Generic bound enabled by feature

[pYtoner]

I have a feature that requires adding a bound to many functions. Something like this:

fn function<T>(_: &T)
where
    T: SomeTrait
{}

Without the feature I do not want that bound. Right now the only way I know how to make this happen is by coping the function twice:

#[cfg(not(feature="add_bound"))]
fn function<T>(_: &T) {}

#[cfg(feature="add_bound")]
fn function<T>(_: &T)
where
    T: SomeTrait
{}

But this requires loads of code duplication. There should be a way to do this better. Is there?

[pYtoner]

I think I could do this:

trait MaybeBounds {}

#[cfg(feature="add_bound")]
impl<T: SomeTrait> MaybeBounds for T {}

#[cfg(not(feature="add_bound"))]
impl<T> MaybeBounds for T {}

fn function<T>(_: &T)
where
    T: MaybeBounds
{}

Or like:

#[cfg(feature="add_bound")]
trait MaybeBounds: SomeTrait {}

#[cfg(not(feature="add_bound"))]
trait MaybeBounds {}

But that would make the function signatures maybe misleading. Is this the only way to do it? (other than macros )

[quinedot]

You could

#[inline(always)]
fn _function<T>(_: &T) {
    same_innards()
}

#[cfg(not(feature="add_bound"))]
pub fn function<T>(a: &T) {
    _function(a)
}

#[cfg(feature="add_bound")]
pub fn function<T>(a: &T) where T: SomeTrait {
    _function(a)
}

[pYtoner]

That makes it better if the function bodies are big but in this case, imagine loads of small functions.

It would be good if something like this would be possible:

fn function<T>(_: &T)
where
    #[cfg(feature="add_bound")]
    T: SomeTrait
{}

[quinedot]

If the bounds are simple enough, you can

pub fn function<
    #[cfg(feature="add_bound")] T: SomeTrait,
    #[cfg(not(feature="add_bound"))] T,
>(_: &T) {
}

I don't know if there are any specific reasons for not accepting attributes on where clause items.

[pYtoner]

This works! Thank you very much.