Top-level statements

The entry file may skip the fn main wrapper. Write statements at file scope and the file becomes the body of an implicit fn main():

println!("Hello World")

Functions and other items declared alongside top-level statements are hoisted to file scope, exactly as if you had written them outside main:

fn greet(name: &String) -> String { format!("hi, {name}") }

let who = "ada"
println!("{}", greet(&who))

Mental model

The entry file is implicitly wrapped in fn main(). This is the model Swift, Kotlin scripts, and Deno/Node ES modules use. Top-level let bindings are locals of that main; share state across functions with const / static.

Return type

If any top-level statement uses ?, the implicit main returns Result<(), errors::Error> so propagation works:

use std::fs

let text = fs::read_to_string(&"config.toml")?
println!("{}", text)

Otherwise main returns (). To set a process exit code, call std::process::exit(n):

use std::process

if bad_input {
    eprintln!("usage: tool <path>")
    process::exit(2)
}

Rules

• Only the entry file may carry top-level statements (the file run directly, or [project] entry). Module / library files contain items only.
• A file uses one entry form: mixing top-level statements with an explicit fn main is an error.