chore(ssa refactor): blackbox array arg support

[joss-aztec]

Description

Problem*

ACIR gen didn't support array arguments to black boxes

Resolves

Summary*

acir_gen/mod Forwards blackbox arguments to acir_variable as a mix of AcirVar and ArrayIds. acir_variable they take responsibility for rationalising the arguments

Example

use dep::std;

fn main(x: Field) {
    let _p1 = std::scalar_mul::fixed_base(x);
    let _p2 = std::hash::pedersen([x]);
}

compiles to

current witness index : 6
public parameters indices : []
return value indices : []
BLACKBOX::FIXED_BASE_SCALAR_MUL [(_1, num_bits: 254)] [ _2, _3]
BLACKBOX::PEDERSEN [(_1, num_bits: 254)] [ _4, _5]

Documentation

• This PR requires documentation updates when merged.

  • I will submit a noir-lang/docs PR.
  • I will request for and support Dev Rel's help in documenting this PR.

Additional Context

PR Checklist*

• I have tested the changes locally.
• I have formatted the changes with Prettier and/or cargo fmt on default settings.

[joss-aztec]

I'm not particularly happy about the inconsistent passing of AcirVar/ArrayIds between acir_gen/mod and acir_variable. Ways this might be rationalised:

• Push the responsibility of destructuring the array args up to acir_gen/mod. (I think this should be fine as long as different blackboxes don't need to do argument position checks.)
• Allow array addresses to be represented as AcirVars such that acir_gen/mod no longer has to manage them. (However this would complicate various acir_variable operations.)

Suggestions welcome.

[jfecher]

I can see what you're saying about this check being a bit odd. Can it be reconciled if we pass inputs to convert_ssa_value such that the function can push multiple inputs if needed? E.g. self.convert_ssa_value(*value_id, dfg, &mut inputs);. Then that function could handle array addresses internally.

[joss-aztec]

Makes sense. Currently call instructions are the only place where we need to gather the contents of an array. Until that changes, I figure it's better not to force that interface change on all the other places that use convert_ssa_value.

[kevaundray]

Left some nits -- Feel free to merge and address in a follow-up PR