Mix scala 2 and 3 macros #395

gregghz · 2024-05-21T21:40:10Z

This follows the pattern shown here: https://docs.scala-lang.org/scala3/guides/migration/tutorial-macro-mixing.html#:~:text=Introduction,during%20the%20macro%20expansion%20phase

This would allow a mixed scala version project to depend on the scala 3 version of enumeratum and build correctly for both scala 2 modules and scala 3 modules. This is particularly helpful when building with bazel since you can't conditionally include a dependency based on the current scala version the same way you would in sbt.

This was brought up previously by @coreywoodfield here: #300 (comment) (and he is the author of this PR -- I just rebased the branch).

lloydmeta · 2024-05-21T23:01:03Z

Thanks for the PR but can you please add a description for this that explains why we want this?

gregghz · 2024-05-21T23:08:23Z

@lloydmeta ah, thanks for taking a look. I've updated the description to clarify what's going on.

gregghz · 2024-09-20T21:52:27Z

@lloydmeta thanks for your patience with this. I just updated things again to hopefully resolve the build failures. I can't remember, were you manually triggering builds?

lloydmeta · 2024-09-21T02:22:36Z

Yea... I think I set something up once that requires me to approve the CI runs 🤦‍♂️

codecov-commenter · 2024-09-22T00:31:30Z

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@@            Coverage Diff             @@
##           master     #395      +/-   ##
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lloydmeta

Thanks for doing this.

There are quite a lot of changes that do not feel necessary, some of which are IMO a step backwards. My request would be to remove any changes that are not absolutely necessary.

.github/workflows/ci.yml

lloydmeta · 2024-09-22T08:11:13Z

.scalafmt.conf

@@ -1,4 +1,4 @@
-version = 3.7.8
+version = 3.7.17


Was this needed?

Seems like no, I'll revert.

I changed this to 3.7.10 because without that update, in sbt ++3.3.4 test:compile fails with:

[error] (coreJS / Compile / scalafmt) org.scalafmt.sbt.ScalafmtSbtReporter$ScalafmtSbtError: scalafmt: /home/gregg/lucid/enumeratum/enumeratum-core/src/main/scala-3/enumeratum/EnumCompat.scala:14: error: [dialect scala3] pattern must be a value or have parens: EnumMacros.findValuesImpl[A] [error] protected inline def findValues: IndexedSeq[A] = ${ EnumMacros.findValuesImpl[A] } [error] ^ [/home/gregg/lucid/enumeratum/enumeratum-core/src/main/scala-3/enumeratum/EnumCompat.scala]

there and a few other places.

lloydmeta · 2024-09-22T08:11:20Z

build.sbt

-lazy val scala_2_13Version = "2.13.11"
-lazy val scala_3Version    = "3.3.0"
+lazy val scala_2_13Version = "2.13.13"
+lazy val scala_3Version    = "3.4.0"


Please revert.

lloydmeta · 2024-09-22T08:11:53Z

build.sbt

-        "org.scala-lang" %% "scala3-compiler" % scalaVersion.value % Provided
+        Seq(
+          "org.scala-lang" %% "scala3-compiler" % scalaVersion.value % Provided,
+          "org.scala-lang" % "scala-reflect" % scala_2_13Version,


Curious if there's a way to make this additional dependency

Contingent on actually using the mix mode macro

Only a compile time constraint?

Unfortunately I think only the consumer of the library knows if the mixed mode macro is being used, from here, sbt only knows that it's a scala3 build.

I believe we could mark this as Provided too, but then all consumers would need to explicitly provide it. Marking it provided here means I also need to add it as Provided in core. It would only need to be added as a normal/runtime dep if the project was using the mixed mode because I think this is only needed for the scala 2 portion of the build.

@lloydmeta let me know if you have a preference here.

Still having a think about this - has there been any better solution than the ones presented here? Adding another runtime dependency is not great.

Personally I think it's fine if the experience is subpar for "mixed mode" but not for Scala 2 or 3 users.

lloydmeta · 2024-09-22T08:12:12Z

build.sbt

    val base = {
      if (scalaBinaryVersion.value == "3") {
-        minimal :+ "-deprecation"
+        minimal :+ "-Wconf:cat=deprecation:s"


Curious why this was needed?

[error] -- Error: /home/gregg/lucid/enumeratum/macros/src/main/scala/enumeratum/compat/EnumMacros.scala:109:32 [error] 109 | val enclosingModule = c.enclosingClass match { [error] | ^^^^^^^^^^^^^^^^ [error] |method enclosingClass in trait Enclosures is deprecated since 2.11.0: c.enclosingTree-style APIs are now deprecated; consult the scaladoc for more information

and a few more identical errors

lloydmeta · 2024-09-22T08:13:12Z

enumeratum-core/src/main/scala-3/enumeratum/EnumCompat.scala

    * aren't using this method... why are you even bothering with this lib?
    */
-  inline def findValues: IndexedSeq[A] = ${ EnumMacros.findValuesImpl[A] }
+  protected inline def findValues: IndexedSeq[A] = ${ EnumMacros.findValuesImpl[A] }


Curious why this is now protected.

I'm not sure. I reverted it.

lloydmeta · 2024-09-22T08:13:54Z

enumeratum-core/src/main/scala/enumeratum/EnumEntry.scala

  def entryName: String = stableEntryName

-  private[this] lazy val stableEntryName: String = toString
+  private lazy val stableEntryName: String = toString


Was this (and below) removals of this needed?

the scala 3 build fails without removing these (deprecated feature)

This change has been reverted. It was originally made because Scala v3.4.0 deprecates [this] access modifiers, but, as we're now using Scala v3.3.5 on master, it's no longer necessary.

lloydmeta · 2024-09-22T08:14:02Z

enumeratum-play/src/main/scala/enumeratum/values/Forms.scala

 object Forms extends FormsCompat {

-  protected[this] def formatter[ValueType, EntryType <: ValueEnumEntry[
+  protected def formatter[ValueType, EntryType <: ValueEnumEntry[


This change has been reverted.

lloydmeta · 2024-09-22T08:14:55Z

macros/src/main/scala/enumeratum/compat/EnumMacros.scala

  /** Finds any [A] in the current scope and returns an expression for a list of them
    */
-  def findValuesImpl[A: c.WeakTypeTag](c: Context): c.Expr[IndexedSeq[A]] = {
+  def findValuesImpl[A: c.WeakTypeTag](c: Context): c.Tree = {


Was this needed? Feels like a step backwards to lose type info, so if it's not required, please revert.

See my comment here:
#395 (comment)

lloydmeta · 2024-09-22T08:15:06Z

macros/src/main/scala/enumeratum/compat/ValueEnumMacros.scala

Same question regarding type change for everything here.

See my comment here:
#395 (comment)

Scala 2 macros can be compiled in scala 3 only if they do not use reification or quasiquotes

This reverts commit 02b74d3.

This reverts commit b33b462.

jadenPete · 2025-09-18T21:22:10Z

Hi, @lloydmeta! I've obtained @gregghz's permission to take over ownership of this PR. I've made a couple changes to it:

I rebased it off of the latest version of master
I reverted a couple commits that weren't necessary
I reverted the changes to library/compiler versions, since those have been updated on master since this PR was originally opened

Regarding the modified return types in the macro methods (c.Expr[???] to c.Tree), this change was made because the Scala 3 compiler is unable to summon WeakTypeTags, so it's incompatible with mixed macro compilation. However, I'm hopeful that with comprehensive testing, this won't be much of a problem. If you disagree, please let me know and we can explore other ways to maintain specific type information.

Would you mind reviewing the PR another time whenever you get the chance? We really appreciate your help here.

Copilot

Pull Request Overview

This PR implements mixed Scala 2 and 3 macro support following the pattern from the Scala 3 migration guide. This allows projects using mixed Scala versions to depend on the Scala 3 version of enumeratum while building correctly for both Scala 2 and 3 modules.

Creates a compatibility layer with macro implementations in the compat package
Updates return types from c.Expr[IndexedSeq[A]] to c.Tree for cross-version compatibility
Adds Scala 2 macro fallback definitions alongside existing Scala 3 inline macros

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
macros/src/main/scala/enumeratum/compat/ValueEnumMacros.scala	Moves ValueEnumMacros to compat package and updates return types to c.Tree
macros/src/main/scala/enumeratum/compat/EnumMacros.scala	Moves EnumMacros to compat package and updates return types to c.Tree
enumeratum-core/src/main/scala-3/enumeratum/values/ValueEnumCompat.scala	Adds Scala 2 macro fallback definitions alongside Scala 3 inline macros
enumeratum-core/src/main/scala-3/enumeratum/EnumCompat.scala	Adds Scala 2 macro fallback definitions alongside Scala 3 inline macros
enumeratum-core/src/main/scala-2/enumeratum/values/ValueEnumCompat.scala	Updates import to use compat package macros
enumeratum-core/src/main/scala-2/enumeratum/EnumCompat.scala	Updates import to use compat package macros
macros/src/test/scala-2/enumeratum/FindValEnums.scala	Adds import for compat package
macros/compat/src/main/scala-3	Creates symlink to scala-2.13 for macro compatibility

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Copilot · 2025-09-20T03:40:48Z

macros/src/main/scala/enumeratum/compat/ValueEnumMacros.scala

+      weakTypeOf[ValueEntryType].baseClasses.contains(
+        c.mirror.staticClass(classOf[AllowAlias].getName)
+      )


[nitpick] The refactored type check using baseClasses.contains and c.mirror.staticClass(classOf[AllowAlias].getName) is more complex than the original <:< subtype check. Consider if this change is necessary for Scala 2/3 compatibility or if the original approach could be preserved.

Suggested change

weakTypeOf[ValueEntryType].baseClasses.contains(

c.mirror.staticClass(classOf[AllowAlias].getName)

)

weakTypeOf[ValueEntryType] <:< weakTypeOf[AllowAlias]

There's a similar problem here: the Scala 3 compiler is unable to summon WeakTypeTags

[error] -- [E172] Type Error: /home/jpeterson/lucid/enumeratum/macros/src/main/scala/enumeratum/compat/ValueEnumMacros.scala:108:61 [error] 108 | if (weakTypeOf[ValueEntryType] <:< weakTypeOf[AllowAlias]) { [error] | ^ [error] | No WeakTypeTag available for enumeratum.values.AllowAlias

Copilot · 2025-09-20T03:40:48Z

macros/src/main/scala/enumeratum/compat/EnumMacros.scala

+  ): c.Tree = {
    import c.universe._
    val resultType = weakTypeOf[A]
+    val indexedSeq = Ident(c.mirror.staticModule(classOf[IndexedSeq[A]].getName))


Using classOf[IndexedSeq[A]].getName with c.mirror.staticModule is more fragile than the original reify(IndexedSeq).tree approach. The classOf[IndexedSeq[A]] will fail at runtime because A is a type parameter. Consider using "scala.collection.immutable.IndexedSeq" as a string literal instead.

Suggested change

val indexedSeq = Ident(c.mirror.staticModule(classOf[IndexedSeq[A]].getName))

val indexedSeq = Ident(c.mirror.staticModule("scala.collection.immutable.IndexedSeq"))

I believe this change was necessary because we no longer have a WeakTypeTag[IndexedSeq[A\] available.

lloydmeta requested changes Sep 22, 2024

View reviewed changes

coreywoodfield and others added 7 commits September 18, 2025 15:43

Update scala 2 macros to not use reification

2ef62be

Scala 2 macros can be compiled in scala 3 only if they do not use reification or quasiquotes

Mix scala 2 and scala 3 macros

b69c89a

Update dependencies to fix builds

02b74d3

Suppress deprecation warning

b33b462

Removes protected

650fd7c

Revert "Update dependencies to fix builds"

3632171

This reverts commit 02b74d3.

Revert "Suppress deprecation warning"

18c59d8

This reverts commit b33b462.

jadenPete force-pushed the mix-scala-2-and-3-macros branch 2 times, most recently from 78ea80e to 18c59d8 Compare September 18, 2025 21:19

lloydmeta requested a review from Copilot September 20, 2025 03:40

Copilot AI reviewed Sep 20, 2025

View reviewed changes

	val indexedSeq = Ident(c.mirror.staticModule(classOf[IndexedSeq[A]].getName))
	val indexedSeq = Ident(c.mirror.staticModule("scala.collection.immutable.IndexedSeq"))

Mix scala 2 and 3 macros #395

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Mix scala 2 and 3 macros #395

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jadenPete commented Sep 18, 2025

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Pull Request Overview

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gregghz commented May 21, 2024 •

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