkdir -p $(OUTDIR)
+
+# Build dynamic version (links with libu-boot.so)
+$(OUTDIR)/demo: $(RUST_SOURCES) $(UBOOT_BUILD)/libu-boot.so | $(OUTDIR)
+	@echo "Building Rust demo (dynamic) with library from $(UBOOT_BUILD)"
+	@echo "OUTDIR=$(OUTDIR), abspath=$(OUTDIR_ABS)"
+	@if [ ! -f "$(UBOOT_BUILD)/libu-boot.so" ]; then \
+		echo "No shared library at $(UBOOT_BUILD)/libu-boot.so" >&2; \
+		echo "Please build U-Boot: make sandbox_defconfig && make" >&2; \
+		exit 1; \
+	fi
+	@UBOOT_DYNAMIC=1 $(CARGO) build --target-dir \
+		$(OUTDIR_ABS)/build-dynamic --release --bin demo -q
+	@cp $(OUTDIR_ABS)/build-dynamic/release/demo $(OUTDIR)/
+
+# Build static version (links with libu-boot.a)
+$(OUTDIR)/demo_static: $(RUST_SOURCES) $(UBOOT_BUILD)/libu-boot.a | $(OUTDIR)
+	@echo "Building Rust demo (static) with library from $(UBOOT_BUILD)"
+	@if [ ! -f "$(UBOOT_BUILD)/libu-boot.a" ]; then \
+		echo "No static library at $(UBOOT_BUILD)/libu-boot.a" >&2; \
+		echo "Please build U-Boot: make sandbox_defconfig && make" >&2; \
+		exit 1; \
+	fi
+	@$(CARGO) build --target-dir $(OUTDIR_ABS)/build-static --release \
+		--bin demo -q
+	@cp $(OUTDIR_ABS)/build-static/release/demo $(OUTDIR)/demo_static
+
+demo: $(OUTDIR)/demo
+
+# Test the programs
+test: $(OUTDIR)/demo $(OUTDIR)/demo_static
+	@echo "Testing Rust demos..."
+	@echo "Testing dynamic version:"
+	@LD_LIBRARY_PATH="$(UBOOT_BUILD)" $(OUTDIR)/demo
+	@echo "Testing static version:"
+	@$(OUTDIR)/demo_static
+
+# Clean build artifacts
+clean:
+	$(CARGO) clean
+	@if [ "$(OUTDIR)" != "." ]; then \
+		rm -rf $(OUTDIR_ABS)/build-dynamic \
+			$(OUTDIR_ABS)/build-static $(OUTDIR)/demo \
+			$(OUTDIR)/demo_static; \
+	else \
+		rm -f demo demo_static; \
+	fi
+
+# Show cargo version and info
+info:
+	@echo "Rust toolchain information:"
+	$(CARGO) --version
+	@echo "U-Boot build directory: $(UBOOT_BUILD)"
+	@echo "U-Boot source tree: $(srctree)"
+
+.PHONY: all test clean info