Refactor RefThenConfig to be an opaque type

main-settings/src/main/scala/sbt/Reference.scala

@@ -14,7 +14,8 @@ import java.net.URI
 import sbt.internal.util.AttributeKey
 import sbt.io.IO
 import sbt.librarymanagement.Configuration
-import sbt.ScopeAxis.{ Select, This, RefThenConfig }
+import sbt.Scope.RefThenConfig
+import sbt.ScopeAxis.{ Select, This }
 
 // in all of these, the URI must be resolved and normalized before it is definitive
 
@@ -25,13 +26,13 @@ sealed trait Reference:
   private[sbt] def asScope: Scope =
     Scope(asScopeAxis, This, This, This)
 
-  def /(c: ConfigKey): RefThenConfig = RefThenConfig(asScope.rescope(c))
+  def /(c: ConfigKey): RefThenConfig = RefThenConfig(asScopeAxis, c)
 
-  def /(c: Configuration): RefThenConfig = RefThenConfig(asScope.rescope(c))
+  def /(c: Configuration): RefThenConfig = RefThenConfig(asScopeAxis, c: ConfigKey)
 
   // This is for handling `Zero / Zero / name`.
   def /(configAxis: ScopeAxis[ConfigKey]): RefThenConfig =
-    RefThenConfig(asScope.copy(config = configAxis))
+    RefThenConfig(asScopeAxis, configAxis)
 
   final def /[K](key: Scoped.ScopingSetting[K]): K = asScope.scope(key)
 

main-settings/src/main/scala/sbt/Scope.scala

@@ -430,4 +430,23 @@ object Scope:
         t <- withZeroAxis(scope.task)
         e <- withZeroAxis(scope.extra)
       } yield Scope(Zero, c, t, e)
+
+  /**
+   * Temporary data structure to capture first two axis using slash syntax.
+   * In theory, we might be able to express this as type parameters of Scope,
+   * like Scope[Select[ThisBuild.type], Select[ConfigKey], This, This] but then
+   * scope becomes more complicated to deal with.
+   */
+  opaque type RefThenConfig = (ScopeAxis[Reference], ScopeAxis[ConfigKey])
+
+  object RefThenConfig:
+    def apply(project: ScopeAxis[Reference], config: ScopeAxis[ConfigKey]): RefThenConfig =
+      (project, config)
+    def apply(project: ScopeAxis[Reference], config: ConfigKey): RefThenConfig =
+      (project, Select(config))
+
+    extension (in: RefThenConfig)
+      def project: ScopeAxis[Reference] = in._1
+      def config: ScopeAxis[ConfigKey] = in._2
+  end RefThenConfig
 end Scope

main-settings/src/main/scala/sbt/ScopeAxis.scala

@@ -10,10 +10,9 @@ package sbt
 
 import sbt.internal.util.Util.*
 import sbt.librarymanagement.Configuration
-import sbt.internal.util.AttributeKey
 
 enum ScopeAxis[+A1]:
-  import ScopeAxis.RefThenConfig
+  import Scope.RefThenConfig
 
   /**
    * Select is a type constructor that is used to wrap type `S`
@@ -57,12 +56,12 @@ enum ScopeAxis[+A1]:
   inline def /[K](key: Scoped.ScopingSetting[K])(using A1 <:< Reference): K = key.rescope(asScope)
 
   inline def /(c: ConfigKey)(using A1 <:< Reference): RefThenConfig =
-    RefThenConfig(asScope.rescope(c))
+    RefThenConfig(this.asInstanceOf, c)
   inline def /(c: Configuration)(using A1 <:< Reference): RefThenConfig =
-    RefThenConfig(asScope.rescope(c))
+    RefThenConfig(this.asInstanceOf, c: ConfigKey)
   // This is for handling `Zero / Zero / name`.
   inline def /(configAxis: ScopeAxis[ConfigKey])(using A1 <:< Reference): RefThenConfig =
-    RefThenConfig(asScope.copy(config = configAxis))
+    RefThenConfig(this.asInstanceOf, configAxis)
 end ScopeAxis
 
 object ScopeAxis:
@@ -72,15 +71,4 @@ object ScopeAxis:
   def fromOption[A1](o: Option[A1]): ScopeAxis[A1] = o match
     case Some(v) => ScopeAxis.Select(v)
     case None    => ScopeAxis.Zero
-
-  /** Temporary data structure to capture first two axis using slash syntax. */
-  class RefThenConfig(val scope: Scope):
-    override def toString(): String = scope.toString()
-    def /[K](key: Scoped.ScopingSetting[K]): K = scope / key
-
-    def /(task: AttributeKey[?]): Scope = scope.copy(task = Select(task))
-
-    /** This is for handling `Zero / Zero / Zero / name`. */
-    def /(taskAxis: ScopeAxis[AttributeKey[?]]): Scope = scope.copy(task = taskAxis)
-  end RefThenConfig
 end ScopeAxis

main-settings/src/main/scala/sbt/SlashSyntax.scala

@@ -11,6 +11,8 @@ package sbt
 import sbt.librarymanagement.Configuration
 import sbt.internal.util.AttributeKey
 import sbt.ScopeAxis.{ Select, This }
+import sbt.Scope.RefThenConfig
+import sbt.Scope.RefThenConfig.{ project, config }
 
 /**
  * SlashSyntax implements part of the slash syntax to scope keys for build.sbt DSL.
@@ -39,6 +41,15 @@ trait SlashSyntax:
     def asScope: Scope = (c: ConfigKey).asScope
     def /[K](key: Scoped.ScopingSetting[K]): K = (c: ConfigKey) / key
     def /(task: AttributeKey[?]): Scope = (c: ConfigKey) / task
+
+  extension (in: RefThenConfig)
+    def asScope: Scope = in.project.asScope.copy(config = in.config)
+    def toString(): String = asScope.toString()
+    def /[K](key: Scoped.ScopingSetting[K]): K = asScope / key
+    def /(task: AttributeKey[?]): Scope = asScope.copy(task = Select(task))
+
+    /** This is for handling `Zero / Zero / Zero / name`. */
+    def /(taskAxis: ScopeAxis[AttributeKey[?]]): Scope = asScope.copy(task = taskAxis)
 end SlashSyntax
 
 private[sbt] object SlashSyntax0 extends SlashSyntax

main/src/main/scala/sbt/coursierint/CoursierArtifactsTasks.scala

@@ -19,6 +19,7 @@ import lmcoursier.definitions.{
 import sbt.librarymanagement._
 import sbt.Keys._
 import sbt.ProjectExtra.extract
+import sbt.SlashSyntax0.*
 
 object CoursierArtifactsTasks {
   def coursierPublicationsTask(

main/src/main/scala/sbt/internal/ClasspathImpl.scala

@@ -18,6 +18,7 @@ import sbt.Def.Initialize
 import sbt.internal.util.{ Attributed, Dag }
 import sbt.librarymanagement.{ Configuration, TrackLevel }
 import sbt.librarymanagement.Configurations.names
+import sbt.SlashSyntax0.*
 import sbt.std.TaskExtra._
 import sbt.util._
 import scala.jdk.CollectionConverters.*

main/src/main/scala/sbt/internal/server/BuildServerProtocol.scala

@@ -18,6 +18,7 @@ import sbt.Keys._
 import sbt.ProjectExtra.*
 import sbt.ScopeFilter.Make._
 import sbt.Scoped.richTaskSeq
+import sbt.SlashSyntax0.*
 import sbt.StandardMain.exchange
 import sbt.internal.bsp._
 import sbt.internal.langserver.ErrorCodes