New lint: early return variables

What it does

detect variables that are defined/initialized but not used before an early return

Lint Name

early-variable

Category

perf

Advantage

• Safes computation time
• Maybe safes variable initialization?

Drawbacks

Users might like to keep the position of their variable declaration

Example

fn main() {
    let x = 5;

    if let Some(arg) = env::args().nth(1) {
        match arg.as_str() {
            "-h" | "--help" => {
                return;
            }
            _ => {}
        }
    }

    dbg!(&x);
}

Could be written as:

fn main() {
    if let Some(arg) = env::args().nth(1) {
        match arg.as_str() {
            "-h" | "--help" => {
                return;
            }
            _ => {}
        }
    }

    let x = 5;

    dbg!(&x);
}

Real world example https://github.com/extrawurst/gitui/pull/660#discussion_r619650270

I tried implementing it, but I have no idea how to detect such a case and a foreign code base does not make it easier

Start with the Rust examples and the linked gitui discussion to clarify the intended lint behavior and its edge cases. Then read Clippy's existing lint implementations and tests to determine how an early return and variable-use analysis should be represented. Done means the lint reliably identifies the shown pattern, avoids false positives, and has coverage for the agreed cases.