Valid slice access fails constraint

[vezenovm]

Aim

Merging slices based off of witness values requires tracking a user facing dynamic slice length (original issue here: #1889). This required adding a slice access check before any slice access. I expect to have accurate error locations and slice length checks.

Expected Behavior

When I do something like this:

fn main(x: Field, y: Field) {
 let slice = merge_slices_mutate(x, y);
 assert(slice.len() == 3);
 assert(slice[3] == 5);
}

fn merge_slices_mutate(x: Field, y: Field) -> [Field] {
    let mut slice = [0; 2];
    if x != y {
        slice = slice.push_back(y);
        slice = slice.push_back(x);
    } else {
        slice = slice.push_back(x);
    }
    slice
}

I should have a constraint failure at assert(slice.len() == 3);

Bug

The line slice.len() == 3 should be where I get a failed constraint. But instead I get a failed constraint here assert(slice[3] == 5);. This is due to us replacing the dynamic value for the length during SSA when it should be checked during runtime.

To Reproduce

1. Use snippet posted above
2. Run nargo execute
3. See output failure at assert(slice[3] == 5);

Installation Method

Compiled from source

Nargo Version

nargo 0.11.1 (git version hash: 3847e9c, is dirty: true)

Additional Context

No response

Would you like to submit a PR for this Issue?

No

Support Needs

No response

[vezenovm]

This is happening after this PR (#2483) which is a useful optimizations in some instances but can lead to mismatched constraints when we are re-using SSA values internally. After removing the changes in #2483 though the error is not much clearer as it looks there is missing location information.

[vezenovm]

#2476 looks to be the cause of the missing location data as we use an iterator which is lazy and is thus doing nothing. So we are just taking self.locations but not inserting anything.