I’m starting to look at adding support for a) codegen to synthetically inject runtime dependencies, and b) “dependency inference” (meaning you can leave off

dependencies

in your BUILD file and Pants will do the right thing based on import statements) -- We need to convert

Dependencies

from a

PrimitiveField

to

AsyncField

so that the engine can do its magic. Call sites would look like:

await Get[Addresses](DependenciesRequest(tgt.get(Dependencies))

This means we have one rule to go from

DependenciesRequest -> Addresses

, just like we have one rule to go from

HydrateSourcesRequest -> HydratedSources

. Fine and good. -- The challenge is with codegen dependency injection. Specifically, that that injection will need to access subsystem options, e.g.

--protoc-runtime-targets

. It will need a

SubsystemRule

. But, we can’t have our generic

DependenciesRequest -> Addresses

rule request that subsystem because it has no idea it’s needed. Instead, in the rule, we need a way to say something like “If there are any injected dependencies for this

Dependencies

field, run the rule to inject those.” This is unions, right? Something like:

injected_deps = await Get[InjectedDependencies](InjectDependenciesRequest, dependencies_field)

And then the Protoc backend would define a rule that goes from

(ProtocDependencies, Protoc) -> InjectedDependencies

, where

ProtocDependencies

belongs to the union?