Hello rustaceans 
,
I am trying to get the following to work:
// Some trait with a function that returns a slice:
pub trait AsSlice {
type Element;
fn as_slice(&self) -> &[Self::Element];
}
// Some type
pub struct A<Cont>(Cont);
// Here we say that if A wraps a slice, then it implements AsSlice
impl<'a, Element> AsSlice for A<&'a [Element]> {
type Element = Element;
fn as_slice(&self) -> &[Self::Element] {
self.0
}
}
impl<Cont> A<Cont> {
// We want this function to work
pub fn failing<Coef>(&self)
where
Self: AsSlice<Element = Coef>,
{
// This works, meaning that A<&[<Self as AsSlice>::Element]> is recognized to be AsSlice.
self.as_ref_a().some_func();
// This does not, meaning that A<&[<Self as AsSlice>::Element]> is not recognized to be
// AsSlice.
self.as_ref_a().as_ref_a();
}
pub fn as_ref_a<Coef>(&self) -> A<&[<Self as AsSlice>::Element]>
where
Self: AsSlice<Element = Coef>,
{
A(self.as_slice())
}
pub fn some_func<Coef>(&self)
where
Self: AsSlice<Element = Coef>,
{
println!("{}", self.as_slice().len());
}
pub fn workaround<Coef>(&self)
where
Self: AsSlice<Element = Coef>,
for<'a> A<&'a [Coef]>: AsSlice<Element = Coef>,
{
self.as_ref_a().some_func();
self.as_ref_a().as_ref_a();
}
}
fn main() {
println!("Hello, world!");
}
The issue is the following: The failing method calls the as_ref_a method to build a A(&[T]) type. Depending on the method that is called next ( as_ref_a again, or some_func ), the A(&[T]) is recognized to implement AsSlice or not. The workaround is to use a HKTB to explicitly state that AsSlice is implemented, but I would prefer to not have it.
My first question is why does it happen? My second question is how to get rid of the HKTB?
Thanks for your help 
