Hooks for macro classloader caching

[retronym]

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Macros implementations are executed with reflection from a classloader created from the entire compilation classpath.

Scala 2.12.6 adds experimental support for caching these classloaders for performance reasons.

In practice, when multiple Globals are being used with differing classpaths, the caching often doesn't help.

It would be good if the compiler offered a programatic hook and/or config options to let users specify the runtime classpath of each macro (which would be a small subset of the full classpath)

[dwijnand]

Did this progress in any way in scala or in zinc?