Debugger Known Limitations

There are currently some limits to what the debugger can observe.

Mutable references

The debugger is currently blind to any state mutated via a mutable reference. For example, in:

let mut x = 1;
let y = &mut x;
*y = 2;

The update on x will not be observed by the debugger. That means, when running vars from the debugger REPL, or inspecting the local variables pane in the VS Code debugger, x will appear with value 1 despite having executed *y = 2;.

Variables of type function or mutable references are opaque

When inspecting variables, any variable of type Function or MutableReference will render its value as <<function>> or <<mutable ref>>.

Debugger instrumentation affects resulting ACIR

In order to make the state of local variables observable, the debugger compiles Noir circuits interleaving foreign calls that track any mutations to them. While this works (except in the cases described above) and doesn't introduce any behavior changes, it does as a side effect produce bigger bytecode. In particular, when running the command opcodes on the REPL debugger, you will notice Unconstrained VM blocks that look like this:

...
5    BRILLIG inputs=[Single(Expression { mul_terms: [], linear_combinations: [], q_c: 2 }), Single(Expression { mul_terms: [], linear_combinations: [(1, Witness(2))], q_c: 0 })]
       |       outputs=[]
  5.0  |   Mov { destination: RegisterIndex(2), source: RegisterIndex(0) }
  5.1  |   Mov { destination: RegisterIndex(3), source: RegisterIndex(1) }
  5.2  |   Const { destination: RegisterIndex(0), value: Value { inner: 0 } }
  5.3  |   Const { destination: RegisterIndex(1), value: Value { inner: 0 } }
  5.4  |   Mov { destination: RegisterIndex(2), source: RegisterIndex(2) }
  5.5  |   Mov { destination: RegisterIndex(3), source: RegisterIndex(3) }
  5.6  |   Call { location: 8 }
  5.7  |   Stop
  5.8  |   ForeignCall { function: "__debug_var_assign", destinations: [], inputs: [RegisterIndex(RegisterIndex(2)), RegisterIndex(RegisterIndex(3))] }
...

If you are interested in debugging/inspecting compiled ACIR without these synthetic changes, you can invoke the REPL debugger with the --skip-instrumentation flag or launch the VS Code debugger with the skipConfiguration property set to true in its launch configuration. You can find more details about those in the Debugger REPL reference and the VS Code Debugger reference.

note

Skipping debugger instrumentation means you won't be able to inspect values of local variables.