[MockProver] bug: query_instance always triggers VerifyFailure::CellNotAssigned.

[therealyingtong]

In the current MockProver, query_instance always results in a VerifyFailure::CellNotAssigned when used in a gate with a selector. This is because we only look for assigned cells in the region where the gate was activated, and instance cells are not in any region.

halo2/halo2_proofs/src/dev.rs

Lines 577 to 593 in 96d9bde

	gate.queried_cells().iter().filter_map(move |cell| {
	// Determine where this cell should have been assigned.
	let cell_row = ((gate_row + n + cell.rotation.0) % n) as usize;
	// Check that it was assigned!
	if r.cells.contains(&(cell.column, cell_row)) {
	None
	} else {
	Some(VerifyFailure::CellNotAssigned {
	gate: (gate_index, gate.name()).into(),
	region: (r_i, r.name.clone()).into(),
	gate_offset: *selector_row,
	column: cell.column,
	offset: cell_row as isize - r.rows.unwrap().0 as isize,
	})
	}
	})

[therealyingtong]

Thank you @nalinbhardwaj for surfacing this bug 🐛

[themighty1]

@str4d , is there a reason why #622 is still pending your review?
I'm finding this bug to slow down development, since I have to wait a long time to start up the real Prover every time I need to debug the circuit.