How to execute Rust code directly on Unix systems? (using the shebang)

This seems to work:

#!/bin/sh
//usr/bin/env rustc $0 -o a.out && ./a.out && rm ./a.out ; exit

fn main() {
    println!("Hello World!");
}

There's cargo-script. That also lets you use dependencies.

After installing cargo-script via cargo install cargo-script, you can create your script file (hello.rs) like this:

#!/usr/bin/env run-cargo-script

fn main() {
    println!("Hello World!");
}

To execute it, you need to:

$ chmod +x hello.rs
$ ./hello.rs
   Compiling hello v0.1.0 (file://~/.cargo/.cargo/script-cache/file-hello-d746fc676c0590b)
    Finished release [optimized] target(s) in 0.80 secs
Hello World!

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To use crates from crates.io, please see the tutorial in the README linked above.