examples: Add Rust example

examples/rust-hello-world/Cargo.toml

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+[package]
+name = "rust-hello-world"
+version = "0.1.0"
+authors = ["Christian Amsüss <chrysn@fsfe.org>"]
+edition = "2018"
+resolver = "2"
+
+[lib]
+crate-type = ["staticlib"]
+
+[dependencies]
+riot-wrappers = "0.7"

examples/rust-hello-world/Makefile

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+# name of your application
+APPLICATION = hello-world
+
+# If no BOARD is found in the environment, use this default:
+BOARD ?= native
+
+# This has to be the absolute path to the RIOT base directory:
+RIOTBASE ?= $(CURDIR)/../..
+
+# Comment this out to disable code in RIOT that does safety checking
+# which is not needed in a production environment but helps in the
+# development process:
+DEVELHELP ?= 1
+
+# Change this to 0 show compiler invocation lines by default:
+QUIET ?= 1
+
+# The name of crate (as per Cargo.toml package name, but with '-' replaced with '_')
+APPLICATION_RUST_MODULE = rust_hello_world
+BASELIBS += $(APPLICATION_RUST_MODULE).module
+
+FEATURES_REQUIRED += rust_target
+
+include $(RIOTBASE)/Makefile.include

examples/rust-hello-world/README.md

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+Hello World!
+============
+
+This is a basic example how to use Rust to write your RIOT application.
+It prints out the famous text `Hello World!`.
+
+This example should foremost give you an overview how an application built
+completely in Rust is structured:
+
+* The Makefile resembles the regular application Makefile, see ../hello-world/
+  for more introduction to that.
+
+* The Cargo.toml file describes the Rust code, and declares its dependencies.
+
+  Prominently, it contains a `[lib]` / `crate-type = ["staticlib"]` section,
+  which is necessary for how RIOT later links together the C and Rust portions.
+
+* The file src/lib.rs (and any modules referenced by it) contain Rust code to
+  be run.
+
+  It uses the `riot_main!` macro provided by the riot-wrappers crate to declare
+  the entry point of the program.
+
+The code itself looks like the usual Rust hello-world example.

examples/rust-hello-world/src/lib.rs

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+// Copyright (C) 2020 Christian Amsüss
+//
+// This file is subject to the terms and conditions of the GNU Lesser
+// General Public License v2.1. See the file LICENSE in the top level
+// directory for more details.
+#![no_std]
+
+use riot_wrappers::riot_main;
+use riot_wrappers::println;
+
+riot_main!(main);
+
+fn main() {
+    println!("Hello Rust!");
+}