Rust: Unexpected error in const generics: constant expression depends on a generic parameter

[qwerty19106]

The code:

#![feature(const_generics)]

struct Stack<const N: usize> {
    stack: [u64; {N / 8}],
}

must be compiled but gives error:

error: constant expression depends on a generic parameter
 --> src/lib.rs:4:5
  |
4 |     stack: [u64; {N / 8}],
  |     ^^^^^^^^^^^^^^^^^^^
  |
  = note: this may fail depending on what value the parameter takes

note: rustc 1.42.0-nightly (8a79d08fa 2020-01-27) running on x86_64-unknown-linux-gnu

playground

It is the same as #61368, but not ICE.

[Evrey]

Here are more cases where that error occurs:

direct link to playground

Test snippet as I don't know for how long the playground will keep the code.

// (╯°□°）╯︵ ┻┻

/*
error: constant expression depends on a generic parameter
   […snip…]
   = note: this may fail depending on what value the parameter takes
*/

#![feature(const_generics)]
#![allow(incomplete_features)]



const fn gud_gaemu() -> usize {
    0xFF7_600D
}



// Example showing a configurable type that takes
// const generics via a struct.
#[derive(Eq, PartialEq)]
struct Magic {
    pub fairy_dust: usize,
}

struct X<const N: Magic> {
    arr: [u32; N.fairy_dust],
}



// Example showing a configurable type that takes
// const generics via a trait.
trait Nightmare {
    const SCAWWY: usize;
}

impl Nightmare for () {
    const SCAWWY: usize = gud_gaemu();
}

struct Y<T: Nightmare> {
    arr: [u32; <T as Nightmare>::SCAWWY],
    _but_i_am_using_t_indirectly: std::marker::PhantomData<T>,
}



// Example showing a configurable type that takes
// bare const generics.
//
// This is the only one that works!
struct Z<const N: usize> {
    arr: [u32; N],
}



// Example showing a configurable type that takes
// const generics via a tuple.
struct W<const N: (usize,)> {
    arr: [u32; N.0],
}



fn main() {
    const M: Magic = Magic { fairy_dust: gud_gaemu() };
    println!("{}B", std::mem::size_of::<X<{           M  }>>());
    println!("{}B", std::mem::size_of::<Y<           ()   >>());
    println!("{}B", std::mem::size_of::<Z<{ gud_gaemu()  }>>());
    println!("{}B", std::mem::size_of::<W<{(gud_gaemu(),)}>>());
}

I'm happy to see this issue being tagged as C-bug. Would've been nice to mark that error message as TODO or something. I feared that to be a permanent anti-feature.

[jonas-schievink]

It looks like this error is currently intentional (added in https://github.com/rust-lang/rust/pull/68388), and needs some language design work to be fixed properly, which is tracked at https://github.com/rust-lang/rust/issues/68436, so closing in favor of that.