Should trait bounds be duplicated in struct and impl?

Question

The following code uses a struct with generic type. While it\'s implementation is only valid for the given trait bound, the struct can be defined with or without the same bo

Answer 1

Trait bounds that apply to every instance of the struct should be applied to the struct:

struct IteratorThing
where
    I: Iterator,
{
    a: I,
    b: Option,
}

Trait bounds that only apply to certain instances should only be applied to the impl block they pertain to:

struct Pair {
    a: T,
    b: T,
}

impl Pair
where
    T: std::ops::Add,
{
    fn sum(self) -> T {
        self.a + self.b
    }
}

impl Pair
where
    T: std::ops::Mul,
{
    fn product(self) -> T {
        self.a * self.b
    }
}

to conform to the DRY principle

The redundancy will be removed by RFC 2089:

Eliminate the need for “redundant” bounds on functions and impls where those bounds can be inferred from the input types and other trait bounds. For example, in this simple program, the impl would no longer require a bound, because it can be inferred from the Foo type:

struct Foo { .. }
impl Foo {
  //    ^^^^^ this bound is redundant
  ...
}