Incorrect implementation of trait causes Noir to panic

[benesjan]

Aim

I was trying to modify Serialize trait in noir-protocol-circuits in aztec-packages such that only 1 trait implementation is allowed. This led me to stumble upon this panic.

Expected Behavior

No panic

Bug

When a Noir trait implementation is ill-defined it causes Noir to panic.

Tagging @jfecher as we've discussed this on Slack.

To Reproduce

Compile the following code:

struct MockStruct {
    value: Field,
}

pub trait Serialize {
    let N: u32;

    fn serialize(self) -> [Field; N];
}

impl Serialize for MockStruct {
    let N = 1;

    fn serialize(self) -> [Field; Self::N] {
        [self.value]
    }
}

pub struct Counted<T> {
    pub inner: T,
    pub counter: u32,
}

impl<T> Counted<T> {
    pub fn new(inner: T, counter: u32) -> Self {
        Self { inner, counter }
    }
}

impl<T> Serialize for Counted<T> where T: Serialize {
    type N = <T as Serialize>::N + 1;

    fn serialize(self) -> [Field; Self::N] {
        array_concat(self.inner.serialize(), [self.counter as Field])
    }
}

// impl<T, let NT: u32> Serialize for Counted<T>
//     where T: Serialize<N = NT>
// {
//     let N = NT + 1;

//     fn serialize(self) -> [Field; NT + 1] {
//         array_concat(self.inner.serialize(), [self.counter as Field])
//     }
// }

pub fn array_concat<T, let N: u32, let M: u32>(array1: [T; N], array2: [T; M]) -> [T; N + M] {
    let mut result = [array1[0]; N + M];
    for i in 1..N {
        result[i] = array1[i];
    }
    for i in 0..M {
        result[i + N] = array2[i];
    }
    result
}

You should get the following panic:

Nargo Version

nargo version = 1.0.0-beta.1 noirc version = 1.0.0-beta.1+74f0a50ed44e5712 (git version hash: 74f0a50ed44e5712, is dirty: false)

[jfecher]

impl<T> Serialize for Counted<T> where T: Serialize {
    type N = <T as Serialize>::N + 1;

    fn serialize(self) -> [Field; Self::N] {
        array_concat(self.inner.serialize(), [self.counter as Field])
    }
}

This should use let N = ... here but we shouldn't panic either way.
From the error message, this looks related to #6383

[benesjan]

This should use let N = ... here but we shouldn't panic either way.

@jfecher I specifically wanted to point out the panic here.

If you think #6383 tracks the issue sufficiently feel free to close this one. Thanks