Because the compiler interface is recompiled against each Scala version in use in your project, its source must stay compatible with all the Scala versions that sbt supports (from Scala 2.8 to the latest version of Scala).
This comes at great cost for both the the sbt maintainers and the Scala compiler authors:
To circumvent this problem, a new mechanism that allows sbt to fetch the version of the sources for the compiler interface that are the most specific for the Scala version in use has been implemented in sbt.
For instance, for a project that is compiled using Scala 2.11.8-M2, sbt will look for the following version of the sources for the compiler interface, in this order:
This new mechanism allows both the Scala compiler and sbt to move forward and enjoy new APIs while being certain than users of older versions of Scala will still be able to use sbt.
Finally, another advantage of this technique is that it relies on Ivy to retrieve the sources of the compiler bridge, but can be easily ported for use with Maven, which is the distribution mechanism that the sbt maintainers would like to use to distribute sbt’s modules.