Trait Solver 2 @BoxyUwU @WaffleLapkin

Warning: this quiz is still "work-in-progress", some questions might not have good explanations (or any at all), formatting/structure/titles/etc are not final, and so on. You might want to return here on a later date.

struct A;
struct B;
struct C;

trait Trait<'a, T> {
    type Assoc;
}

trait Bound {}

impl Bound for B {}
impl Bound for C {}

impl<'a> Trait<'a, B> for A {
    type Assoc = B;
}

impl<'a> Trait<'a, C> for A {
    type Assoc = C;
}

fn f<T>(_: T)
where
    for<'a> A: Trait<'a, T>,
    for<'a> <A as Trait<'a, T>>::Assoc: Bound,
{
}

fn main() {
    f(B);
}

Solution

error[E0277]: the trait bound `for<'a> <A as Trait<'a, _>>::Assoc: Bound` is not satisfied
  --> examples/trait_solver_2.rs:30:7
   |
30 |     f(B);
   |     - ^ the trait `for<'a> Bound` is not implemented for `<A as Trait<'a, _>>::Assoc`
   |     |
   |     required by a bound introduced by this call
   |
   = help: the following other types implement trait `Bound`:
             B
             C
note: this is a known limitation of the trait solver that will be lifted in the future
  --> examples/trait_solver_2.rs:30:7
   |
30 |     f(B);
   |     --^-
   |     | |
   |     | the trait solver is unable to infer the generic types that should be inferred from this argument
   |     add turbofish arguments to this call to specify the types manually, even if it's redundant
note: required by a bound in `f`
  --> examples/trait_solver_2.rs:25:41
   |
22 | fn f<T>(_: T)
   |    - required by a bound in this function
...
25 |     for<'a> <A as Trait<'a, T>>::Assoc: Bound,
   |                                         ^^^^^ required by this bound in `f`

For more information about this error, try `rustc --explain E0277`.
error: could not compile `code` (example "trait_solver_2") due to 1 previous error

Unfair Rust Quiz

Trait Solver 2 @BoxyUwU @WaffleLapkin