chore: enable explicit-nulls in the stdlib

Author: hamzaremmal

project/Build.scala

@@ -1619,7 +1619,8 @@ object Build {
       // NOTE: The only difference here is that we drop `-Werror` and semanticDB for now
       Compile / scalacOptions := Seq("-deprecation", "-feature", "-unchecked", "-encoding", "UTF8", "-language:implicitConversions"),
       Compile / scalacOptions += "-Yno-stdlib-patches",
-      Compile / scalacOptions ++= Seq(
+      Compile / scalacOptions += "-Yexplicit-nulls",
+      (Compile / scalacOptions) ++= Seq(
         // Needed so that the library sources are visible when `dotty.tools.dotc.core.Definitions#init` is called
         "-sourcepath", (Compile / sourceDirectories).value.map(_.getCanonicalPath).distinct.mkString(File.pathSeparator),
       ),
@@ -1701,6 +1702,7 @@ object Build {
       // NOTE: The only difference here is that we drop `-Werror` and semanticDB for now
       Compile / scalacOptions :=  Seq("-deprecation", "-feature", "-unchecked", "-encoding", "UTF8", "-language:implicitConversions"),
       Compile / scalacOptions += "-Yno-stdlib-patches",
+      Compile / scalacOptions += "-Yexplicit-nulls",
       Compile / scalacOptions ++= Seq(
         // Needed so that the library sources are visible when `dotty.tools.dotc.core.Definitions#init` is called
         "-sourcepath", (Compile / sourceDirectories).value.map(_.getCanonicalPath).distinct.mkString(File.pathSeparator),