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

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 :)

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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

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.

Seems to be a problem still:

const trait Trait {
    type Type: Copy + [const] Clone;
}

#[derive(Copy)]
#[derive_const(Clone)] // unsatisfied trait bound introduced here
struct Struct<T: Trait>(T::Type); // note: required for `Struct<T>` to implement `[const] Clone`

#[derive(Copy)]
#[derive_const(Clone)]
struct StructStruct<T: Trait>(Struct<T>); // error[E0277]: the trait bound `T: [const] Trait` is not satisfied

@nxsaken

@nxsaken The generated code from the derive never makes a T: [const] Trait bound: (play)

#![feature(const_clone)]
#![feature(const_trait_impl)]
#![feature(derive_const)]

const trait Trait {
    type Type: Copy + [const] Clone;
}

#[derive(Copy)]
// #[derive_const(Clone)]
struct Struct<T: Trait>(T::Type);

impl<T: [const] Clone + Trait> const Clone for Struct<T>
where T::Type: [const] Clone {
    fn clone(&self) -> Self { Struct(self.0.clone()) }
}

#[derive(Copy)]
// #[derive_const(Clone)]
struct StructStruct<T: Trait>(Struct<T>); 

impl<T: [const] Clone + Trait> const Clone for StructStruct<T> {
    fn clone(&self) -> Self { StructStruct(self.0.clone()) }
}

And [const] is only implied via [const] bounds. If you want it to always be applicable, use type Type: Copy + const Clone, or use T: [const] Trait on a manual impl

Closing the original issue as won't fix. T::Assoc can be assumed to have [const] ConstableTrait if we have T: [const] ExTrait. The original code snippet does not have T: [const] ExTrait so this cannot be assumed.

To fix: either use type Assoc: const ConstableTrait always const bound on the assoc, or bound T: [const] ExTrait.

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rustc doesn't seem to understand ~const bounds on associated types #88655

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rustc doesn't seem to understand ~const bounds on associated types #88655

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fee1-dead commented on Sep 9, 2021

nxsaken commented on Nov 16, 2025

fee1-dead commented on Nov 16, 2025

fee1-dead commented on Nov 16, 2025

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