cannot use constants which depend on generic parameters in types

[JonathanWoollett-Light]

My original stack overflow post on the issue.
Rust Playground

I was trying to write a data-structure such that it is generic over different dimensionality.

The beginning is:

/// A field of adjacent sections in a given dimensionality.
///  With 1, you have `|left|center|right|`.
///  With 2 dimensions you have:
///  ```
///    up_left│    up│up_right
///  ─────────┼──────┼──────────
///       left│center│right
///  ─────────┼──────┼──────────
///  down_left│  down│down_right
///  ```
///  Etc.
#[derive(Debug)]
struct Adjacents<T, const D: u32>([T; size(D)])

const fn size(d: u32) -> usize { 3u32.pow(d) as usize }

When trying to write implementations like:

impl<T> Adjacents<T,1u32> { /*...*/ }

I'm encountering the severe warning (in a few instances):

warning: cannot use constants which depend on generic parameters in types
  --> src/main.rs:26:9
   |
26 | impl<T> Adjacents<T,1u32> {
   |         ^^^^^^^^^^^^^^^^^
   |
   = note: `#[warn(const_evaluatable_unchecked)]` on by default
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #76200 <https://github.com/rust-lang/rust/issues/76200>

In my use case certain functionality can be generic across the stored data type (T) but not generic across dimensionality (D), so it seems only inevitable I must use Adjacents<T,1u32> throughout.

Given D does not depend on T, I would not expect such a warning to arise.

Meta

rustc --version --verbose:

rustc 1.58.0-nightly (82af160c2 2021-11-10)
binary: rustc
commit-hash: 82af160c2cb9c349a0373cba98d8ad7f911f0d34
commit-date: 2021-11-10
host: x86_64-unknown-linux-gnu
release: 1.58.0-nightly
LLVM version: 13.0.0

Backtrace

Since this is not technically an error I have not included a backtrace.

[fmease]

Current output on stable:

error: generic parameters may not be used in const operations
 --> src/lib.rs:2:44
  |
2 | struct Adjacents<T, const D: u32>([T; size(D)]);
  |                                            ^ cannot perform const operation using `D`
  |
  = help: const parameters may only be used as standalone arguments, i.e. `D`

Current output on nightly:

error: generic parameters may not be used in const operations
 --> src/lib.rs:2:44
  |
2 | struct Adjacents<T, const D: u32>([T; size(D)]);
  |                                            ^ cannot perform const operation using `D`
  |
  = help: const parameters may only be used as standalone arguments, i.e. `D`
  = help: add `#![feature(generic_const_exprs)]` to allow generic const expressions

[fmease]

You need to enable the experimental feature generic_const_exprs to make this pass:

#![feature(generic_const_exprs)]

#[derive(Debug)]
struct Adjacents<T, const D: u32>([T; size(D)])
where
    [(); size(D)]:;

const fn size(d: u32) -> usize { 3u32.pow(d) as usize }

[fmease]

Closing in favor of the tracking issue for generic_const_exprs #76560.