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.

Solution 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
```

Solution 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);

Solution 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.

Solution 4:

Put these two lines on the top of the file.

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

Solution 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