scala/scala3

Slow performance in a combination of features

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#18,763 opened on Oct 25, 2023

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 (12 comments) (0 reactions) (0 assignees)Scala (6,247 stars) (1,159 forks)batch import
area:inferarea:typerbackloghelp wanteditype:enhancementitype:performancestat:needs minimization

Description

Compiler version

3.3.1, 3.4.0-RC1-bin-20231024-15033c7-NIGHTLY

Minimized example

//> using scala "3.4.0-RC1-bin-20231024-15033c7-NIGHTLY"
//> using lib "org.typelevel::cats-effect:3.5.2"
package demo

import cats.effect.IO
import cats.effect.kernel.Resource
import cats.implicits._

val demo: Resource[IO, Unit] = Resource.unit.as(???)

Output

Takes ~7s to compile on an M1 Max machine, ~18s in Scastie.

Expectation

Compiles as fast as this:

//> using scala "3.4.0-RC1-bin-20231024-15033c7-NIGHTLY"
//> using lib "org.typelevel::cats-effect:3.5.2"
package demo

import cats.effect.IO
import cats.effect.kernel.Resource
import cats.implicits._

val demo: Resource[IO, Unit] = Resource.unit[IO].as(???)

which is under a second on my machine.

Extra context

  • unit's signature is def unit[F[_]]: Resource[F, Unit]
  • as is a method coming from an implicit conversion, which is brought in via cats.implicits._
  • The code doesn't compile on Scala 2.13 because unit's parameter is simply not inferred at all (I suppose it's just the new inference algorithm)

I believe the slowdown is a combination of trying to infer unit's type parameter (which is a type constructor, potentially complicating inference) from a Scala 2 implicit conversion.

The main problem with this issue is that it's not only slowing down the actual compilation process (for my runs or tests), but also affecting all Metals functionality - completions timing out and similar.

Contributor guide

Tech stack
scala
Domain
performancebackend
Issue type
performance
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
5
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
over 1 week
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
stale
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Scala compiler internalstype inferenceimplicit conversions
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
10
Research direction
Investigate the Scala 3 compiler's type inference algorithm, specifically the interaction between implicit conversions and type constructor inference. Look at the compiler source files related to implicit resolution and type parameter inference, such as the Typer or Implicits classes. Profile the compilation to identify where the time is spent. Compare the fast version (explicit type application) vs slow version to understand the difference in compiler phases.