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How to disable unused code warnings in Rust?

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.

like image 576
Andrew Wagner Avatar asked Sep 16 '14 19:09

Andrew Wagner


6 Answers

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
    
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Arjan Avatar answered Nov 12 '22 06:11

Arjan


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);
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antoyo Avatar answered Nov 12 '22 06:11

antoyo


Put these two lines on the top of the file.

#![allow(dead_code)]
#![allow(unused_variables)]
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M. Hamza Rajput Avatar answered Nov 12 '22 04:11

M. Hamza Rajput


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.

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Victor Basso Avatar answered Nov 12 '22 06:11

Victor Basso


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

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Muhammed Moussa Avatar answered Nov 12 '22 05:11

Muhammed Moussa


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)]

like image 4
PPP Avatar answered Nov 12 '22 05:11

PPP