Enable dynamic index argument for slice insert and slice remove

[vezenovm]

Problem

Support for dynamic indices is included in this PR (#2446). However, it still has a missing feature with handling dynamic indices for the SliceInsert and SliceRemove intrinsics. Implementing SliceInsert and SliceRemove with non-const inputs is not as straightforward as the other slice intrinsics in ACIR gen.

A more involved codegen will be required. In both SSA gen and ACIR gen, slice intrinsicssimplifications could be performed in most cases with the im::Vector builtins. Without a constant index, the im::Vector builtin cannot be used, and an equivalent codegen of the insert and remove methods will most likely be required.

Happy Case

We can use insert and remove on slices with witness values.

For example, this should be enabled:

fn main(x: Field) {
    let mut slice = [];
    for i in 0..5 {
        slice = slice.push_back(i);
    }
    
    slice = slice.insert(x - 2, 20);
    assert( ... );
}

Alternatives Considered

If this is not implemented the feature will just be missing.

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[jfecher]

I think an approach I'd like for this bug and one direction I strongly want to move towards in the future is to have a compiler error for unimplemented features like this so that users do not randomly encounter these crashes.

For example, for here I think we can add a std::assert_constant(index) within our slice insert and remove methods in the standard library. This should give users a nicer error while we're trying to implement it in the meantime.

[vezenovm]

This was closed as per #3624