Severe performance regression in `neotypes/neotypes`

[WojciechMazur]

Based on OpenCB build in neotypes/neotypes - build logs

When using Scala 3.6.4-RC1 the compilation finishes under 3-4 minutes
In 3.7.0-NIGHTLY it takes over 3 hours

Bisect (based on 5 minut timeout for building the whole project) points to c65d45d

Needs minimization

[Gedochao]

cc @som-snytt

c65d45d +2622-1230 lines changed

Ah yes, this will be tricky to pinpoint...

[Gedochao]

@som-snytt I assigned you for now, since you probably have the best chance of finding the root cause, but let me know if we should get someone else to take a look.

[som-snytt]

It turned out to be easy to isolate, as there is a generated source of the form

final def productNamed[A <: Product, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W]
(b: (String, ResultMapper[B]), c: (String, ResultMapper[C]), d: (String, ResultMapper[D]), e: (String, ResultMapper[E]), f: (String, ResultMapper[F]), g: (String, ResultMapper[G]), h: (String, ResultMapper[H]), i: (String, ResultMapper[I]), j: (String, ResultMapper[J]), k: (String, ResultMapper[K]), l: (String, ResultMapper[L]), m: (String, ResultMapper[M]), n: (String, ResultMapper[N]), o: (String, ResultMapper[O]), p: (String, ResultMapper[P]), q: (String, ResultMapper[Q]), r: (String, ResultMapper[R]), s: (String, ResultMapper[S]), t: (String, ResultMapper[T]), u: (String, ResultMapper[U]), v: (String, ResultMapper[V]), w: (String, ResultMapper[W]))
(fun: (B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W) => A): ResultMapper[A] =
fromFunctionNamed(
b._1, c._1, d._1, e._1, f._1, g._1, h._1, i._1, j._1, k._1, l._1, m._1, n._1, o._1, p._1, q._1, r._1, s._1, t._1, u._1, v._1, w._1
)(fun)(
b._2, c._2, d._2, e._2, f._2, g._2, h._2, i._2, j._2, k._2, l._2, m._2, n._2, o._2, p._2, q._2, r._2, s._2, t._2, u._2, v._2, w._2
)
}

sbt-typelevel turns on parameter checking.

[som-snytt]

Apparently a =:=, top of the stack shown:

        at dotty.tools.dotc.core.TypeComparer.loop$1(TypeComparer.scala:1357)
        at dotty.tools.dotc.core.TypeComparer.isMatchingApply$1(TypeComparer.scala:1371)
        at dotty.tools.dotc.core.TypeComparer.compareAppliedType2$1(TypeComparer.scala:1441)
        at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:649)
        at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:573)
        at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:432)
        at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1599)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:229)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:239)
        at dotty.tools.dotc.core.TypeComparer.isSameType(TypeComparer.scala:2359)
        at dotty.tools.dotc.core.TypeComparer.isSubArg$1(TypeComparer.scala:1852)
        at dotty.tools.dotc.core.TypeComparer.recurArgs$1(TypeComparer.scala:1862)
        at dotty.tools.dotc.core.TypeComparer.isSubArgs(TypeComparer.scala:1872)
        at dotty.tools.dotc.core.TypeComparer.loop$1(TypeComparer.scala:1357)
        at dotty.tools.dotc.core.TypeComparer.isMatchingApply$1(TypeComparer.scala:1371)
        at dotty.tools.dotc.core.TypeComparer.compareAppliedType2$1(TypeComparer.scala:1441)
        at dotty.tools.dotc.core.TypeComparer.thirdTry$1(TypeComparer.scala:649)
        at dotty.tools.dotc.core.TypeComparer.secondTry$1(TypeComparer.scala:573)
        at dotty.tools.dotc.core.TypeComparer.firstTry$1(TypeComparer.scala:432)
        at dotty.tools.dotc.core.TypeComparer.recur(TypeComparer.scala:1599)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:229)
        at dotty.tools.dotc.core.TypeComparer.isSubType(TypeComparer.scala:239)
        at dotty.tools.dotc.core.TypeComparer.isSameType(TypeComparer.scala:2359)
        at dotty.tools.dotc.core.TypeComparer$.isSameType(TypeComparer.scala:3400)
        at dotty.tools.dotc.core.Types$Type.$eq$colon$eq(Types.scala:1135)
        at dotty.tools.dotc.transform.CheckUnused$.dotty$tools$dotc$transform$CheckUnused$Resolved$$_$hasRecord$$anonfun$1(CheckUnused.scala:470)
        at dotty.tools.dotc.transform.CheckUnused$Resolved$$Lambda/0x00007f725f4ca100.apply(Unknown Source)
        at scala.collection.immutable.List.find(List.scala:414)
        at dotty.tools.dotc.transform.CheckUnused$Resolved.hasRecord(CheckUnused.scala:470)
        at dotty.tools.dotc.transform.CheckUnused.resolveUsage(CheckUnused.scala:340)
        at dotty.tools.dotc.transform.CheckUnused.transformSelect(CheckUnused.scala:78)
        at dotty.tools.dotc.transform.CheckUnused.transformSelect(CheckUnused.scala:67)
        at dotty.tools.dotc.transform.MegaPhase.goSelect(MegaPhase.scala:636)

for types such as

CMP neotypes.mappers.ResultMapper[(((((((((B, C), D), E), F), G), H), I), J), K)] =:= neotypes.mappers.ResultMapper[(
  (
    (((((((((((((((B, C), D), E), F), G), H), I), J), K), L), M), N), O), P), Q)
      ,
  R),
S)]

This is comparing prefixes of a selection and import, which was the motivating problem.

Edit: per the PR, it should not have been performing these comparisons, so it is moot whether they are expensive.