Lints - The rustc book

# Lints

In software, a "lint" is a tool used to help improve your source code. The Rust compiler contains a number of lints, and when it compiles your code, it will also run the lints. These lints may produce a warning, an error, or nothing at all, depending on how you've configured things.

Here's a small example:

    $ cat main.rs
    fn main() {
        let x = 5;
    }
    $ rustc main.rs
    warning: unused variable: `x`
     --> main.rs:2:9
      |
    2 |     let x = 5;
      |         ^
      |
      = note: #[warn(unused_variables)] on by default
      = note: to avoid this warning, consider using `_x` instead

This is the `unused_variables` lint, and it tells you that you've introduced a variable that you don't use in your code. That's not wrong, so it's not an error, but it might be a bug, so you get a warning.