Remove allocations from StringIn when returning a string

bench/src/main/scala/cats/parse/bench/StringInBench.scala

@@ -39,7 +39,9 @@ private[parse] class StringInBenchmarks {
 
   var radixNode: RadixNode = _
 
-  var stringIn: Parser[Unit] = _
+  var stringInV: Parser[Unit] = _
+
+  var stringInS: Parser[String] = _
 
   var oneOf: Parser[Unit] = _
 
@@ -60,13 +62,18 @@ private[parse] class StringInBenchmarks {
     }
 
     radixNode = RadixNode.fromStrings(stringsToMatch)
-    stringIn = Parser.stringIn(stringsToMatch).void
+    stringInS = Parser.stringIn(stringsToMatch)
+    stringInV = stringInS.void
     oneOf = Parser.oneOf(stringsToMatch.map(Parser.string(_)))
   }
 
   @Benchmark
-  def stringInParse(): Unit =
-    inputs.foreach(stringIn.parseAll(_))
+  def stringInVParse(): Unit =
+    inputs.foreach(stringInV.parseAll(_))
+
+  @Benchmark
+  def stringInSParse(): Unit =
+    inputs.foreach(stringInS.parseAll(_))
 
   @Benchmark
   def oneOfParse(): Unit =

build.sbt

@@ -135,18 +135,33 @@ lazy val core = crossProject(JSPlatform, JVMPlatform)
       if (isScala211) Set.empty else mimaPreviousArtifacts.value
     },
     mimaBinaryIssueFilters ++= {
+      /*
+       * It is okay to filter anything in Impl or RadixNode which are private
+       */
       if (tlIsScala3.value)
         List(
-          ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#State.error"),
-          ProblemFilters.exclude[IncompatibleMethTypeProblem]("cats.parse.Parser#State.error_="),
-          ProblemFilters.exclude[IncompatibleMethTypeProblem]("cats.parse.RadixNode.this"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.Parser#Impl.mergeCharIn"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.Parser#Impl.mergeStrIn"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.RadixNode.children"),
           ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.RadixNode.fsts"),
           ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.RadixNode.prefixes"),
-          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.RadixNode.children"),
           ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.RadixNode.word"),
-          ProblemFilters.exclude[FinalClassProblem]("cats.parse.RadixNode")
+          ProblemFilters.exclude[FinalClassProblem]("cats.parse.RadixNode"),
+          ProblemFilters.exclude[IncompatibleMethTypeProblem]("cats.parse.Parser#State.error_="),
+          ProblemFilters.exclude[IncompatibleMethTypeProblem]("cats.parse.RadixNode.this"),
+          ProblemFilters
+            .exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#Impl#StringIn.parseMut"),
+          ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#Impl.stringIn"),
+          ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#State.error")
+        )
+      else
+        List(
+          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.Parser#Impl.mergeCharIn"),
+          ProblemFilters.exclude[DirectMissingMethodProblem]("cats.parse.Parser#Impl.mergeStrIn"),
+          ProblemFilters
+            .exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#Impl#StringIn.parseMut"),
+          ProblemFilters.exclude[IncompatibleResultTypeProblem]("cats.parse.Parser#Impl.stringIn")
         )
-      else Nil
     }
   )
   .jsSettings(