[sbt 0.13] Support sourceGenerators += Def.task { ... }

[eed3si9n]

This is a backport of #2942

Adds an Append instance that extracts taskValue

This adds a macro-level hack to support += op for sourceGenerators and resourceGenerators using RHS of Initialize[Task[Seq[File]]].
When the types match up, the macro now calls .taskValue automatically.

Before:

sourceGenerators in Compile += buildInfo.taskValue,
sourceGenerators in Compile += (Def.task { Nil }).taskValue

After:

sourceGenerators in Compile += buildInfo,
sourceGenerators in Compile += Def.task { Nil }

[jvican]

Note that this only works for settings.

The macro code to implement this looks scary. Are we sure we want to special case this for settings and +=? This is IMO a big hack that can confuse people even more since they won't know where they should actually do .taskValue or not...

@eed3si9n Can you comment on this?

[eed3si9n]

What we colloquially call "tasks" are Def.Initialize[Task[A]], which TaskKey[A] also extends. Importantly, it's not just Task[A]. The build user expects sourceGenerators to also collect reference to "tasks", and the fact that its type is SettingKey[Seq[Task[Seq[File]]]] is not that important. It's just the matter of matching types.

See discussion in #2942 for more details.

[jvican]

I'm well aware of those details.

My comment is that this only works for setting keys (source generators) but it won't work for task keys. We're still forcing users to match the types in that case.

[eed3si9n]

Sure. But why would anyone wrap Task in a Task?