I stumbled across the ScalaCL project and its compiler plugin that has an awesome loop optimizer.
This made me wonder:
Which compiler plugins are available for scala?
Plugins need not be performance improving plugins; any type of plugin is eligible for this list.
(I have done Google searches but the SNR is low for this query.)
A few plugins are linked from this thread on the mailing list (autoproxy, browse, enhanced strings, avro).
From the Scala team, there is the existing delimited continuations plugin, and a current effort toward an effect system plugin.
Documentation on writing a plugin is here.