When checking, output unified diff if requested

build.sbt

@@ -41,6 +41,7 @@ lazy val plugin = project
   .settings(
     moduleName := "sbt-scalafmt",
     libraryDependencies ++= List(
+      "com.googlecode.java-diff-utils" % "diffutils" % "1.3.0",
       "org.scalameta" %% "scalafmt-sysops" % scalafmtVersion,
       "org.scalameta" %% "scalafmt-dynamic" % scalafmtVersion
     ),

plugin/src/main/scala/org/scalafmt/sbt/DiffUtils.scala

@@ -0,0 +1,23 @@
+package org.scalafmt.sbt
+
+import org.scalafmt.CompatCollections.JavaConverters._
+
+import difflib.DiffUtils._
+
+object DiffUtils {
+
+  def unifiedDiff(file: String, input: String, output: String): String = {
+    def jList(code: String, addEol: Boolean) = {
+      val last = if (addEol) Iterator.single("") else Iterator.empty
+      (code.linesIterator ++ last).toIndexedSeq.asJava
+    }
+    val a = jList(input, false)
+    // output always has EOL; if input doesn't, pretend output has extra line
+    val inputNoEol = input.lastOption.forall(x => x != '\n' && x != '\r')
+    val b = jList(output, inputNoEol)
+    val patch =
+      generateUnifiedDiff(s"a$file", s"b$file", a, diff(a, b), 1)
+    if (patch.isEmpty) "" else patch.iterator().asScala.mkString("\n")
+  }
+
+}

plugin/src/main/scala/org/scalafmt/sbt/ErrorHandling.scala

@@ -1,6 +1,7 @@
 package org.scalafmt.sbt
 
 private[sbt] class ErrorHandling(
+    val printDiff: Boolean,
     val logOnEachError: Boolean,
     val failOnErrors: Boolean,
     val detailedErrorEnabled: Boolean

plugin/src/main/scala/org/scalafmt/sbt/ScalafmtPlugin.scala

@@ -77,6 +77,9 @@ object ScalafmtPlugin extends AutoPlugin {
     val scalafmtFailOnErrors = settingKey[Boolean](
       "Controls whether to fail in case of formatting errors."
     )
+    val scalafmtPrintDiff = settingKey[Boolean](
+      "Enables full diff output when running check."
+    )
   }
 
   import autoImport._
@@ -306,7 +309,11 @@ object ScalafmtPlugin extends AutoPlugin {
       }
       val unformatted = Set.newBuilder[File]
       withFormattedSources(Unit, sources) { (_, file, input, output) =>
-        log.warn(s"$file isn't formatted properly!")
+        val diff = if (errorHandling.printDiff) {
+          DiffUtils.unifiedDiff("/" + asRelative(file).getPath, input, output)
+        } else ""
+        val suffix = if (diff.isEmpty) "" else '\n' + diff
+        log.warn(s"$file isn't formatted properly!$suffix")
         unformatted += file
         Unit
       }
@@ -448,6 +455,7 @@ object ScalafmtPlugin extends AutoPlugin {
         thisProject.value,
         scalafmtFilter.value,
         new ErrorHandling(
+          scalafmtPrintDiff.value,
           scalafmtLogOnEachError.value,
           scalafmtFailOnErrors.value,
           scalafmtDetailedError.value
@@ -492,6 +500,7 @@ object ScalafmtPlugin extends AutoPlugin {
         thisProject.value,
         "",
         new ErrorHandling(
+          scalafmtPrintDiff.value,
           scalafmtLogOnEachError.value,
           scalafmtFailOnErrors.value,
           scalafmtDetailedError.value
@@ -524,6 +533,7 @@ object ScalafmtPlugin extends AutoPlugin {
       scalafmtOnCompile := false,
       scalafmtLogOnEachError := false,
       scalafmtFailOnErrors := true,
+      scalafmtPrintDiff := false,
       scalafmtDetailedError := false
     )