Do not allow const generics to depend on type parameters

[varkor]

Fixes #60264. In https://github.com/rust-lang/rust/pull/58191/files/b534cf992d0189032207f395c27ed092c89b40c7#diff-aeb0880081a991f34aef2ab889e1fb7a, it was suggested that there might be a better place for this error, but as this bug already affects stable, it's probably worth merging this now, and refactoring afterwards (I can open an issue for this).

[rust-highfive]

r? @cramertj

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[cramertj]

@bors r+

[bors]

📌 Commit 908a639 has been approved by cramertj

[varkor]

See #60286 for the follow up.

[Pratyush]

Is there a type theoretic reason why we want to disallow this?

[varkor]

@Pratyush: no, we're just being conservative at this point with the types we allow to be used in const generics (see the RFC for more details). I would expect this constraint to be relaxed in the future.

[Pratyush]

Ahh I see the structural match requirement now. Thanks!

[varkor]

@bors r=cramertj

[bors]

📌 Commit 6d7c794 has been approved by cramertj

[astrojhgu]

Is there a plan to allow const generics to depend on type parameters in the (near) future?
I find the following code cannot be compiled:

#![feature(const_generics)]
#![feature(specialization)]

struct S<const ND:usize>{}

trait Type{
    type tt;
}

default impl<const ND:usize> Type for SSS<ND>{
    type tt=<SSS<{ND-1}> as Type>::tt;
}

impl Type for SSS<0>{
    type tt=i32;
}


fn main() {
    let a=SSS::<3>{};
}

[varkor]

Is there a plan to allow const generics to depend on type parameters in the (near) future?

Not in the near future, but it's something we'd like to support eventually. It's not in the original RFC, though, so will require some design discussion.