unused-variable — Unused variable

Why this fires

A let-bound name (or a function parameter) is never read after it is introduced. Either the binding is leftover from an earlier version of the code, or it shadows a name the rest of the body actually wanted to use.

fn greet(name: str) {
    let greeting = "Hello"     // warning: unused-variable `greeting`
    print("Hi, ${name}")
}

Fix it

There are three sensible responses, depending on the intent:

1. Delete the binding

If the value is not needed, remove it:

fn greet(name: str) {
    print("Hi, ${name}")
}

2. Use it

If the variable was supposed to be used and the lint surfaced a typo or a forgotten reference, fix the reference:

fn greet(name: str) {
    let greeting = "Hello"
    print("${greeting}, ${name}")
}

3. Mark it intentionally unused with `_`

Parameters required by an interface or trait — and let bindings whose side effects matter even if the value does not — should be prefixed with _. The lint treats _-prefixed names as deliberately unused:

fn on_event(_event: Event, ctx: Context) {
    ctx.tick()
}

For a discarded RHS, the bare _ pattern also works:

let _ = compute_and_log()

Why `_` instead of suppressing the rule

_-prefix is a permanent code-level signal that the next reader sees, while @diagnostic(off, "unused-variable") hides the lint and (more importantly) hides the next time you accidentally orphan a binding. Prefer _.

Unused variable