Trait *definition* is rejected if assoc type has bou... | rust-lang/rust

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Imagine you have the following (cf. https://docs.rs/imply-hack, and/or https://docs.rs/implied-bounds):

// 0.
mod implied_bounds {
    pub trait ImpliedPredicate<X: ?Sized> : HasAssociatedImpls<X, Impls = X> {}
    pub trait HasAssociatedImpls<X: ?Sized> { type Impls: ?Sized; }
    
    impl<T: ?Sized, X: ?Sized> ImpliedPredicate<X> for T {}
    impl<T: ?Sized, X: ?Sized> HasAssociatedImpls<X> for T { type Impls = X; }
}
use implied_bounds::ImpliedPredicate;

trait SomeBound {}

// 1.
trait HasImpliedBoundOnSelf
:
    ImpliedPredicate<Self ,Impls: SomeBound> // <- the specific bound does not matter
{}

// 2. HEREIN LIES THE ISSUE
trait HasAssocWithImplyingBoundSuperTrait {
    // ERROR, missing `SomeBound` on `Self::Assoc`.
    type Assoc: HasImpliedBoundOnSelf;
}

I expected to see this happen: trait declarations (with no "bad" impls) to be allowed.

Instead, this happened: the trait HasAssocWithImplyingBoundSuperTrait definition is rejected.

error[E0277]: the trait bound `<Self as HasAssocWithImplyingBoundSuperTrait>::Assoc: SomeBound` is not satisfied
  --> <source>:21:17
   |
21 |     type Assoc: HasImpliedBoundOnSelf; // <- ERROR, missing `Send` on `Self::Assoc`.
   |                 ^^^^^^^^^^^^^^^^^^^^^ the trait `SomeBound` is not implemented for `<Self as HasAssocWithImplyingBoundSuperTrait>::Assoc`
   |
note: required by a bound in `HasImpliedBoundOnSelf`
  --> <source>:16:35
   |
14 | trait HasImpliedBoundOnSelf
   |       --------------------- required by a bound in this trait
15 | :
16 |     ImpliedPredicate<Self ,Impls: SomeBound> // <- the specific bound does not matter
   |                                   ^^^^^^^^^ required by this bound in `HasImpliedBoundOnSelf`
help: consider further restricting the associated type
   |
20 | trait HasAssocWithImplyingBoundSuperTrait where <Self as HasAssocWithImplyingBoundSuperTrait>::Assoc: SomeBound {
   |                                           +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

Rationale

Click to see

As to some rationale behind the odd pattern (e.g., rather than a mere super-trait on Self in this instance): https://docs.rs/named-generics-bundle.

There, I want a trait to imply certain non-dyn-safe bounds such as Clone (to work around non-perfect derives later on with type-level-only generics), but I also want the trait, or a close version thereof, to be dyn-safe.

I achieve this by doing the following:

trait TargetTrait<Helper = Self>: ImpliedPredicate<Helper ,Impls: Clone> {
    type Assoc : …;
    // …
}

This way:

dyn TargetTrait<(), …> is well-formed;
X: TargetTrait entails : Clone.

From there, I can do the following:

impl<Dyn : ?Sized + TargetTrait<()>> TargetTrait for PhantomData<Dyn> {
    type Assoc = Dyn::Assoc;
}

// So that `PhantomData<dyn TargetTrait<(), Assoc = …>> : TargetTrait`!

Final, ergonomic touch:

macro_rules! TargetTrait {( $($assoc:tt)* ) => (
    ::core::marker::PhantomData<
        dyn TargetTrait<(), $($assoc)*>
    >
)}

And we get the final result:

fn some_api<T: TargetTrait>() {
    let _: T::SomeAssoc; // It Works!
}

some_api::<TargetTrait![SomeAssoc = …]>() // Named generic params!

But doing all this runs into issues the moment someone does:

trait AnotherTrait {
    type Assoc: TargetTrait; // <- Error, `Self::Assoc : Clone` is not fulfilled!
}

which is what this very issue is about.

Start by compiling the reproducer with Rust 1.79 and current nightly, then compare how the associated type bound is checked in the trait declaration. Done means the trait definition is accepted without the suggested explicit bound while invalid implementations remain rejected; no source files or tests are named in the issue.

Written by the indexing model from the issue text.

Trait definition is rejected if assoc type has bound with an implied predicate on itself

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Trait *definition* is rejected if assoc type has bound with an implied predicate on itself

Description

Meta

Rationale

Contributor guide

First steps

Research direction

Assessment

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Trait definition is rejected if assoc type has bound with an implied predicate on itself