Struct with trait bounds definition

[zhak]

If I want to define a structure to hold only data types with specific trait bounds, do I need to define trait bounds for both struct itself and its impl? I met this investigating sources of the volatile_register crate.

pub struct RO<T>
    where T: Copy
{
    register: VolatileCell<T>,
}

impl<T> RO<T>
    where T: Copy
{
    #[inline(always)]
    pub fn read(&self) -> T {
        self.register.get()
    }
}

As I understand it, impl cannot go by itself, so probably it's enough to specify trait bounds only for the struct (or enum, or whatever impl is for)?

[17cupsofcoffee]

If you have trait bounds on the struct, you also need the same (or more restrictive) bounds on the impl:

struct Test<T>(T)
where
    T: Copy;

// works fine
impl<T> Test<T> where T: Copy {}

// compile error!
impl<T> Test<T> {}

The opposite is not true, however!

struct Test<T>(T);

// works fine - impl will only apply in cases where T is Copy
impl<T> Test<T> where T: Copy {}

// also works fine - impl will always apply!
impl<T> Test<T> {}

[zhak]

Well, looks reasonable. Thank you