There are a few compiler passes right now related to peephole optimization:

• Collect2qBlocks
• ConsolidateBlocks
• Optimize1qGates
• Optimize1qDecomposition
• Add CollectMultiQBlocks pass to find multiqubit blocks in a circuit #4747

We should unify these into one general pass which is able to walk the circuit and collect+consolidate these gates. This should rely on the DAGDependency graph otherwise it will miss some opportunities when gates commute but are not next to each other. This should also accept a filter function that tells the pass to collect those gates that meet a certain condition (I believe retworkx can already do this). The filter functions could be:

• gate.num_qubits == 1
• gate.num_qubits <= 2
• gate.is_clifford
• etc.

Also I think we should just combine the collection phase with the consolidation phase. We can call it CollapseChains. I haven't found a use case for collecting a chain of gates but not collapsing them into an equivalent unitary.