to_le_bytes/to_be_bytes return fixed size arrays when applied to constants

[ggiraldez]

Aim

The documentation states that these Field methods should return slices, but that is not the case when applied to constants. The compiler will apply an optimization to precompute the result but it will return it in an array instead of a slice.

I also think the documentation is a bit outdated on to_le_bits/to_be_bits (see here) because it says it returns an array, but the stdlib definition returns a slice.

Expected Behavior

to_be_bytes/to_le_bytes should return a slice consistently, regardless if it's applied to a constant or not.

Bug

Given this program:

use dep::std;

fn main() {
    let x = 2040124;
    let mut a = x.to_le_bytes(7);
    let mut a1 = a.push_back(123);

    std::println(a.len());
    std::println(a1.len());
    std::println(a1[7]);
}

Fails compilation with this error:

error: Index out of bounds, array has size 7, but index was 7
   ┌─ /home/ggiraldez/Scratchpad/to_radix/src/main.nr:10:18
   │
10 │     std::println(a1[7]);
   │                  -----
   │
   = Call stack:
     1. /home/ggiraldez/Scratchpad/to_radix/src/main.nr:10:18

Aborting due to 1 previous error

But this program (with the appropriate inputs defined):

use dep::std;

fn main(x: Field) {
    let mut a = x.to_le_bytes(7);
    let mut a1 = a.push_back(123);

    std::println(a.len());
    std::println(a1.len());
    std::println(a1[7]);
}

Produce the expected output:

0x07
0x08
123
[to_radix] Circuit witness successfully solved

To Reproduce

Attempting to compile or execute the above program will fail with the error mentioned above.

Installation Method

None

Nargo Version

No response

Additional Context

As stated above, this happens because the compiler will apply an optimization when resolving a ToRadix intrinsic used on a constant value. The relevant code is here.

Would you like to submit a PR for this Issue?

Yes

Support Needs

No response