Implement into modifier on parameter types (#14514)

in the thread
https://contributors.scala-lang.org/t/proposed-changes-and-restrictions-for-implicit-conversions/4923
we discussed two changes that would make implicit conversions largely
redundant. One proposed change was implemented in #14497. The other is
implemented here. It's based on #14497.

The idea of this PR is to have some way to specify that a method
parameter accepts implicit conversions on its arguments. Then the
feature warning on implicit conversion use would be suppressed in this
case. In the previous thread I proposed
`~` to mark convertible arguments but I now feel this is too cryptic.
Instead there is a `into` prefix in the
parameter type. E.g.
```scala
  def f(x: into T) = ...
```
For Scala 2, we introduce an annotation on the parameter that expresses
the same thing:
```scala
  // proposed Scala-2 syntax:
  def f(@allowConversions x: T) = 
```
A larger example:
```scala
given Conversion[String, Text] = Text(_)

@main def Test =

  def f(x: into Text, y: => into Text, zs: into Text*) =
    println(s"${x.str} ${y.str} ${zs.map(_.str).mkString(" ")}")

  f("abc", "def")  // ok, no feature warning
  f("abc", "def", "xyz", "uvw")  // also ok
  f("abc", "def", "xyz", Text("uvw"))  // also ok
```
A larger example is a parser combinator library that works without
unrestricted implicit conversions:

[tests/run/Parser.scala](https://github.com/lampepfl/dotty/blob/0f113da1bacc01603740cde4a5bafbfae39895f0/tests/run/Parser.scala)

Author: KacperFKorban

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

@@ -6,6 +6,7 @@ import core._
 import util.Spans._, Types._, Contexts._, Constants._, Names._, NameOps._, Flags._
 import Symbols._, StdNames._, Trees._, ContextOps._
 import Decorators._, transform.SymUtils._
+import Annotations.Annotation
 import NameKinds.{UniqueName, EvidenceParamName, DefaultGetterName, WildcardParamName}
 import typer.{Namer, Checking}
 import util.{Property, SourceFile, SourcePosition, Chars}
@@ -165,32 +166,41 @@ object desugar {
    *
    *  Generate setter where needed
    */
-  def valDef(vdef0: ValDef)(using Context): Tree = {
+  def valDef(vdef0: ValDef)(using Context): Tree =
     val vdef @ ValDef(_, tpt, rhs) = vdef0
-    val mods = vdef.mods
-
     val valName = normalizeName(vdef, tpt).asTermName
-    val vdef1 = cpy.ValDef(vdef)(name = valName)
+    var mods1 = vdef.mods
+
+    def dropInto(tpt: Tree): Tree = tpt match
+      case Into(tpt1) =>
+        mods1 = vdef.mods.withAddedAnnotation(
+          TypedSplice(
+            Annotation(defn.AllowConversionsAnnot).tree.withSpan(tpt.span.startPos)))
+        tpt1
+      case ByNameTypeTree(tpt1) =>
+        cpy.ByNameTypeTree(tpt)(dropInto(tpt1))
+      case PostfixOp(tpt1, op) if op.name == tpnme.raw.STAR =>
+        cpy.PostfixOp(tpt)(dropInto(tpt1), op)
+      case _ =>
+        tpt
+
+    val vdef1 = cpy.ValDef(vdef)(name = valName, tpt = dropInto(tpt))
+      .withMods(mods1)
 
-    if (isSetterNeeded(vdef)) {
-      // TODO: copy of vdef as getter needed?
-      // val getter = ValDef(mods, name, tpt, rhs) withPos vdef.pos?
-      // right now vdef maps via expandedTree to a thicket which concerns itself.
-      // I don't see a problem with that but if there is one we can avoid it by making a copy here.
+    if isSetterNeeded(vdef) then
       val setterParam = makeSyntheticParameter(tpt = SetterParamTree().watching(vdef))
       // The rhs gets filled in later, when field is generated and getter has parameters (see Memoize miniphase)
       val setterRhs = if (vdef.rhs.isEmpty) EmptyTree else unitLiteral
       val setter = cpy.DefDef(vdef)(
-        name    = valName.setterName,
-        paramss = (setterParam :: Nil) :: Nil,
-        tpt     = TypeTree(defn.UnitType),
-        rhs     = setterRhs
-      ).withMods((mods | Accessor) &~ (CaseAccessor | GivenOrImplicit | Lazy))
-       .dropEndMarker() // the end marker should only appear on the getter definition
+          name    = valName.setterName,
+          paramss = (setterParam :: Nil) :: Nil,
+          tpt     = TypeTree(defn.UnitType),
+          rhs     = setterRhs
+        ).withMods((vdef.mods | Accessor) &~ (CaseAccessor | GivenOrImplicit | Lazy))
+        .dropEndMarker() // the end marker should only appear on the getter definition
       Thicket(vdef1, setter)
-    }
     else vdef1
-  }
+  end valDef
 
   def makeImplicitParameters(tpts: List[Tree], implicitFlag: FlagSet, forPrimaryConstructor: Boolean = false)(using Context): List[ValDef] =
     for (tpt <- tpts) yield {

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

@@ -117,6 +117,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   case class ContextBounds(bounds: TypeBoundsTree, cxBounds: List[Tree])(implicit @constructorOnly src: SourceFile) extends TypTree
   case class PatDef(mods: Modifiers, pats: List[Tree], tpt: Tree, rhs: Tree)(implicit @constructorOnly src: SourceFile) extends DefTree
   case class ExtMethods(paramss: List[ParamClause], methods: List[Tree])(implicit @constructorOnly src: SourceFile) extends Tree
+  case class Into(tpt: Tree)(implicit @constructorOnly src: SourceFile) extends Tree
   case class MacroTree(expr: Tree)(implicit @constructorOnly src: SourceFile) extends Tree
 
   case class ImportSelector(imported: Ident, renamed: Tree = EmptyTree, bound: Tree = EmptyTree)(implicit @constructorOnly src: SourceFile) extends Tree {
@@ -649,6 +650,9 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
     def ExtMethods(tree: Tree)(paramss: List[ParamClause], methods: List[Tree])(using Context): Tree = tree match
       case tree: ExtMethods if (paramss eq tree.paramss) && (methods == tree.methods) => tree
       case _ => finalize(tree, untpd.ExtMethods(paramss, methods)(tree.source))
+    def Into(tree: Tree)(tpt: Tree)(using Context): Tree = tree match
+      case tree: Into if tpt eq tree.tpt => tree
+      case _ => finalize(tree, untpd.Into(tpt)(tree.source))
     def ImportSelector(tree: Tree)(imported: Ident, renamed: Tree, bound: Tree)(using Context): Tree = tree match {
       case tree: ImportSelector if (imported eq tree.imported) && (renamed eq tree.renamed) && (bound eq tree.bound) => tree
       case _ => finalize(tree, untpd.ImportSelector(imported, renamed, bound)(tree.source))
@@ -718,6 +722,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         cpy.PatDef(tree)(mods, transform(pats), transform(tpt), transform(rhs))
       case ExtMethods(paramss, methods) =>
         cpy.ExtMethods(tree)(transformParamss(paramss), transformSub(methods))
+      case Into(tpt) =>
+        cpy.Into(tree)(transform(tpt))
       case ImportSelector(imported, renamed, bound) =>
         cpy.ImportSelector(tree)(transformSub(imported), transform(renamed), transform(bound))
       case Number(_, _) | TypedSplice(_) =>
@@ -777,6 +783,8 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
         this(this(this(x, pats), tpt), rhs)
       case ExtMethods(paramss, methods) =>
         this(paramss.foldLeft(x)(apply), methods)
+      case Into(tpt) =>
+        this(x, tpt)
       case ImportSelector(imported, renamed, bound) =>
         this(this(this(x, imported), renamed), bound)
       case Number(_, _) =>

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

@@ -30,6 +30,7 @@ object Feature:
   val saferExceptions = experimental("saferExceptions")
   val pureFunctions = experimental("pureFunctions")
   val captureChecking = experimental("captureChecking")
+  val into = experimental("into")
 
   val globalOnlyImports: Set[TermName] = Set(pureFunctions, captureChecking)
 

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

@@ -78,7 +78,7 @@ object Decorators {
   /** Implements filterConserve, zipWithConserve methods
    *  on lists that avoid duplication of list nodes where feasible.
    */
-  implicit class ListDecorator[T](val xs: List[T]) extends AnyVal {
+  extension [T](xs: List[T])
 
     final def mapconserve[U](f: T => U): List[U] = {
       @tailrec
@@ -207,11 +207,7 @@ object Decorators {
     }
 
     /** Union on lists seen as sets */
-    def | (ys: List[T]): List[T] = xs ::: (ys filterNot (xs contains _))
-
-    /** Intersection on lists seen as sets */
-    def & (ys: List[T]): List[T] = xs filter (ys contains _)
-  }
+    def setUnion (ys: List[T]): List[T] = xs ::: ys.filterNot(xs contains _)
 
   extension [T, U](xss: List[List[T]])
     def nestedMap(f: T => U): List[List[U]] = xss match

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

@@ -644,6 +644,8 @@ class Definitions {
 
   @tu lazy val RepeatedParamClass: ClassSymbol = enterSpecialPolyClass(tpnme.REPEATED_PARAM_CLASS, Covariant, Seq(ObjectType, SeqType))
 
+  @tu lazy val IntoType: TypeSymbol = enterAliasType(tpnme.INTO, HKTypeLambda(TypeBounds.empty :: Nil)(_.paramRefs(0)))
+
   // fundamental classes
   @tu lazy val StringClass: ClassSymbol = requiredClass("java.lang.String")
   def StringType: Type = StringClass.typeRef
@@ -973,6 +975,7 @@ class Definitions {
   @tu lazy val RefiningAnnotationClass: ClassSymbol = requiredClass("scala.annotation.RefiningAnnotation")
 
   // Annotation classes
+  @tu lazy val AllowConversionsAnnot: ClassSymbol = requiredClass("scala.annotation.allowConversions")
   @tu lazy val AnnotationDefaultAnnot: ClassSymbol = requiredClass("scala.annotation.internal.AnnotationDefault")
   @tu lazy val BeanPropertyAnnot: ClassSymbol = requiredClass("scala.beans.BeanProperty")
   @tu lazy val BooleanBeanPropertyAnnot: ClassSymbol = requiredClass("scala.beans.BooleanBeanProperty")
@@ -2005,6 +2008,7 @@ class Definitions {
       orType,
       RepeatedParamClass,
       ByNameParamClass2x,
+      IntoType,
       AnyValClass,
       NullClass,
       NothingClass,

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

@@ -15,8 +15,8 @@ import scala.annotation.internal.sharable
 object Names {
   import NameKinds._
 
-  /** Things that can be turned into names with `totermName` and `toTypeName`
-   *  Decorators defines implements these as extension methods for strings.
+  /** Things that can be turned into names with `toTermName` and `toTypeName`.
+   *  Decorators implements these as extension methods for strings.
    */
   type PreName = Name | String
 

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

@@ -634,7 +634,7 @@ class OrderingConstraint(private val boundsMap: ParamBounds,
     case param: TypeParamRef if contains(param) =>
       param :: (if (isUpper) upper(param) else lower(param))
     case tp: AndType if isUpper  =>
-      dependentParams(tp.tp1, isUpper) | (dependentParams(tp.tp2, isUpper))
+      dependentParams(tp.tp1, isUpper).setUnion(dependentParams(tp.tp2, isUpper))
     case tp: OrType if !isUpper =>
       dependentParams(tp.tp1, isUpper).intersect(dependentParams(tp.tp2, isUpper))
     case EtaExpansion(tycon) =>

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

@@ -128,6 +128,7 @@ 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>"
@@ -500,6 +501,7 @@ object StdNames {
     val info: N                 = "info"
     val inlinedEquals: N        = "inlinedEquals"
     val internal: N             = "internal"
+    val into: N                 = "into"
     val isArray: N              = "isArray"
     val isDefinedAt: N          = "isDefinedAt"
     val isDefinedAtImpl: N      = "$isDefinedAt"

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

@@ -397,6 +397,10 @@ object Types {
     def isRepeatedParam(using Context): Boolean =
       typeSymbol eq defn.RepeatedParamClass
 
+    /** Is this a parameter type that allows implicit argument converson? */
+    def isConvertibleParam(using Context): Boolean =
+      typeSymbol eq defn.IntoType
+
     /** Is this the type of a method that has a repeated parameter type as
      *  last parameter type?
      */
@@ -536,7 +540,7 @@ object Types {
       case tp: ClassInfo =>
         tp.cls :: Nil
       case AndType(l, r) =>
-        l.parentSymbols(include) | r.parentSymbols(include)
+        l.parentSymbols(include).setUnion(r.parentSymbols(include))
       case OrType(l, r) =>
         l.parentSymbols(include) intersect r.parentSymbols(include) // TODO does not conform to spec
       case _ =>
@@ -1864,6 +1868,11 @@ object Types {
 
     def dropRepeatedAnnot(using Context): Type = dropAnnot(defn.RepeatedAnnot)
 
+    /** A translation from types of original parameter ValDefs to the types
+     *  of parameters in MethodTypes.
+     *  Translates `Seq[T] @repeated` or `Array[T] @repeated` to `<repeated>[T]`.
+     *  That way, repeated arguments are made manifest without risk of dropped annotations.
+     */
     def annotatedToRepeated(using Context): Type = this match {
       case tp @ ExprType(tp1) =>
         tp.derivedExprType(tp1.annotatedToRepeated)
@@ -3948,27 +3957,48 @@ object Types {
      *  and inline parameters:
      *   - replace @repeated annotations on Seq or Array types by <repeated> types
      *   - add @inlineParam to inline parameters
+     *   - add @erasedParam to erased parameters
+     *   - wrap types of parameters that have an @allowConversions annotation with Into[_]
      */
-    def fromSymbols(params: List[Symbol], resultType: Type)(using Context): MethodType = {
-      def translateInline(tp: Type): Type = tp match {
-        case ExprType(resType) => ExprType(AnnotatedType(resType, Annotation(defn.InlineParamAnnot)))
-        case _ => AnnotatedType(tp, Annotation(defn.InlineParamAnnot))
-      }
-      def translateErased(tp: Type): Type = tp match {
-        case ExprType(resType) => ExprType(AnnotatedType(resType, Annotation(defn.ErasedParamAnnot)))
-        case _ => AnnotatedType(tp, Annotation(defn.ErasedParamAnnot))
-      }
-      def paramInfo(param: Symbol) = {
+    def fromSymbols(params: List[Symbol], resultType: Type)(using Context): MethodType =
+      def addAnnotation(tp: Type, cls: ClassSymbol): Type = tp match
+        case ExprType(resType) => ExprType(addAnnotation(resType, cls))
+        case _ => AnnotatedType(tp, Annotation(cls))
+
+      def wrapConvertible(tp: Type) =
+        AppliedType(defn.IntoType.typeRef, tp :: Nil)
+
+      /** Add `Into[..] to the type itself and if it is a function type, to all its
+       *  curried result type(s) as well.
+       */
+      def addInto(tp: Type): Type = tp match
+        case tp @ AppliedType(tycon, args) if tycon.typeSymbol == defn.RepeatedParamClass =>
+          tp.derivedAppliedType(tycon, addInto(args.head) :: Nil)
+        case tp @ AppliedType(tycon, args) if defn.isFunctionType(tp) =>
+          wrapConvertible(tp.derivedAppliedType(tycon, args.init :+ addInto(args.last)))
+        case tp @ RefinedType(parent, rname, rinfo) if defn.isFunctionOrPolyType(tp) =>
+          wrapConvertible(tp.derivedRefinedType(parent, rname, addInto(rinfo)))
+        case tp: MethodOrPoly =>
+          tp.derivedLambdaType(resType = addInto(tp.resType))
+        case ExprType(resType) =>
+          ExprType(addInto(resType))
+        case _ =>
+          wrapConvertible(tp)
+
+      def paramInfo(param: Symbol) =
         var paramType = param.info.annotatedToRepeated
-        if (param.is(Inline)) paramType = translateInline(paramType)
-        if (param.is(Erased)) paramType = translateErased(paramType)
+        if param.is(Inline) then
+          paramType = addAnnotation(paramType, defn.InlineParamAnnot)
+        if param.is(Erased) then
+          paramType = addAnnotation(paramType, defn.ErasedParamAnnot)
+        if param.hasAnnotation(defn.AllowConversionsAnnot) then
+          paramType = addInto(paramType)
         paramType
-      }
 
       apply(params.map(_.name.asTermName))(
          tl => params.map(p => tl.integrate(params, paramInfo(p))),
          tl => tl.integrate(params, resultType))
-    }
+    end fromSymbols
 
     final def apply(paramNames: List[TermName])(paramInfosExp: MethodType => List[Type], resultTypeExp: MethodType => Type)(using Context): MethodType =
       checkValid(unique(new CachedMethodType(paramNames)(paramInfosExp, resultTypeExp, self)))

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -1917,6 +1917,13 @@ object Parsers {
       else
         core()
 
+    private def maybeInto(tp: () => Tree) =
+      if in.isIdent(nme.into)
+          && in.featureEnabled(Feature.into)
+          && canStartTypeTokens.contains(in.lookahead.token)
+      then atSpan(in.skipToken()) { Into(tp()) }
+      else tp()
+
     /** FunArgType ::=  Type
      *               |  `=>' Type
      *               |  [CaptureSet] `->' Type
@@ -1929,10 +1936,10 @@ object Parsers {
      */
     def paramType(): Tree = paramTypeOf(paramValueType)
 
-    /** ParamValueType ::= Type [`*']
+    /** ParamValueType ::= [`into`] Type [`*']
      */
     def paramValueType(): Tree = {
-      val t = toplevelTyp()
+      val t = maybeInto(toplevelTyp)
       if (isIdent(nme.raw.STAR)) {
         in.nextToken()
         atSpan(startOffset(t)) { PostfixOp(t, Ident(tpnme.raw.STAR)) }