delegation: do not always inherit ConstArgHasType predicates

[aerooneqq]

The explanation can be found in comment in inherit_predicates_for_delegation_item.

So the problem is that we have two ConstArgHasType predicates in param_env which tell that a single const has different types. But the function find_const_ty_from_env panics in this situation. This assert is here for a long time, and I do not have enough knowledge to say if it is correct or not, however I have thoughts that having contradictory predicates should not lead to panic, instead an error from trait solver or whatever engine that solves those predicates should be reported. Do not want to dive too deep into researching and possibly rewriting find_const_ty_from_env, so fixed it on delegation level.

Fixes #158675. Part of #118212.
r? @petrochenkov

[petrochenkov]

r? types

[oli-obk]

r? @BoxyUwU

[rustbot]

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[BoxyUwU]

i am quite lost reading this comment & generally with what's going on at a high level with delegation 😅

Why do we wind up "merging" the ParamEnv of Trait::foo with the ParamEnv of the inherent impl? Is this because you dont want to have people need to write something like: reuse foo::function::<T> where T: Bound?

View changes since the review

[aerooneqq]

When generating delegation's generic params we inherit predicates from delegation signature, so if we have:

trait Trait<T: Clone> {
  fn foo() { .. }
}

reuse Trait::foo;

// We will generate with inherited predicate: T: Clone
fn foo<Self, T>() {
  <Self as Trait<T>>:foo();
}

// If we specify generic args:
reuse Trait::<usize>::foo;
// We will generate with inherited predicate: usize: Clone (after mapping T into usize)
fn foo<Self>() {
  <Self as Trait<usize>>:foo();
}

It works fine with types, but with consts if we have two ConstArgHasType predicates in env it panics.

After looking at it again and reading your comment maybe we should not inherit predicates at all for specified generic args, so we will not inherit usize: Clone, I will experiment with it and rewrite the fix in case of success.

[BoxyUwU]

assuming that the merged environments are intentional (it seems so), you will (in the long term) probably want to do a more involved refactor here to remove find_const_ty_from_env entirely and instead solely rely on the trait solver for finding the types of const parameters.

though having two ConstArgHasType clauses in an env is weird conceptually, you basically cannot ever satisfy both of them unless the clauses are the same 🤔 (in which case why have both). regardless, seems fine to me :3

@bors r+ rollup

[rust-bors]

📌 Commit 4d1093d has been approved by BoxyUwU

It is now in the queue for this repository.