Scalafix sbt task hangs when invoked as part of compile task

[bpholt]

Prior to Scalafix 0.12.1, we had many repos where we apply Scalafix rules on every compile by chaining the scalafix task as part of the compile task. As of 0.12.1, applying any semantic rules in this way causes sbt to hang in such a way that it can only be killed from the outside (e.g. using kill -9 ${sbt pid}).

As a minimal example, use this build.sbt:

Compile / compile := Def.taskDyn {
  val output = (Compile / compile).value
  Def.task {
    (Compile / scalafix).toTask(" ExplicitResultTypes").value
    output
  }
}.value

project/plugins.sbt:

addSbtPlugin("ch.epfl.scala" % "sbt-scalafix" % "0.12.1")

and project/build.properties:

sbt.version=1.10.0

Example output:

[info] welcome to sbt 1.10.0 (Eclipse Adoptium Java 11.0.18)
[info] loading settings for project global-plugins from global.sbt ...
[info] loading global plugins from ~/.sbt/1.0/plugins
[info] loading settings for project sbt-scalafix-issue-build from plugins.sbt ...
[info] loading project definition from ~/sbt-scalafix-issue/project
[info] loading settings for project sbt-scalafix-issue from build.sbt ...
[info] set current project to sbt-scalafix-issue (in build file:~/sbt-scalafix-issue/)
sbt:sbt-scalafix-issue> compile
^C
[warn] Canceling execution...
^Z
[1]+  Stopped                 sbt
~/sbt-scalafix-issue $ kill -9 %1
[1]+  Killed: 9               sbt

If we comment out the (Compile / scalafix).toTask(" ExplicitResultTypes").value line in build.sbt and run the tasks separately, they work fine:

[info] welcome to sbt 1.10.0 (Eclipse Adoptium Java 11.0.18)
[info] loading settings for project global-plugins from global.sbt ...
[info] loading global plugins from ~/.sbt/1.0/plugins
[info] loading settings for project sbt-scalafix-issue-build from plugins.sbt ...
[info] loading project definition from ~/sbt-scalafix-issue/project
[info] loading settings for project sbt-scalafix-issue from build.sbt ...
[info] set current project to sbt-scalafix-issue (in build file:~/sbt-scalafix-issue/)
sbt:sbt-scalafix-issue> compile
[success] Total time: 0 s, completed Jun 10, 2024, 4:56:53 PM
sbt:sbt-scalafix-issue> scalafix ExplicitResultTypes
[success] Total time: 1 s, completed Jun 10, 2024, 4:57:02 PM

Furthermore, using one of the built-in syntactic rules works fine as well.

Replacing Scalafix 0.12.1 with 0.12.0 also works fine.