The plugin should create a file
.settings/org.scala-ide.sdt.core.prefs with the following contents:
scala.compiler.additionalParams=\ -Xsource\:2.10 -Ymacro-expand\:none scala.compiler.installation=2.10 scala.compiler.sourceLevel=2.10 scala.compiler.useProjectSettings=true
(replace 2.10 with whatever Scala version the project actually uses)
Without this, Scala 2.10 projects will get tons of errors. Scala 2.11 projects should work just fine without this since Scala IDE uses Scala 2.11 by default.
The code that should be changed is the same code edited in this PR: https://github.com/gradle/gradle/pull/99/files