Using a const Associated Type in trait function signature

[OliverFM]

I have been trying to figure out how to specify the a function in a trait where the return type depends on an associated type. Specifically, for a const associated type.

pub trait FancyArray {
    const Length: usize;

    fn pad(&self) -> [u32; Self::Length] {
        [0; Self::Length]
    }
}

However this code does not compile with this error:

❯ cargo check
    Checking category_theory_rust v0.1.0 (/Users/oliver/code/category_theory_rust)
error: generic parameters may not be used in const operations
 --> src/lending_iterator.rs:3:28
  |
3 |     fn pad(&self) -> [u32; Self::Length] {
  |                            ^^^^^^^^^^^^ cannot perform const operation using `Self`
  |
  = note: type parameters may not be used in const expressions

error: constant expression depends on a generic parameter
 --> src/lending_iterator.rs:4:13
  |
4 |         [0; Self::Length]
  |             ^^^^^^^^^^^^
  |
  = note: this may fail depending on what value the parameter takes

This seems really odd, since Self::Length is specified at compile time, so it feels like this should be possible. Could anyone tell me where to find out why this does not work?

[xodmq]

You can do it on nightly with the generic_const_exprs feature (see issue)

[OliverFM]

Thanks! Adding #![feature(generic_const_exprs)] to the src/main.rs solved this issue!