fix: ensured hints are relabelled under a label combinator

Author: j-mie6

parsley/shared/src/main/scala/parsley/internal/machine/instructions/ErrorInstrs.scala

@@ -35,7 +35,8 @@ private [internal] final class RelabelHints(labels: Iterable[String]) extends In
 private [internal] final class RelabelErrorAndFail(labels: Iterable[String]) extends Instr {
     override def apply(ctx: Context): Unit = {
         ensureHandlerInstruction(ctx)
-        //ctx.restoreHints() //FIXME: I'm not sure this was meant to be there in the first place
+        // this has the effect of relabelling all hints since the start of the label combinator
+        ctx.restoreHints()
         ctx.errs.error = ctx.useHints {
             // only use the label if the error message is generated at the same offset
             // as the check stack saved for the start of the `label` combinator.

parsley/shared/src/test/scala/parsley/ErrorTests.scala

@@ -125,6 +125,13 @@ class ErrorTests extends ParsleyTest {
     }
 
     it should "replace everything under the label" in {
+        val p = (optional('a') *> 'b').label("hi")
+        inside(p.parse("e")) {
+            case Failure(TestError((1, 1), VanillaError(unex, exs, rs, 1))) =>
+                unex should contain (Raw("e"))
+                exs should contain.only(Named("hi"))
+                rs shouldBe empty
+        }
         val s = (optional('a') *> optional('b')).label("hi") *> 'c'
         inside(s.parse("e")) {
             case Failure(TestError((1, 1), VanillaError(unex, exs, rs, 1))) =>