add --triggered support to scalafixOnCompile

src/main/scala/scalafix/sbt/ScalafixPlugin.scala

@@ -45,7 +45,8 @@ object ScalafixPlugin extends AutoPlugin {
 
     val scalafixOnCompile: SettingKey[Boolean] =
       settingKey[Boolean](
-        "Run Scalafix rule(s) declared in .scalafix.conf on compilation and fail on lint errors. " +
+        "Run Scalafix rule(s) declared in .scalafix.conf on compilation and fail on lint errors," +
+          "with overlaying configuration by `triggered` sections if defined in .scalafix.conf file." +
           "Off by default. When enabled, caching will be automatically activated, " +
           "but can be disabled with `scalafixCaching := false`."
       )
@@ -107,7 +108,7 @@ object ScalafixPlugin extends AutoPlugin {
               compile.value // evaluated first, before the potential scalafix evaluation
             if (scalafixOnCompile.value)
               scalafix
-                .toTask("")
+                .toTask(" --triggered")
                 .map(_ => oldCompile)
             else Def.task(oldCompile)
           }.value,
@@ -529,13 +530,29 @@ object ScalafixPlugin extends AutoPlugin {
                 // custom tool classpaths might contain directories for which we would need to stamp all files, so
                 // just disable caching for now to keep it simple and to be safe
                 throw StampingImpossible
+              case "--triggered" =>
+                // If there is a triggered section in a default config file, --triggered flag should be accounted.
+                // If not, --triggered can share the same cache.
+                checkIfTriggeredSectionExists
               case _ => true
             }
             write(cacheKeys)
           case Arg.NoCache =>
             throw StampingImpossible
         }
 
+        private lazy val checkIfTriggeredSectionExists: Boolean = {
+          val confInArgs = interface.args
+            .collect { case Arg.Config(conf) => conf }
+            .flatten
+            .lastOption
+          val configFile = confInArgs.fold(file(".scalafix.conf"))(_.toFile())
+          if (configFile.exists()) {
+            val lines = IO.readLines(configFile)
+            lines.exists(_.startsWith("triggered"))
+          } else false
+        }
+
         def stampFile(file: File): String = {
           // ensure the file exists and is not a directory
           if (file.isFile)

src/sbt-test/sbt-scalafix/scalafixOnCompile/.scalafix.conf

@@ -1,2 +1,9 @@
 rules = [DisableSyntax, RemoveUnused]
-DisableSyntax.noNulls = true
+DisableSyntax.noNulls = true
+
+DisableSyntax.noVars = true
+RemoveUnused.privates = false
+triggered = {
+    DisableSyntax.noVars = false
+    RemoveUnused.privates = true
+}

src/sbt-test/sbt-scalafix/scalafixOnCompile/args/src/test/scala/Variables.scala

@@ -0,0 +1,4 @@
+object Variables {
+  private var a = 1
+  def b: Int = return 2
+}

src/sbt-test/sbt-scalafix/scalafixOnCompile/build.sbt

@@ -5,7 +5,7 @@ inThisBuild(
     scalaVersion := Versions.scala212,
     scalacOptions ++= List(
       "-Yrangepos",
-      "-Ywarn-unused-import"
+      "-Ywarn-unused"
     )
   )
 )
@@ -21,3 +21,8 @@ lazy val rewrite = project
     addCompilerPlugin(scalafixSemanticdb)
   )
   .settings(scalafixConfigSettings(IntegrationTest): _*)
+
+lazy val args = project
+  .settings(
+    addCompilerPlugin(scalafixSemanticdb)
+  )

src/sbt-test/sbt-scalafix/scalafixOnCompile/test

@@ -23,4 +23,11 @@
 
 # check configuration granularity for scalafixOnCompile
 > set scalafixOnCompile.in(lint, Test) := false
-> lint/test:compile
+> lint/test:compile
+
+# check that triggered rules are respected on compilation, and ignored on explicit invocation.
+#   `private var` is detected by `DisableSyntax.noVars = true` on explicit invocation,
+#   and removed by `triggered.RemoveUnused.privates = true` on compilation.
+-> args/test:scalafix
+> args/test:compile
+> args/test:scalafix

src/sbt-test/skip-windows/caching/files/Triggered.scalafix.conf

@@ -0,0 +1,4 @@
+rules = [DisableSyntax]
+DisableSyntax.noVars = true
+
+triggered.rules = [ProcedureSyntax]

src/sbt-test/skip-windows/caching/test

@@ -106,6 +106,54 @@ $ exec chmod 000 src/main/scala/InitiallyValid.scala
 -> scalafix --check ProcedureSyntax
 $ delete src/main/scala
 
+# files should be re-checked in scalafix run without --triggered even after scalafix runs onCompile, if a triggered configuration is defined
+> set scalafixConfig := Some(file("files/Triggered.scalafix.conf"))
+> set scalafixOnCompile := true
+$ mkdir src/main/scala
+$ copy-file files/Valid.scala src/main/scala/Valid.scala
+> compile
+$ exec chmod 000 src/main/scala/Valid.scala
+> scalafix --triggered --check
+-> scalafix --check
+> set scalafixOnCompile := false
+$ delete src/main/scala
+
+# files should be re-checked in scalafix --triggered run even after scalafix run without --triggered, if a triggered configuration is defined
+> set scalafixConfig := Some(file("files/Triggered.scalafix.conf"))
+$ mkdir src/main/scala
+$ copy-file files/Valid.scala src/main/scala/Valid.scala
+> scalafix
+$ exec chmod 000 src/main/scala/Valid.scala
+> scalafix --check
+-> scalafix --triggered --check
+$ delete src/main/scala
+
+# files should not be re-checked in scalafix explicit invocation after scalafix runs onCompile if any triggered configuration is not defined
+> set scalafixConfig := Some(file("files/DisableSyntaxVar.scalafix.conf"))
+> set scalafixOnCompile := true
+$ mkdir src/main/scala
+$ copy-file files/Valid.scala src/main/scala/Valid.scala
+> compile
+$ exec chmod 000 src/main/scala/Valid.scala
+> scalafix --triggered --check
+> scalafix --check
+> set scalafixOnCompile := false
+$ delete src/main/scala
+
+# a file patched during the last run with --triggered should not also be cached.
+> set scalafixConfig := Some(file("files/Triggered.scalafix.conf"))
+> set scalafixOnCompile := true
+$ mkdir src/main/scala
+$ copy-file files/ProcedureSyntax.scala src/main/scala/ToPatch.scala
+-> scalafix --triggered --check
+> compile
+-$ must-mirror files/ProcedureSyntax.scala src/main/scala/ToPatch.scala
+$ exec chmod 000 src/main/scala/ToPatch.scala
+-> scalafix --triggered --check
+-> scalafix --check
+> set scalafixOnCompile := false
+$ delete src/main/scala
+
 # files should not be re-checked if all requested rules have previously been run across invocations
 > set scalafixConfig := Some(file("files/DisableSyntaxVar.scalafix.conf"))
 $ mkdir src/test/scala