feat: support Scala 3 nightly versions in sbt

api/src/main/scala/org/scastie/api/ScalaTarget.scala

@@ -255,7 +255,12 @@ case class Scala3(scalaVersion: String) extends SbtScalaTarget {
     else renderSbtDouble(lib)
   }
 
-  def sbtConfig: String = sbtConfigScalaVersion
+  def sbtConfig: String =
+    if (ScalaVersions.isNightly(scalaVersion))
+      // TODO: change this to "resolvers += Resolver.scalaNightlyRepository" after upgrading to sbt 1.11.5
+      s"""|$sbtConfigScalaVersion
+          |resolvers += "scala-nightly" at "https://repo.scala-lang.org/artifactory/maven-nightlies"""".stripMargin
+    else sbtConfigScalaVersion
 
   def sbtPluginsConfig: String = ""
 

api/src/main/scala/org/scastie/api/ScalaVersions.scala

@@ -129,4 +129,6 @@ object ScalaVersions {
 
   def find(tpe: ScalaTargetType, sv: String): String =
     allVersions(tpe).find(_.startsWith(sv)).getOrElse(sv)
+
+  def isNightly(version: String): Boolean = version.endsWith("-NIGHTLY")
 }

client/src/main/scala/org/scastie/client/components/VersionSelector.scala

@@ -6,6 +6,8 @@ import japgolly.scalajs.react._
 
 import vdom.all._
 import org.scastie.buildinfo.BuildInfo
+import org.scastie.client.scalacli.ScalaVersionUtil
+import scala.scalajs.concurrent.JSExecutionContext.Implicits.queue
 
 case class VersionSelector(scalaTarget: SbtScalaTarget, onChange: ScalaTarget ~=> Callback) {
   @inline def render: VdomElement = VersionSelector.versionSelectorHook(this)
@@ -16,7 +18,11 @@ object VersionSelector {
   val versionSelectorHook =
     ScalaFnComponent
       .withHooks[VersionSelector]
-      .render(props => {
+      .useState(Option.empty[String])
+      .useEffectOnMountBy((_, nightly) =>
+        Callback.future(ScalaVersionUtil.resolveScala3Nightly.map(nightly.setState))
+      )
+      .render((props, nightly) => {
         def versionSelectors(scalaVersion: String) =
           props.scalaTarget match {
             case d: Scala2       => Scala2.apply(scalaVersion)
@@ -49,12 +55,29 @@ object VersionSelector {
               )
             }
             .toTagMod,
+          TagMod.when(props.scalaTarget.targetType == ScalaTargetType.Scala3)(
+            nightly.value match {
+              case Some(nightlyVersion) =>
+                li(
+                  input(
+                    `type` := "radio",
+                    id := "scala-nightly",
+                    name := "scalaV",
+                    onChange --> props.onChange(Scala3(nightlyVersion)),
+                    checked := ScalaVersions.isNightly(props.scalaTarget.scalaVersion)
+                  ),
+                  label(`for` := "scala-nightly", className := "radio", role := "button", "Nightly")
+                )
+              case None => EmptyVdom
+            }
+          ),
           li(
             label(
               div(cls := "select-wrapper"){
                 val isRecommended = ScalaVersions
                     .suggestedScalaVersions(props.scalaTarget.targetType)
-                    .contains(props.scalaTarget.scalaVersion)
+                    .contains(props.scalaTarget.scalaVersion) ||
+                  ScalaVersions.isNightly(props.scalaTarget.scalaVersion)
 
                 select(
                   name := "scalaVersion",

client/src/main/scala/org/scastie/client/scalacli/ScalaVersionUtil.scala

@@ -6,6 +6,7 @@ import org.scalajs.dom
 import scala.util.matching.Regex
 import scala.scalajs.concurrent.JSExecutionContext.Implicits.queue
 import org.scastie.buildinfo.BuildInfo
+import org.scastie.api.ScalaVersions
 
 object ScalaVersionUtil {
     val location = dom.window.location
@@ -102,4 +103,7 @@ object ScalaVersionUtil {
               Future.successful(version)
           }
     }
+
+    def resolveScala3Nightly: Future[Option[String]] =
+        resolveVersion(scala3Nightly).map(v => Option.when(ScalaVersions.isNightly(v))(v))
 }

instrumentation/src/main/scala/org/scastie/instrumentation/Instrument.scala

@@ -61,6 +61,8 @@ object Instrument {
 
   def instrumentOne(term: Term, tpeTree: Option[Type], offset: Int, isScalaJs: Boolean): Patch = {
 
+    // TODO: rename `$t` to a `$`-free identifier to silence the warning:
+    // "The identifier `$t` should not contain `$`, which is reserved for internal compiler use."
     val treeQuote =
       tpeTree match {
         case None      => s"val $$t = $term"