Implement code generation example

Main goal is to create simple crate that generates code while compilation (see `build.rs`)
and includes it via `include!(concat!(env!("OUT_DIR"), "/generated-code.rs"));`.
It should help to create proper tests in intellij-rust plugin

Author: Undin

.gitignore

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+/target
+**/*.rs.bk
+.idea
+*.iml

Cargo.lock

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+[package]
+name = "code-generation-example"
+version = "0.1.0"
+authors = ["Arseniy Pendryak <[email protected]>"]
+edition = "2018"
+
+license = "MIT"
+description = "Simple lib with code generation example"
+homepage = "https://github.com/intellij-rust/code-generation-example"
+repository = "https://github.com/intellij-rust/code-generation-example"
+documentation = "https://docs.rs/code-generation-example"

Cargo.toml

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+use std::env;
+use std::fs::File;
+use std::io::Write;
+use std::path::Path;
+
+fn main() {
+    let out_dir = env::var("OUT_DIR").unwrap();
+    let dest_path = Path::new(&out_dir).join("hello.rs");
+    let mut f = File::create(&dest_path).unwrap();
+
+    f.write_all(b"
+        pub fn message() -> &'static str {
+            \"Hello, World!\"
+        }",
+    )
+    .unwrap();
+}

build.rs

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+use code_generation_example::message;
+
+fn main() {
+    println!("{}", message());
+}

examples/example.rs

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+include!(concat!(env!("OUT_DIR"), "/hello.rs"));