How to allow multiple implementations of a trait on various types of IntoIterator items?

4
votes

Rust doesn't seem to distinguish between different implementations of a trait only if they differ by an associated type.

How can I implement a method on all kinds of collections/iterators, but have specific implementations for each concrete type they contain?

error: conflicting implementations for trait Foo [E0119]

The code:

trait Foo { fn foo(self); }

impl<T> Foo for T 
    where T: IntoIterator<Item=u32> 
{
    fn foo(self) {
        self.into_iter();
    }
}

impl<T> Foo for T 
    where T: IntoIterator<Item=u16> 
{
    fn foo(self) {
        self.into_iter();
    }
}

fn main() {
    vec![0u32].foo();
    vec![0u16].foo();
}
1
iteratorrusttraits

1 Answers

6
votes

You can not do the generic form directly, which is issue #20400. You'll have to introduce either a trait that can be used as a bound on T::Item to merge the two implementations, or wrapper types. E.g. the first one might look like:

trait FooIterItem {
    // behaviours needed for Foo impl
}
impl FooIterItem for u32 { ... }
impl FooIterItem for u16 { ... }

impl<T> Foo for T
    where T: IntoIterator, T::Item: FooIterItem
{
    ...
}