rustc doesn't seem to understand ~const bounds on associated types

[Techcable]

I tried this code (playground]:

#![feature(const_trait_impl, const_fn_trait_bound)]

pub trait ConstableTrait {
    fn foo(&self) -> u32;
}

pub trait ExTrait {
    type Assoc: ~const ConstableTrait;
}

struct ShouldBeInferred<T: ExTrait> {
    a: T::Assoc
}
impl<T: ExTrait> ShouldBeInferred<T> {
    pub const fn should_infer(&self) -> u32 {
        self.a.foo()
    }
}

I expected to see this happen:
ShouldBeInferred::should_infer should understand that T::AssocTrait: ~const ConstableTrait and self.a.foo() is a const-fn.

Instead, this happened:
The compiler doesn't seem to infer the T::AssocTrait: ~const ConstableTrait bound gives an error self.a.foo() is not callable in a constant context.

I might be misunderstanding the purpose of ~const bounds on associated types. Please correct me if I am :)

Also great work so far on const fn. It's really starting to shine :)

Meta

rustc --version --verbose:

rustc 1.56.0-nightly (b7404c898 2021-09-03)
binary: rustc
commit-hash: b7404c898a1a6933b71c72428a6dce551bcc1be7
commit-date: 2021-09-03
host: x86_64-unknown-linux-gnu
release: 1.56.0-nightly
LLVM version: 13.0.0

[fee1-dead]

With the change to ~const, these implicitness have been removed. So you can't rely on the inherent impl method being const, to assume that the impl's bounds are all inferred to be ~const.

And currently I don't think it is possible to achieve what you want. But the ideal solution is to use where clauses on the const fn and bound T: ~const ExTrait. See also #88424, and we will allow these where clauses on const fns on inherent impls once we land #88558.