This is a compiler plugin that adds additional lint checks to protect against sharp corners in the Scala compiler and standard libraries.

It's a work in progress. For an overview of writing compiler plugins, see http://www.scala-lang.org/node/140

Add it as a compiler plugin in your project by editing your build.sbt file. For example, once published:

addCompilerPlugin("com.foursquare.lint" %% "linter" % "x.y.z")

Or, until published:

scalacOptions += "-Xplugin:..path-to-jar../linter.jar"

Optionally, run sbt console in this project to see it in action.

scala> Nil == None
<console>:29: warning: Comparing with == on instances of different types (object Nil, object None) will probably return false.
              Nil == None
                  ^

scala> List(1, 2, 3).contains("4")
<console>:29: warning: SeqLike[Int].contains(java.lang.String) will probably return false.
              List(1, 2, 3).contains("4")
                            ^

scala> import scala.collection.JavaConversions._
<console>:29: warning: Conversions in scala.collection.JavaConversions._ are dangerous.
       import scala.collection.JavaConversions._
                               ^

scala> import scala.collection.JavaConversions._
<console>:10: warning: Wildcard imports should be avoided.
       import scala.reflect.generic._

scala> Option(1).get
<console>:29: warning: Calling .get on Option will throw an exception if the Option is None.
              Option(1).get
                        ^

• Add more warnings
• Pick and choose which warnings you want
• Choose whether they should be warnings or errors

Feel free to implement these, or add your own ideas. Pull requests welcome!

• Modularize the wildcard import warnings like the "Avoid Star Import" configuration of checkstyle (http://checkstyle.sourceforge.net/config_imports.html)
• Require explicit override whenever a method is being overwritten
• Implicit methods should always have explicit return types
• Expressions spanning multiple lines should be enclosed in parentheses
• Unused method argument warnings
• Warn on unrestricted catch clauses (case e => ...)
• Traversable#head, Traversable#last, Traversable#maxBy
• Warn on shadowing variables, especially those of the same type
• Warn on inexhaustive pattern matching