Unable to see constraint traces

[oskarth]

Problem

When debugging circuits, it is very useful to see what constraints haven't been met and why.

Details

This should be possible with ConstraintLayer but I haven't managed to get to work properly:

In poseidon_tornado with

RUST_LOG="poseidon_tornado=TRACE,ark_relations=TRACE" cargo run

We get

Constraint trace requires enabling `ConstraintLayer`
thread 'main' panicked at 'assertion failed: {\n    use ark_relations::r1cs::{ConstraintSynthesizer, ConstraintSystem};\n    let cs = ConstraintSystem::<E::Fr>::new_ref();\n    circom.clone().generate_constraints(cs.clone()).unwrap();\n    cs.is_satisfied().unwrap()\n}', /home/oskarth/repos/github.com/gakonst/ark-circom/src/circom/builder.rs:82:9
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace

The panic happens because constraints aren't met, which is expected. This then panics because of the debug_assert here https://github.com/gakonst/ark-circom/blob/master/src/circom/builder.rs#L82-L87

The problem is the lack of tracing, even with 9b1fa85

Tracing instrumentation based on example here https://github.com/arkworks-rs/r1cs-tutorial/blob/main/merkle-tree-example/src/constraints.rs#L109-L113

Same when trying to do tracing instrumentation in various ways directly in ark-circom etc.

Error from https://github.com/arkworks-rs/snark/blob/master/relations/src/r1cs/constraint_system.rs#L609

Probably missing something simple here but haven't been to figure out what's wrong.

Acceptance criteria

Failing constraints should produce some form of useful debugging information re which constraints are failing.

[oskarth]

This is actually quite a big problem, e.g. a typo "nullifer" vs "nullifier" had me stumped for quite a while. Right now there's zero useful feedback here. Some kind of type assertions would be useful here? Probably requires looking into proof/witness WASM format a bit more.