Fix #23: use synthetic val instead for unused vars

Synthetic vals just work for ignoring any warnings,
including with wartremover and would still trigger
side effects if needed

Author: oleg-py

better-monadic-for/src/main/scala/com/olegpy/bm4/NoTupleBinding.scala

@@ -18,11 +18,13 @@ trait NoTupleBinding extends TreeUtils {
           // Synthetic vals are generated by destructuring. We don't want to touch them
           case v @ ValDef(mods, _, _, _) if mods.hasFlag(Flag.SYNTHETIC) => v
 
-          // we wrap RHS in locally { } because we cannot check if it's pure or not without typer
+          // we wrap RHS in synthetic val ourselves because we cannot check if it's pure or not without typer
           // (pure exprs will be generated by destructuring) but we don't want warnings to be issued on
           // expressions like (_, x) = tmp because a temp variable underscore is written to is unused
-          case ValDef(_, TermName(s), _, rhs) if !usedVal(s) =>
-            replaceTree(rhs, q"scala.Predef.locally($rhs)")
+          case ValDef(_, TermName(s), tp, rhs) if !usedVal(s) =>
+            val name = freshTermName("$suppressedUnused$")(currentFreshNameCreator)
+            val defn = ValDef(Modifiers() | Flag.SYNTHETIC, name, tp, rhs)
+            replaceTree(rhs, defn)
 
           case a => a
         }

build.sbt

@@ -98,3 +98,13 @@ lazy val scalazTests = (project in file("scalaz-tests"))
     )
   )
   .settings(testSettings)
+
+lazy val wartRemoverTests = (project in file("wartremover-tests"))
+  .dependsOn(pluginTests % "compile->compile;test->test")
+  .settings(
+    name := "wartremover-tests",
+    crossScalaVersions := List(scala212),
+    addCompilerPlugin("org.wartremover" %% "wartremover" % "2.4.2"),
+    scalacOptions += "-P:wartremover:traverser:org.wartremover.warts.NonUnitStatements"
+  )
+  .settings(testSettings)

wartremover-tests/src/test/scala/com/olegpy/bm4/WartremoverWarnings.scala

@@ -0,0 +1,16 @@
+package com.olegpy.bm4
+
+import org.scalatest.freespec.AnyFreeSpec
+
+
+class WartremoverWarnings extends AnyFreeSpec {
+  "Wartremover should not complain" in {
+    def right[A, B](b: B): Either[A, B] = Right(b)
+    case class Foo(a: Long)
+    for {
+      Foo(a) <- right[String, Foo](Foo(1))
+      b <- right(Foo(2))
+      Foo(c) = b
+    } yield a + c
+  }
+}