Support arrays as template parameters

[tim-hoffman]

Current handling of template parameters in llzk_backend/src/module.rs is as follows:

    // Insert Operations to read templated struct parameters into an SSA Value in each function.
    // This ensures the struct parameter is available as a Value in the block context.
    for name in struct_params.into_iter() {
        compute_ctx.block_ctx.declare_name_if_not_present(name, || {
            Ok(poly::read_const(struct_loc, name, codegen.felt_type().into()))
        })?;
        constrain_ctx.block_ctx.declare_name_if_not_present(name, || {
            Ok(poly::read_const(struct_loc, name, codegen.felt_type().into()))
        })?;
    }

However, template parameters can also be arrays:

pragma circom 2.0.0;

template T(N) {
    signal output out <== N[1][1];
}

component main = T([[1,2],[3,4]]);

[tim-hoffman]

Adding direct support for this in LLZK does not seem like the best solution because it would require adding an array literal attribute that could be added to the parameter list of StructType which would end up being extremely verbose. The alternative is to convert array-type template parameters to a real parameters of the struct functions.

[iangneal]

@tim-hoffman one issue I see with this is that elements of an array parameter can technically be used to parameterize elements within a template:

pragma circom 2.0.0;

template Foo(arr) {
    signal input inp[arr[0]];
    signal output outp[arr[1]] <== [inp[0], inp[0]];
}

component main = Foo([1, 2]);

However, I haven't actually found any real-world examples of this, so it's probably fine to ignore.