Auto return doesn't work with arrays

[Savio-Sou]

Description

Aim

Auto returning the value of an array without the need to specify it in Prover.toml before proving.

The feature was introduced in #360.

Expected behavior

main.nr

fn main(x : Field, y : pub Field) -> pub [Field; 2] {
    [x + y, y]
}

Prover.toml

x = "1"
y = "2"
return = []

Expect Verifier.toml to be auto-filled as:

Verifier.toml

y = "0x0000000000000000000000000000000000000000000000000000000000000002"
return = [
    "0x0000000000000000000000000000000000000000000000000000000000000003",
    "0x0000000000000000000000000000000000000000000000000000000000000002",
]

Bug

Instead, was prompted with the error below when proving:

The parameters in the main do not match the parameters in the Prover.toml file. 
Please check `return` parameter

To reproduce

1. Fill main.nr as above
2. Run nargo build
3. Fill Prover.toml as above
4. Run nargo prove p

Environment

• OS: Linux (x86_64-pc-linux-gnu)

For nargo users

• noir-lang/noir commit cloned: e885d42
• Proving backend
  • default
    • Clang: 10.0.0-4ubuntu1
  • wasm-base

[vezenovm]

I changed this to enhancement as this is not supported yet with the auto-return feature:

noir/crates/nargo/src/cli/prove_cmd.rs

Line 96 in 1d1b592

//We do not support undefined arrays for now - TODO

The error makes sense as you did specify an empty return array.

To add some more context if you had specified return = "", you would see the error:

Values of array returned from main must be specified in prover toml file