Inlay hints for deriving strategies

[michaelpj]

Is your enhancement request related to a problem? Please describe.

When you use deriving without deriving strategies, the choice of how the instance is derived is implicit. We could make this explicit with inlay hints.

Describe the solution you'd like

data Foo ...
  deriving <hint>stock</hint> (Eq, Ord)

The tricky bit will be if we have a list of derived classes which use different strategies! I would suggest something like:

data Foo ...
   deriving <hint>stock,anyclass</hint> (Eq, ToJSON)

Triggering this would of course produce

data Foo ..
   deriving stock Eq
   deriving anyclass ToJSON

[michaelpj]

This would be useful as a code action also, so we should make sure to expose it as a code action handler too. "Make deriving strategies explicit" or something.