Decide upon Compiler Plugin architecture

[felixmulder]

No description provided.

[liufengyun]

I'd like to work on the plugin architecture, which will open the door for a lot of experiments & research on Dotty without changing the compiler.

There are mainly two categories of plugins in Scalac: Phase plugins & Typer plugins.

For Phase Plugins, we need them as well, no question here. We can more or less follow the same design in Scalac for phase plugins.

There are also Typer Plugins: AnalyzerPlugin and MacroPlugin. We definitely don't want MacroPlugin. For AnalyzerPlugin, maybe we can begin with something simple without polluting too much the Typer:

1. the opportunity to expand untyped trees in Namer
   • usage: transform macro definitions, annotation based code generation
2. the last effort to work on type adaptation failure (?)
3. the last effort to work on implicit search failure (?)
4. the last effort to work on subtyping failure (?)

The last 3 items are contentious, they are just for discussion. Initially, I think it's fine to not support them.

With only PhasePlugin and NamerExpansion, a blackbox macro system can be supported without changing the compiler.

WDYT? cc/ @odersky