Can't disable no_effect

[vihdzp]

I'm using a struct with a custom DerefMut implementation that internally updates the object every time it's mutably de-referenced, as a way to check whether it has changed. To force an update, I'm using the following code:

#[allow(clippy::no_effect)]
&mut *custom_deref;

Despite appearances, the second line is not actually a no-op. I can't blame Clippy for not being able to figure that out. However, I'm not able to disable the lint warning, even with the #[allow(clippy::no_effect)] line.

The following code replicates this bug. Note how &mut *custom_deref; is not a no-op, and note how the warning still shows despite disabling it.

struct CustomDeref(i32);

impl std::ops::Deref for CustomDeref {
    type Target = i32;

    fn deref(&self) -> &Self::Target {
        println!("Derefenced!");
        &self.0
    }
}

impl std::ops::DerefMut for CustomDeref {
    fn deref_mut(&mut self) -> &mut Self::Target {
        println!("Mutably dereferenced!");
        &mut self.0
    }
}

fn main() {
    let mut custom_deref = CustomDeref(0);

    #[allow(clippy::no_effect)]
    &mut *custom_deref;
}

Output after cargo clippy:

warning: statement with no effect
  --> src\main.rs:23:5
   |
23 |     &mut *custom_deref;
   |     ^^^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(clippy::no_effect)]` on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#no_effect

warning: 1 warning emitted

[camsteffen]

I can't blame Clippy for not being able to figure that out.

Actually you can. We can detected custom derefs.