Add -Yimports compiler flag

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

@@ -281,6 +281,7 @@ 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 Yimports: Setting[List[String]] = MultiStringSetting("-Yimports", helpArg="", "Custom root imports. If set, none of scala.*, java.lang.*, or Predef.* will be imported unless explicitly included.")
   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

@@ -1558,12 +1558,21 @@ class Definitions {
   private val PredefImportFns: RootRef =
     RootRef(() => ScalaPredefModule.termRef, isPredef=true)
 
-  @tu private lazy val JavaRootImportFns: List[RootRef] =
-    if ctx.settings.YnoImports.value then Nil
-    else JavaImportFns
+  @tu private lazy val YimportsImportFns: List[RootRef] = ctx.settings.Yimports.value.map { name =>
+    val denot =
+      getModuleIfDefined(name).suchThat(_.is(Module)) `orElse`
+      getPackageClassIfDefined(name).suchThat(_.is(Package))
+    if !denot.exists then
+      report.error(s"error: bad preamble import $name")
+    val termRef = denot.symbol.termRef
+    RootRef(() => termRef)
+  }
+
+  @tu private lazy val JavaRootImportFns: List[RootRef] = JavaImportFns
 
   @tu private lazy val ScalaRootImportFns: List[RootRef] =
-    if ctx.settings.YnoImports.value then Nil
+    if !ctx.settings.Yimports.isDefault then YimportsImportFns
+    else if ctx.settings.YnoImports.value then Nil
     else if ctx.settings.YnoPredef.value then ScalaImportFns
     else ScalaImportFns :+ PredefImportFns
 

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

@@ -151,9 +151,6 @@ class CompilationTests {
       compileFile("tests/neg-custom-args/ovlazy.scala", scala2CompatMode.and("-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/newline-braces.scala", scala2CompatMode.and("-Xfatal-warnings")),
       compileFile("tests/neg-custom-args/autoTuplingTest.scala", defaultOptions.andLanguageFeature("noAutoTupling")),
-      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/i1650.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i3882.scala", allowDeepSubtypes),
       compileFile("tests/neg-custom-args/i4372.scala", allowDeepSubtypes),

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/neg/noimports-additional.scala

@@ -0,0 +1,4 @@
+// scalac: -Yno-imports -Yimports:scala.annotation,scala.util.matching
+class annotation extends Annotation
+val s: String = "str" // error
+val regex: Regex = new Regex("str")

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

@@ -1,3 +1,4 @@
+// scalac: -Yno-imports
 object Test {
   val t: Int = 1 // error: not found Int
 }

tests/neg-custom-args/noimports2.scala → tests/neg/noimports2.scala

@@ -1,3 +1,4 @@
+// scalac: -Yno-imports
 object Test {
   assert("asdf" == "asdf") // error: not found assert
 }

tests/neg/nopredef-additional.scala

@@ -0,0 +1,4 @@
+// scalac: -Yno-predef -Yimports:java.lang,scala.annotation,scala.util.matching
+class annotation extends Annotation
+val s: String = "str"
+val regex: Regex = s.r // error

tests/neg/nopredef.scala

@@ -1,5 +1,4 @@
-import Predef.{assert as _}
-
+// scalac: -Yno-predef
 object Test {
   assert("asdf" == "asdf") // error: not found assert
 }

tests/neg-custom-args/nopredef.scala → tests/neg/unimport-Predef-assert.scala

@@ -1,3 +1,5 @@
+import Predef.{assert as _}
+
 object Test {
   assert("asdf" == "asdf") // error: not found assert
 }

tests/neg/yimports-custom.check

@@ -0,0 +1,7 @@
+
+-- [E006] Not Found Error: tests/neg/yimports-custom/C_2.scala:5:16 ----------------------------------------------------
+5 |  def greet() = println("hello, world!") // error
+  |                ^^^^^^^
+  |                Not found: println
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/yimports-custom/C_2.scala

@@ -0,0 +1,6 @@
+// scalac: -Yimports:hello.world.minidef
+
+class C {
+  val v: Numb = Magic
+  def greet() = println("hello, world!") // error
+}

tests/neg/yimports-custom/minidef_1.scala

@@ -0,0 +1,7 @@
+
+package hello.world
+
+object minidef {
+  type Numb = Int
+  final val Magic = 42
+}

tests/neg/yimports-nojava.check

@@ -0,0 +1,12 @@
+-- [E006] Not Found Error: tests/neg/yimports-nojava.scala:5:16 --------------------------------------------------------
+5 |  def g() = new Integer(42) // error
+  |                ^^^^^^^
+  |                Not found: type Integer
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/yimports-nojava.scala:6:16 --------------------------------------------------------
+6 |  def sleep() = Thread.sleep(42000L) // error
+  |                ^^^^^^
+  |                Not found: Thread
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/yimports-nojava.scala

@@ -0,0 +1,7 @@
+// scalac: -Yimports:scala,scala.Predef
+
+trait T {
+  def f() = println("hello, world!")
+  def g() = new Integer(42) // error
+  def sleep() = Thread.sleep(42000L) // error
+}

tests/neg/yimports-nosuch.check

@@ -0,0 +1,2 @@
+error: bad preamble import skala
+error: bad preamble import scala.Predeff

tests/neg/yimports-nosuch.scala

@@ -0,0 +1,5 @@
+// scalac: -Yimports:skala,scala.Predeff
+//
+class C
+// nopos-error
+// nopos-error

tests/neg/yimports-order.check

@@ -0,0 +1,16 @@
+-- [E006] Not Found Error: tests/neg/yimports-order.scala:9:16 ---------------------------------------------------------
+9 |      def f() = Map("hello" -> "world") // error // error
+  |                ^^^
+  |                Not found: Map
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E008] Not Found Error: tests/neg/yimports-order.scala:9:28 ---------------------------------------------------------
+9 |      def f() = Map("hello" -> "world") // error // error
+  |                    ^^^^^^^^^^
+  |                    value -> is not a member of String
+-- [E006] Not Found Error: tests/neg/yimports-order.scala:10:16 --------------------------------------------------------
+10 |      def g() = println(f()) // error
+   |                ^^^^^^^
+   |                Not found: println
+   |
+   | longer explanation available when compiling with `-explain`

tests/neg/yimports-order.scala

@@ -0,0 +1,13 @@
+
+package top {
+  package middle {
+    class C {
+      def c() = println("hello, world")
+    }
+    import Predef.{Map => _}
+    object Test {
+      def f() = Map("hello" -> "world") // error // error
+      def g() = println(f()) // error
+    }
+  }
+}

tests/neg/yimports-predef.check

@@ -0,0 +1,4 @@
+-- [E008] Not Found Error: tests/neg/yimports-predef.scala:6:21 --------------------------------------------------------
+6 |  def f[A](x: A) = x + 42 // error
+  |                   ^^^
+  |                   value + is not a member of A

tests/neg/yimports-predef.scala

@@ -0,0 +1,7 @@
+// scalac: -Yimports:scala,scala.Predef
+//
+import Predef.{any2stringadd => _, _}
+
+class classic {
+  def f[A](x: A) = x + 42 // error
+}

tests/neg/yimports-stable.check

@@ -0,0 +1,14 @@
+
+error: bad preamble import hello.world.potions
+-- [E006] Not Found Error: tests/neg/yimports-stable/C_2.scala:4:9 -----------------------------------------------------
+4 |  val v: Numb = magic // error // error
+  |         ^^^^
+  |         Not found: type Numb
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E006] Not Found Error: tests/neg/yimports-stable/C_2.scala:4:16 ----------------------------------------------------
+4 |  val v: Numb = magic // error // error
+  |                ^^^^^
+  |                Not found: magic
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/yimports-stable/C_2.scala

@@ -0,0 +1,7 @@
+// scalac: -Yimports:scala,scala.Predef,hello.world.potions
+//
+class C {
+  val v: Numb = magic // error // error
+  def greet() = println("hello, world!")
+}
+// nopos-error

tests/neg/yimports-stable/minidef_1.scala

@@ -0,0 +1,11 @@
+
+package hello
+
+trait stuff {
+  type Numb = Int
+  val magic = 42
+}
+
+object world {
+  val potions = new stuff {}
+}

tests/pending/neg/yimports-custom-b.check

@@ -0,0 +1,10 @@
+
+C_2.scala:8: error: not found: type Numb
+  val v: Numb = Answer
+         ^
+-- [E006] Not Found Error: tests/neg/yimports-custom-b/C_2.scala:9:16 --------------------------------------------------
+9 |  def greet() = println("hello, world!") // error
+  |                ^^^^^^^
+  |                Not found: println
+  |
+  | longer explanation available when compiling with `-explain`

tests/pending/neg/yimports-custom-b/C_2.scala

@@ -0,0 +1,10 @@
+// scalac: -Yimports:hello.world.minidef
+
+import hello.{world => hw}
+import hw.minidef.{Magic => Answer}
+
+// Finds the answer, but dumb to forget Numb
+class C {
+  val v: Numb = Answer // error
+  def greet() = println("hello, world!") // error
+}

tests/pending/neg/yimports-custom-b/minidef_1.scala

@@ -0,0 +1,8 @@
+// scalac: -Yimports:scala
+
+package hello.world
+
+object minidef {
+  type Numb = Int
+  final val Magic = 42
+}

tests/pending/neg/yimports-masked.check

@@ -0,0 +1,10 @@
+
+C_2.scala:11: error: not found: type Numb
+    val v: Numb = Answer
+           ^
+-- [E006] Not Found Error: tests/neg/yimports-masked/C_2.scala:12:18 ---------------------------------------------------
+12 |    def greet() = println("hello, world!") // error
+   |                  ^^^^^^^
+   |                  Not found: println
+   |
+   | longer explanation available when compiling with `-explain`

tests/pending/neg/yimports-masked/C_2.scala

@@ -0,0 +1,14 @@
+// scalac: -Yimports:scala,hello.world.minidef
+
+// import at top level or top of package disables implicit import.
+// the import can appear at any statement position, here, end of package.
+// Update: with new trick, the import has to be completed before usages.
+
+import hello.world.minidef.{Magic => Answer}
+
+package p {
+  class C {
+    val v: Numb = Answer // error
+    def greet() = println("hello, world!") // error
+  }
+}

tests/pending/neg/yimports-masked/minidef_1.scala

@@ -0,0 +1,7 @@
+
+package hello.world
+
+object minidef {
+  type Numb = Int
+  final val Magic = 42
+}

tests/pos/multiple-additional-imports.scala

@@ -0,0 +1,5 @@
+// scalac: -Yimports:scala,java.lang,scala.Predef,scala.annotation,scala.util.matching
+
+class annotation extends Annotation
+val s: String = "str"
+val regex: Regex = s.r

tests/pos/single-additional-import.scala

@@ -0,0 +1,2 @@
+// scalac: -Yimports:scala.annotation
+class annotation extends Annotation