How to disable unused code warnings in Rust?

Question

struct SemanticDirection;

fn main() {}

warning: struct is never used: `SemanticDirection`
 --> src/main.rs:1:1
  |
1 | struct SemanticDirection;
  | ^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: #[warn(dead_code)] on by default

I will turn these warnings back on for anything serious, but I am just tinkering with the language and this is driving me bats.

I tried adding #[allow(dead_code)] to my code, but that did not work.

Answer 1

You can either:

• Add an allow attribute on a struct, module, function, etc.:

  #[allow(dead_code)]
  struct SemanticDirection;
  

• Add a crate-level allow attribute; notice the !:

  #![allow(dead_code)]
  

• Pass it to rustc:

  rustc -A dead_code main.rs
  

• Pass it using cargo via the RUSTFLAGS environment variable:

  RUSTFLAGS="$RUSTFLAGS -A dead_code" cargo build
  

Answer 2

Another way to disable this warning is to prefix the identifier by _:

struct _UnusedStruct {
    _unused_field: i32,
}

fn main() {
    let _unused_variable = 10;
}

This can be useful, for instance, with an SDL window:

let _window = video_subsystem.window("Rust SDL2 demo", 800, 600);

Prefixing with an underscore is different from using a lone underscore as the name. Doing the following will immediately destroy the window, which is unlikely to be the intended behavior.

let _ = video_subsystem.window("Rust SDL2 demo", 800, 600);

Answer 3

Making the code public also stops the warnings; you'll need to make the enclosing mod's public too.

This makes sense when you're writing a library: your code is "unused" internally because it's intended to be used by client code.