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

Put these two lines on the top of the file.

#![allow(dead_code)]
#![allow(unused_variables)]

Answer 4

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.

Answer 5

also as an addition: rust provides four levels of lints (allow, warn, deny, forbid).

https://doc.rust-lang.org/rustc/lints/levels.html#lint-levels

Answer 6

• Directly way to just put the following in the head of the file

  #![allow(dead_code, unused_variables)]

  • The dead_code lint detects unused, unexported items.
  • The unused_variables lint detects variables which are not used in any way.

• More simple way is to put the following in the head of the file

  #![allow(unused)]

Ref: rust lint list

Answer 7

You can add the #[allow(dead_code)] attribute to the struct definition like so:

#[allow(dead_code)]
struct SemanticDirection;

Or you can disable the warning for the entire file by adding the attribute at the top of the file, like so:

#![allow(dead_code)]

struct SemanticDirection;

But these attributes only work if you have the dead_code lint enabled. By default, the dead_code lint is enabled in Rust, but you can disable it by adding the following to the top of your code:

#![deny(dead_code)]

This will disable the dead_code lint for the entire file.

It's generally a good idea to keep the dead_code lint enabled, as it can help you catch mistakes in your code and ensure that you are not introducing unnecessary code into your project. However, it can be annoying when you are just experimenting and trying out different things, so it's understandable if you want to disable it in those cases.

Answer 8

For unused functions, you should make the function public, but watch out. If the struct isn't public, then you'll still get the error as in here:

//this should be public also
struct A{
   A{}
}

impl A {
    pub fn new() -> A {

    }
}

Or if you don't want it to be public, you should put #[allow(unused)]

Answer 9

You can always disable unused variables/functions by adding an (_) to the variable name, like so:

let _variable = vec![0; 10];

Answer 10

At the top of *.rs file:

#![allow(unused)]  // FIXME

Answer 11

add this in 1st line of code

#![allow(unused_variables)]

all warnings solutions:

https://doc.rust-lang.org/rustc/lints/listing/warn-by-default.html

Answer 12

using features

#[cfg(feature = "dead_code")]

note: "dead_code" can be replaced by any word.