Rust: 상속 된 특성 경계가 완전히 확인되지 않았습니다.

원래 토론 : https://users.rust-lang.org/t/unexpected-behaviors-of-trait-bounds/12286/10

이 오래된 문제와 관련된 것 같습니다 : https://github.com/rust-lang/rust/issues/29859

기본적 갖는 trait Complete 필요 trait Partial 및 trait Partial 필요 trait PartialEq (또는 다른 특성), 유형 ( struct TypeB 포함) impl Complete 는 버그처럼 보이는 impl PartialEq 을 충족 할 필요가 없습니다.

흥미롭게도 <TypeB as Partial> 이 보일 때마다 검사가 수행되고 적절한 오류가 발생합니다.

다음은 재현입니다.

pub trait Partial: PartialEq {
    fn foo() -> Option<bool>;
}

pub trait Complete: Partial {
    fn foo() -> bool;
}

impl<T> Partial for T
where
    T: Complete,
{
    fn foo() -> Option<bool> {
        Some(<Self as Complete>::foo())
    }
}


// ----

#[derive(PartialEq)]
pub struct TypeA {}

impl Partial for TypeA {
    fn foo() -> Option<bool> {
        None
    }
}


// ----

// BUG: No compile warning about `PartialEq` not being implemented
// #[derive(PartialEq)]
pub struct TypeB {}

impl Complete for TypeB {
    fn foo() -> bool {
        true
    }
}


// ----

pub fn main() {
    println!("{:?}", TypeA::foo());

    // This works, but shouldn't!
    println!("{:?}", <TypeB as Complete>::foo());

    // This would trigger the issue, though.
    //println!("{:?}", <TypeB as Partial>::foo());
    /* Result in:
    error[E0277]: the trait bound `TypeB: std::cmp::PartialEq` is not satisfied
      --> src/bin/cyclic_traits.rs:48:22
       |
    48 |     println!("{:?}", <TypeB as Partial>::foo());
       |                      ^^^^^^^^^^^^^^^^^^^^^^^ can't compare `TypeB` with `TypeB`
       |
       = help: the trait `std::cmp::PartialEq` is not implemented for `TypeB`
       = note: required by `Partial::foo`
    */
}