Add -Yadditional-imports compiler flag

compiler/src/dotty/tools/dotc/config/ScalaSettings.scala

@@ -282,6 +282,7 @@ private sealed trait YSettings:
   val Yscala2Unpickler: Setting[String] = StringSetting("-Yscala2-unpickler", "", "Control where we may get Scala 2 symbols from. This is either \"always\", \"never\", or a classpath.", "always")
 
   val YnoImports: Setting[Boolean] = BooleanSetting("-Yno-imports", "Compile without importing scala.*, java.lang.*, or Predef.")
+  val YadditionalImports: Setting[List[String]] = MultiStringSetting("-Yadditional-imports", helpArg="", "Custom root imports, appended to any existing default imports")
   val YnoGenericSig: Setting[Boolean] = BooleanSetting("-Yno-generic-signatures", "Suppress generation of generic signatures for Java.")
   val YnoPredef: Setting[Boolean] = BooleanSetting("-Yno-predef", "Compile without importing Predef.")
   val Yskip: Setting[List[String]] = PhasesSetting("-Yskip", "Skip")

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1536,14 +1536,18 @@ class Definitions {
   private val PredefImportFns: RootRef =
     RootRef(() => ScalaPredefModule.termRef, isPredef=true)
 
+  @tu private lazy val YadditionalImportsImportFns: List[RootRef] = ctx.settings.YadditionalImports.value.map { imp =>
+    RootRef(() => requiredPackageRef(imp), isPredef = false)
+  }
+
   @tu private lazy val JavaRootImportFns: List[RootRef] =
     if ctx.settings.YnoImports.value then Nil
     else JavaImportFns
 
   @tu private lazy val ScalaRootImportFns: List[RootRef] =
-    if ctx.settings.YnoImports.value then Nil
+    (if ctx.settings.YnoImports.value then Nil
     else if ctx.settings.YnoPredef.value then ScalaImportFns
-    else ScalaImportFns :+ PredefImportFns
+    else ScalaImportFns :+ PredefImportFns) ++ YadditionalImportsImportFns
 
   @tu private lazy val JavaRootImportTypes: List[TermRef] = JavaRootImportFns.map(_.refFn())
   @tu private lazy val ScalaRootImportTypes: List[TermRef] = ScalaRootImportFns.map(_.refFn())

compiler/test/dotty/tools/dotc/CompilationTests.scala

@@ -65,6 +65,8 @@ class CompilationTests {
       compileFile("tests/pos-custom-args/i10383.scala", defaultOptions.and("-source", "future", "-deprecation", "-Xfatal-warnings")),
       compileFile("tests/pos-custom-args/i13044.scala", defaultOptions.and("-Xmax-inlines:33")),
       compileFile("tests/pos-custom-args/jdk-8-app.scala", defaultOptions.and("-release:8")),
+      compileFile("tests/pos-custom-args/single-additional-import.scala", defaultOptions.and("-Yadditional-imports:scala.annotation")),
+      compileFile("tests/pos-custom-args/multiple-additional-imports.scala", defaultOptions.and("-Yadditional-imports:scala.annotation,scala.util.matching")),
     ).checkCompile()
   }
 
@@ -154,6 +156,8 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/nopredef.scala", defaultOptions.and("-Yno-predef")),
       compileFile("tests/neg-custom-args/noimports.scala", defaultOptions.and("-Yno-imports")),
       compileFile("tests/neg-custom-args/noimports2.scala", defaultOptions.and("-Yno-imports")),
+      compileFile("tests/neg-custom-args/noimports-additional.scala", defaultOptions.and("-Yno-imports", "-Yadditional-imports:scala.annotation,scala.util.matching")),
+      compileFile("tests/neg-custom-args/nopredef-additional.scala", defaultOptions.and("-Yno-predef", "-Yadditional-imports:scala.annotation,scala.util.matching")),
       compileFile("tests/neg-custom-args/i1650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes),

tests/neg-custom-args/noimports-additional.scala

@@ -0,0 +1,4 @@
+
+class annotation extends Annotation
+val s: String = "str" // error
+val regex: Regex = new Regex("str")

tests/neg-custom-args/nopredef-additional.scala

@@ -0,0 +1,3 @@
+class annotation extends Annotation
+val s: String = "str"
+val regex: Regex = s.r // error

tests/neg/missing-import.scala

@@ -0,0 +1,3 @@
+class annotation extends Annotation // error
+val s: String = "str"
+val regex: Regex = s.r // error

tests/pos-custom-args/multiple-additional-imports.scala

@@ -0,0 +1,4 @@
+
+class annotation extends Annotation
+val s: String = "str"
+val regex: Regex = s.r

tests/pos-custom-args/single-additional-import.scala

@@ -0,0 +1 @@
+class annotation extends Annotation