Add an as(Unit) linter

Author: TonioGela

modules/cats/input/src/main/scala/fix/AsTests.scala

@@ -3,6 +3,8 @@ rule = TypelevelAs
  */
 package fix
 
+import cats.syntax.functor.toFunctorOps
+
 object AsTests {
 
   def listMapLit = {
@@ -17,11 +19,18 @@ object AsTests {
     .map(_ => ()) can be replaced by .void */
   }
 
+  def listAsUnit = {
+    List(1, 2, 3).as(()) /* assert: TypelevelAs.as
+    ^^^^^^^^^^^^^^^^^^^^
+    .as(()) can be replaced by .void */
+  }
+
   def shouldBeIgnored = {
     def f = "a"
     List(1, 2, 3).map(_ => f)
     List(1, 2, 3).map(i => i)
     List(1, 2, 3).map(println(_))
+    List(1, 2, 3).as(1)
   }
 
 }

modules/cats/rules/src/main/scala/org/typelevel/fix/As.scala

@@ -26,6 +26,8 @@ class As extends SemanticRule("TypelevelAs") {
       case tree @ AnonymousMap(Lit.Unit()) => Patch.lint(VoidDiagnostic(tree))
       // fa.map(_ => 1)
       case tree @ AnonymousMap(_: Lit) => Patch.lint(AsDiagnostic(tree))
+      // fa.as(())
+      case tree @ As(Lit.Unit()) => Patch.lint(AsUnitDiagnostic(tree))
     }.asPatch
 
 }
@@ -41,6 +43,17 @@ object AnonymousMap {
   }
 }
 
+object As {
+  def unapply(term: Term): Option[Term] = term match {
+    case Term.Apply.Initial(
+          Term.Select(_, Term.Name("as")),
+          List(value)
+        ) =>
+      Some(value)
+    case _ => None
+  }
+}
+
 final case class AsDiagnostic(t: Tree) extends Diagnostic {
   override def message: String    = ".map(_ => f) can be replaced by .as(f)"
   override def position: Position = t.pos
@@ -52,3 +65,9 @@ final case class VoidDiagnostic(t: Tree) extends Diagnostic {
   override def position: Position = t.pos
   override def categoryID: String = "as"
 }
+
+final case class AsUnitDiagnostic(t: Tree) extends Diagnostic {
+  override def message: String    = ".as(()) can be replaced by .void"
+  override def position: Position = t.pos
+  override def categoryID: String = "as"
+}