What should we add?

Right now we have a mix of the legacy approach to modeling a backend in the transpiler (basis_gates, CouplingMap, etc) and the new approach, the Target class. This causes friction when adding new passes or maintaining existing ones because we have to support dual paths for everything. We unify all of the internals on the new model because in almost every case we can convert from the legacy model to the target without issue.

• Add support for Target to all transpiler passes that need backend constraints (even if it's just to get a legacy view, like a CouplingMap from the target)
  • Add support for BackendV2 and Target to transpiler #7113 tracks a subset of passes
  • Make all Layout and Routing passes target aware #9263
• Document that moving forward all new passes only will get a Target (both contributor guidelines along with preset pass managers)
• Update preset pass manager construction to only pass a Target to passes
  • Stage plugins initialization will need to fill out all the fields in the PassManagerConfig from the Target for backwards compatibility. We can stop doing this as we deprecate non-target constraint fields in PassManagerConfig
• Update transpile() to convert BackendV1 inputs to BackendV2 with BackendV2Converter:
  • Standardize behavior of transpile with BackendV2: Fix custom constraints in transpile with BackendV2 #12042
  • Update transpile() to convert BackendV1 inputs to BackendV2 with BackendV2Converter #11996
• Update transpile() to convert loose input of constraints to a Target with Target.from_configuration()
  • constructor added in: Add constructor to build target from legacy transpiler model #9255
  • Update transpile() and generate_preset_pass_manager to convert loose input of constraints to a Target with Target.from_configuration() #12185
• Deprecate anything we previously could represent that is lost in conversion to Target (or alternatively augment Target's data model to support these edge cases). The cases currently known where we're unable to convert from the legacy model to the Target are:
  • coupling_map without basis_gates (represent solely a connectivity constraint).

After the above is done and released then we can start the deprecation process for the legacy inputs to the transpiler passes along with the PassManager config.