Store the JSDefinitions in a custom platform SJSPlatform.

This required the ability to instantiate a different `Platform`
depending on settings, which, in turn, required to defer the
initialization of `ContextBase.platform`.

Author: sjrd

src/dotty/tools/backend/sjs/JSDefinitions.scala

@@ -8,22 +8,12 @@ import Symbols._
 import StdNames._
 import Decorators._
 
+import dotty.tools.dotc.config.SJSPlatform
+
 object JSDefinitions {
-  @dotty.tools.sharable
-  private val cache = new java.util.WeakHashMap[ContextBase, JSDefinitions]
-
-  // TODO Figure out where best to define this
-  def jsdefn(implicit ctx: Context): JSDefinitions = cache.synchronized {
-    val baseCtx = ctx.base
-    val cached = cache.get(baseCtx)
-    if (cached != null) {
-      cached
-    } else {
-      val newJSDefinitions = new JSDefinitions()
-      cache.put(baseCtx, newJSDefinitions)
-      newJSDefinitions
-    }
-  }
+  /** The Scala.js-specific definitions for the current context. */
+  def jsdefn(implicit ctx: Context): JSDefinitions =
+    ctx.platform.asInstanceOf[SJSPlatform].jsDefinitions
 }
 
 final class JSDefinitions()(implicit ctx: Context) {

src/dotty/tools/dotc/Compiler.scala

@@ -102,6 +102,7 @@ class Compiler {
    *    imports      For each element of RootImports, an import context
    */
   def rootContext(implicit ctx: Context): Context = {
+    ctx.initPlatform()(ctx)
     ctx.definitions.init(ctx)
     val actualPhases = if (ctx.settings.scalajs.value) {
       phases
@@ -124,6 +125,7 @@ class Compiler {
       .setMode(Mode.ImplicitsEnabled)
       .setTyperState(new MutableTyperState(ctx.typerState, ctx.typerState.reporter, isCommittable = true))
     ctx.definitions.init(start) // set context of definitions to start
+    ctx.initPlatform()(start) // re-initialize the platform with start
     def addImport(ctx: Context, refFn: () => TermRef) =
       ctx.fresh.setImportInfo(ImportInfo.rootImport(refFn)(ctx))
     (start.setRunInfo(new RunInfo(start)) /: defn.RootImportFns)(addImport)

src/dotty/tools/dotc/config/SJSPlatform.scala

@@ -0,0 +1,13 @@
+package dotty.tools.dotc.config
+
+import dotty.tools.dotc.core._
+import Contexts._
+
+import dotty.tools.backend.sjs.JSDefinitions
+
+class SJSPlatform()(implicit ctx: Context) extends JavaPlatform {
+
+  /** Scala.js-specific definitions. */
+  val jsDefinitions: JSDefinitions = new JSDefinitions()
+
+}

src/dotty/tools/dotc/core/Contexts.scala

@@ -26,7 +26,7 @@ import reporting._
 import collection.mutable
 import collection.immutable.BitSet
 import printing._
-import config.{Settings, ScalaSettings, Platform, JavaPlatform}
+import config.{Settings, ScalaSettings, Platform, JavaPlatform, SJSPlatform}
 import language.implicitConversions
 import DenotTransformers.DenotTransformer
 
@@ -514,8 +514,23 @@ object Contexts {
     /** The symbol loaders */
     val loaders = new SymbolLoaders
 
+    /** The platform, initialized by `initPlatform()`. */
+    private var _platform: Platform = _
+
     /** The platform */
-    val platform: Platform = new JavaPlatform
+    def platform: Platform = {
+      if (_platform == null) {
+        throw new IllegalStateException(
+            "initPlatform() must be called before accessing platform")
+      }
+      _platform
+    }
+
+    def initPlatform()(implicit ctx: Context): Unit = {
+      _platform =
+        if (settings.scalajs.value) new SJSPlatform()
+        else new JavaPlatform
+    }
 
     /** The loader that loads the members of _root_ */
     def rootLoader(root: TermSymbol)(implicit ctx: Context): SymbolLoader = platform.rootLoader(root)