Add trait objects with static dispatch

language design
4

My proposal is not to generate one enum per Trait for the whole project, but generate different enums: one per usage site, and to define the number of variants in them based on the context of use.

Let's consider we have 3 different structs that implement some trait:

struct A { /* fields omitted */ }
struct B { /* fields omitted */ }
struct C { /* fields omitted */ }

trait Trait { /* body omitted */ }

impl Trait for A { /* impls omitted */ }
impl Trait for B { /* impls omitted */ }
impl Trait for C { /* impls omitted */ }

fn main() {
    let mut foo = Vec::<static Trait>::new();
    // static Trait is being translated into enum with 2 variants (A, B)
    foo.push(A::new());
    foo.push(B::new());

    let mut bar = Vec::<static Trait>::new();
    // static Trait is being translated into enum with 3 variants (A, B, C)
    bar.push(A::new());
    bar.push(B::new());
    bar.push(C::new())
}

Here, foo's inner type would be an enum with 2 variants (A and B) and bar's one would have 3 variants (A, B and C). Because of the context.