Change into scheme to be fully type-based (#23014)

A new version of the `into` scheme to allow certain implicit conversions
without requiring a language import.

This is a lot simpler than previous schemes since it makes use of the
power of the type system instead of building up a parallel structure
based on modifiers. It is also considerably more flexible than the
previous scheme. One open question might be whether it's _too_ flexible.

Author: odersky

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -652,6 +652,8 @@ object desugar {
         tdef, evidenceBuf,
         (tdef.mods.flags.toTermFlags & AccessFlags) | Lazy | DeferredGivenFlags,
         inventGivenName, Nil)
+    if tdef.mods.flags.is(Into, butNot = Opaque) then
+      report.error(ModifierNotAllowedForDefinition(Into), flagSourcePos(tdef, Into))
     if evidenceBuf.isEmpty then result else Thicket(result :: evidenceBuf.toList)
 
   /** The expansion of a class definition. See inline comments for what is involved */
@@ -2268,11 +2270,8 @@ object desugar {
           assert(ctx.mode.isExpr || ctx.reporter.errorsReported || ctx.mode.is(Mode.Interactive), ctx.mode)
           Select(t, op.name)
       case PrefixOp(op, t) =>
-        if op.name == tpnme.into then
-          Annotated(t, New(ref(defn.IntoAnnot.typeRef), Nil :: Nil))
-        else
-          val nspace = if (ctx.mode.is(Mode.Type)) tpnme else nme
-          Select(t, nspace.UNARY_PREFIX ++ op.name)
+        val nspace = if (ctx.mode.is(Mode.Type)) tpnme else nme
+        Select(t, nspace.UNARY_PREFIX ++ op.name)
       case ForDo(enums, body) =>
         makeFor(nme.foreach, nme.foreach, enums, body) orElse tree
       case ForYield(enums, body) =>

compiler/src/dotty/tools/dotc/ast/TreeInfo.scala

@@ -10,6 +10,7 @@ import NameKinds.ContextBoundParamName
 import typer.ConstFold
 import reporting.trace
 import config.Feature
+import util.SrcPos
 
 import Decorators.*
 import Constants.Constant
@@ -536,6 +537,10 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
       if id.span == result.span.startPos => Some(result)
       case _ => None
   end ImpureByNameTypeTree
+
+  /** The position of the modifier associated with given flag in this definition. */
+  def flagSourcePos(mdef: DefTree, flag: FlagSet): SrcPos =
+    mdef.mods.mods.find(_.flags == flag).getOrElse(mdef).srcPos
 }
 
 trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -236,6 +236,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
 
     case class Tracked()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Tracked)
 
+    case class Into()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Into)
+
     /** Used under pureFunctions to mark impure function types `A => B` in `FunctionWithMods` */
     case class Impure()(implicit @constructorOnly src: SourceFile) extends Mod(Flags.Impure)
   }
@@ -565,12 +567,6 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     ValDef(nme.syntheticParamName(n), if (tpt == null) TypeTree() else tpt, EmptyTree)
       .withFlags(flags)
 
-  def isInto(t: Tree)(using Context): Boolean = t match
-    case PrefixOp(Ident(tpnme.into), _) => true
-    case Function(_, res) => isInto(res)
-    case Parens(t) => isInto(t)
-    case _ => false
-
   def lambdaAbstract(params: List[ValDef] | List[TypeDef], tpt: Tree)(using Context): Tree =
     params match
       case Nil               => tpt

compiler/src/dotty/tools/dotc/cc/CaptureOps.scala

@@ -526,19 +526,6 @@ class CleanupRetains(using Context) extends TypeMap:
         RetainingType(tp, Nil, byName = annot.symbol == defn.RetainsByNameAnnot)
       case _ => mapOver(tp)
 
-/** A typemap that follows aliases and keeps their transformed results if
- *  there is a change.
- */
-trait FollowAliasesMap(using Context) extends TypeMap:
-  var follow = true    // Used for debugging so that we can compare results with and w/o following.
-  def mapFollowingAliases(t: Type): Type =
-    val t1 = t.dealiasKeepAnnots
-    if follow && (t1 ne t) then
-      val t2 = apply(t1)
-      if t2 ne t1 then t2
-      else t
-    else mapOver(t)
-
 /** An extractor for `caps.reachCapability(ref)`, which is used to express a reach
  *  capability as a tree in a @retains annotation.
  */

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

@@ -753,6 +753,9 @@ class Definitions {
   @tu lazy val StringBuilderClass: ClassSymbol = requiredClass("scala.collection.mutable.StringBuilder")
   @tu lazy val MatchErrorClass   : ClassSymbol = requiredClass("scala.MatchError")
   @tu lazy val ConversionClass   : ClassSymbol = requiredClass("scala.Conversion").typeRef.symbol.asClass
+  @tu lazy val ConversionModule  : Symbol = ConversionClass.companionModule
+  @tu lazy val ConversionModuleClass: ClassSymbol = ConversionModule.moduleClass.asClass
+    @tu lazy val Conversion_into : Symbol = ConversionModuleClass.requiredType("into")
 
   @tu lazy val StringAddClass    : ClassSymbol = requiredClass("scala.runtime.StringAdd")
     @tu lazy val StringAdd_+ : Symbol = StringAddClass.requiredMethod(nme.raw.PLUS)
@@ -1037,8 +1040,6 @@ class Definitions {
   @tu lazy val ImplicitNotFoundAnnot: ClassSymbol = requiredClass("scala.annotation.implicitNotFound")
   @tu lazy val InferredDepFunAnnot: ClassSymbol = requiredClass("scala.caps.internal.inferredDepFun")
   @tu lazy val InlineParamAnnot: ClassSymbol = requiredClass("scala.annotation.internal.InlineParam")
-  @tu lazy val IntoAnnot: ClassSymbol = requiredClass("scala.annotation.into")
-  @tu lazy val IntoParamAnnot: ClassSymbol = requiredClass("scala.annotation.internal.$into")
   @tu lazy val ErasedParamAnnot: ClassSymbol = requiredClass("scala.annotation.internal.ErasedParam")
   @tu lazy val MainAnnot: ClassSymbol = requiredClass("scala.main")
   @tu lazy val MappedAlternativeAnnot: ClassSymbol = requiredClass("scala.annotation.internal.MappedAlternative")
@@ -1056,6 +1057,7 @@ class Definitions {
   // @tu lazy val ScalaStrictFPAnnot: ClassSymbol = requiredClass("scala.annotation.strictfp")
   @tu lazy val ScalaStaticAnnot: ClassSymbol = requiredClass("scala.annotation.static")
   @tu lazy val SerialVersionUIDAnnot: ClassSymbol = requiredClass("scala.SerialVersionUID")
+  @tu lazy val SilentIntoAnnot: ClassSymbol = requiredClass("scala.annotation.internal.$into")
   @tu lazy val TailrecAnnot: ClassSymbol = requiredClass("scala.annotation.tailrec")
   @tu lazy val ThreadUnsafeAnnot: ClassSymbol = requiredClass("scala.annotation.threadUnsafe")
   @tu lazy val ConstructorOnlyAnnot: ClassSymbol = requiredClass("scala.annotation.constructorOnly")
@@ -1115,7 +1117,7 @@ class Definitions {
 
   // Set of annotations that are not printed in types except under -Yprint-debug
   @tu lazy val SilentAnnots: Set[Symbol] =
-    Set(InlineParamAnnot, ErasedParamAnnot, RefineOverrideAnnot)
+    Set(InlineParamAnnot, ErasedParamAnnot, RefineOverrideAnnot, SilentIntoAnnot)
 
   // A list of annotations that are commonly used to indicate that a field/method argument or return
   // type is not null. These annotations are used by the nullification logic in JavaNullInterop to
@@ -1385,6 +1387,9 @@ class Definitions {
   final def isNamedTuple_From(sym: Symbol)(using Context): Boolean =
     sym.name == tpnme.From && sym.owner == NamedTupleModule.moduleClass
 
+  final def isInto(sym: Symbol)(using Context): Boolean =
+    sym.name == tpnme.into && sym.owner == ConversionModuleClass
+
   private val compiletimePackageAnyTypes: Set[Name] = Set(
     tpnme.Equals, tpnme.NotEquals, tpnme.IsConst, tpnme.ToString
   )

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

@@ -252,7 +252,7 @@ object Flags {
   /** A field generated for a primary constructor parameter (no matter if it's a 'val' or not),
    *  or an accessor of such a field.
    */
-  val (_, ParamAccessor @ _, _) = newFlags(14, "<paramaccessor>")
+  val (ParamAccessorOrInto @ _, ParamAccessor @ _, Into @ _) = newFlags(14, "<paramaccessor>", "into")
 
   /** A value or class implementing a module */
   val (Module @ _, ModuleVal @ _, ModuleClass @ _) = newFlags(15, "module")
@@ -452,7 +452,7 @@ object Flags {
     commonFlags(Private, Protected, Final, Case, Implicit, Given, Override, JavaStatic, Transparent, Erased)
 
   val TypeSourceModifierFlags: FlagSet =
-    CommonSourceModifierFlags.toTypeFlags | Abstract | Sealed | Opaque | Open
+    CommonSourceModifierFlags.toTypeFlags | Abstract | Sealed | Opaque | Open | Into
 
   val TermSourceModifierFlags: FlagSet =
     CommonSourceModifierFlags.toTermFlags | Inline | AbsOverride | Lazy | Tracked
@@ -467,7 +467,7 @@ object Flags {
    *  TODO: Should check that FromStartFlags do not change in completion
    */
   val FromStartFlags: FlagSet = commonFlags(
-    Module, Package, Deferred, Method, Case, Enum, Param, ParamAccessor,
+    Module, Package, Deferred, Method, Case, Enum, Param, ParamAccessorOrInto,
     Scala2SpecialFlags, MutableOrOpen, Opaque, Touched, JavaStatic,
     OuterOrCovariant, LabelOrContravariant, CaseAccessor, Tracked,
     Extension, NonMember, Implicit, Given, Permanent, Synthetic, Exported,

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

@@ -315,4 +315,11 @@ object NamerOps:
             ann.tree match
               case ast.tpd.WitnessNamesAnnot(witnessNames) =>
                 addContextBoundCompanionFor(sym, witnessNames, Nil)
+
+  /** if `sym` is a term parameter or parameter accessor, map all occurrences of
+   *  `into[T]` in its type to `T @$into`.
+   */
+  extension (tp: Type)
+    def suppressIntoIfParam(sym: Symbol)(using Context): Type =
+      if sym.isOneOf(TermParamOrAccessor) then TypeOps.suppressInto(tp) else tp
 end NamerOps

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

@@ -132,7 +132,6 @@ object StdNames {
     val EXCEPTION_RESULT_PREFIX: N    = "exceptionResult"
     val EXPAND_SEPARATOR: N           = str.EXPAND_SEPARATOR
     val IMPORT: N                     = "<import>"
-    val INTO: N                       = "$into"
     val MODULE_SUFFIX: N              = str.MODULE_SUFFIX
     val OPS_PACKAGE: N                = "<special-ops>"
     val OVERLOADED: N                 = "<overloaded>"

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

@@ -18,6 +18,7 @@ import typer.ForceDegree
 import typer.Inferencing.*
 import typer.IfBottom
 import reporting.TestingReporter
+import Annotations.Annotation
 import cc.{CapturingType, derivedCapturingType, CaptureSet, captureSet, isBoxed, isBoxedCapturing}
 import CaptureSet.{IdentityCaptRefMap, VarState}
 
@@ -944,6 +945,28 @@ object TypeOps:
   class StripTypeVarsMap(using Context) extends TypeMap:
     def apply(tp: Type) = mapOver(tp).stripTypeVar
 
+  /** Map no-flip covariant occurrences of `into[T]` to `T @$into` */
+  def suppressInto(using Context) = new FollowAliasesMap:
+    def apply(t: Type): Type = t match
+      case AppliedType(tycon: TypeRef, arg :: Nil) if variance >= 0 && defn.isInto(tycon.symbol) =>
+        AnnotatedType(arg, Annotation(defn.SilentIntoAnnot, util.Spans.NoSpan))
+      case _: MatchType | _: LazyRef =>
+        t
+      case _ =>
+        mapFollowingAliases(t)
+
+  /** Map no-flip covariant occurrences of `T @$into` to `into[T]` */
+  def revealInto(using Context) = new FollowAliasesMap:
+    def apply(t: Type): Type = t match
+      case AnnotatedType(t1, ann) if variance >= 0 && ann.symbol == defn.SilentIntoAnnot =>
+        AppliedType(
+          defn.ConversionModule.termRef.select(defn.Conversion_into), // the external reference to the opaque type
+          t1 :: Nil)
+      case _: MatchType | _: LazyRef =>
+        t
+      case _ =>
+        mapFollowingAliases(t)
+
   /** Apply [[Type.stripTypeVar]] recursively. */
   def stripTypeVars(tp: Type)(using Context): Type =
     new StripTypeVarsMap().apply(tp)