Implement SIP-61 @unroll annotation (#21693)

The main implementation follows
[com-lihaoyi/unroll](https://github.com/com-lihaoyi/unroll) but with
some changes:
- `@unroll` annotation is `@experimental`
- run before pickling
- increase validation checks for incorrect usage of `@unroll`
- `Underscore` not `EmptyTree` in pattern match default case
- ensure correct spans in TASTy
- ensure symbols are correctly substituted in types

There is one main library addition: `scala.annotation.unroll`, i.e. the
`@unroll` annotation that appears on parameters,

fixes #21728

Author: sjrd

compiler/src/dotty/tools/dotc/CompilationUnit.scala

@@ -59,6 +59,9 @@ class CompilationUnit protected (val source: SourceFile, val info: CompilationUn
 
   var hasMacroAnnotations: Boolean = false
 
+  def hasUnrollDefs: Boolean = unrolledClasses.nonEmpty
+  var unrolledClasses: Set[Symbol] = Set.empty
+
   /** Set to `true` if inliner added anonymous mirrors that need to be completed */
   var needsMirrorSupport: Boolean = false
 

compiler/src/dotty/tools/dotc/Compiler.scala

@@ -40,6 +40,7 @@ class Compiler {
     List(new sbt.ExtractDependencies) :: // Sends information on classes' dependencies to sbt via callbacks
     List(new semanticdb.ExtractSemanticDB.ExtractSemanticInfo) :: // Extract info into .semanticdb files
     List(new PostTyper) ::          // Additional checks and cleanups after type checking
+    List(new UnrollDefinitions) ::  // Unroll annotated methods if detected in PostTyper
     List(new sjs.PrepJSInterop) ::  // Additional checks and transformations for Scala.js (Scala.js only)
     List(new SetRootTree) ::        // Set the `rootTreeOrProvider` on class symbols
     Nil

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

@@ -1036,6 +1036,7 @@ class Definitions {
   @tu lazy val MigrationAnnot: ClassSymbol = requiredClass("scala.annotation.migration")
   @tu lazy val NowarnAnnot: ClassSymbol = requiredClass("scala.annotation.nowarn")
   @tu lazy val UnusedAnnot: ClassSymbol = requiredClass("scala.annotation.unused")
+  @tu lazy val UnrollAnnot: ClassSymbol = requiredClass("scala.annotation.unroll")
   @tu lazy val TransparentTraitAnnot: ClassSymbol = requiredClass("scala.annotation.transparentTrait")
   @tu lazy val NativeAnnot: ClassSymbol = requiredClass("scala.native")
   @tu lazy val RepeatedAnnot: ClassSymbol = requiredClass("scala.annotation.internal.Repeated")

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

@@ -1071,7 +1071,9 @@ object Denotations {
     def filterDisjoint(denots: PreDenotation)(using Context): SingleDenotation =
       if (denots.exists && denots.matches(this)) NoDenotation else this
     def filterWithFlags(required: FlagSet, excluded: FlagSet)(using Context): SingleDenotation =
-      val realExcluded = if ctx.isAfterTyper then excluded else excluded | Invisible
+      val realExcluded =
+        if ctx.isAfterTyper || ctx.mode.is(Mode.ResolveFromTASTy) then excluded
+        else excluded | Invisible
       def symd: SymDenotation = this match
         case symd: SymDenotation => symd
         case _ => symbol.denot

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

@@ -125,6 +125,9 @@ object Mode {
   /** Read original positions when unpickling from TASTY */
   val ReadPositions: Mode = newMode(17, "ReadPositions")
 
+  /** We are resolving a SELECT name from TASTy */
+  val ResolveFromTASTy: Mode = newMode(18, "ResolveFromTASTy")
+
   /** We are elaborating the fully qualified name of a package clause.
    *  In this case, identifiers should never be imported.
    */

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

@@ -617,7 +617,7 @@ object SymDenotations {
       case _ =>
         // Otherwise, no completion is necessary, see the preconditions of `markAbsent()`.
         (myInfo `eq` NoType)
-        || is(Invisible) && ctx.isTyper
+        || (is(Invisible) && !ctx.mode.is(Mode.ResolveFromTASTy)) && ctx.isTyper
         || is(ModuleVal, butNot = Package) && moduleClass.isAbsent(canForce)
     }
 

compiler/src/dotty/tools/dotc/core/tasty/TreeUnpickler.scala

@@ -1563,7 +1563,7 @@ class TreeUnpickler(reader: TastyReader,
                *  - sbt-test/tasty-compat/remove-override
                *  - sbt-test/tasty-compat/move-method
                */
-              def lookupInSuper =
+              def lookupInSuper(using Context) =
                 val cls = ownerTpe.classSymbol
                 if cls.exists then
                   cls.asClass.classDenot
@@ -1572,14 +1572,18 @@ class TreeUnpickler(reader: TastyReader,
                 else
                   NoDenotation
 
-              val denot =
+
+              def searchDenot(using Context): Denotation =
                 if owner.is(JavaAnnotation) && name == nme.CONSTRUCTOR then
                   // #19951 Fix up to read TASTy produced before 3.5.0 -- ignore the signature
                   ownerTpe.nonPrivateDecl(name).asSeenFrom(prefix)
                 else
                   val d = ownerTpe.decl(name).atSignature(sig, target)
                   (if !d.exists then lookupInSuper else d).asSeenFrom(prefix)
 
+              val denot = inContext(ctx.addMode(Mode.ResolveFromTASTy)):
+                searchDenot // able to resolve Invisible members
+
               makeSelect(qual, name, denot)
             case REPEATED =>
               val elemtpt = readTpt()

compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala

@@ -220,6 +220,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case DeprecatedInfixNamedArgumentSyntaxID // errorNumber: 204
   case GivenSearchPriorityID // errorNumber: 205
   case EnumMayNotBeValueClassesID // errorNumber: 206
+  case IllegalUnrollPlacementID // errorNumber: 207
 
   def errorNumber = ordinal - 1
 

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3334,14 +3334,14 @@ final class QuotedTypeMissing(tpe: Type)(using Context) extends StagingMessage(Q
 
   private def witness = defn.QuotedTypeClass.typeRef.appliedTo(tpe)
 
-  override protected def msg(using Context): String = 
+  override protected def msg(using Context): String =
     i"Reference to $tpe within quotes requires a given ${witness} in scope"
 
   override protected def explain(using Context): String =
-    i"""Referencing `$tpe` inside a quoted expression requires a `${witness}` to be in scope. 
+    i"""Referencing `$tpe` inside a quoted expression requires a `${witness}` to be in scope.
         |Since Scala is subject to erasure at runtime, the type information will be missing during the execution of the code.
-        |`${witness}` is therefore needed to carry `$tpe`'s type information into the quoted code. 
-        |Without an implicit `${witness}`, the type `$tpe` cannot be properly referenced within the expression. 
+        |`${witness}` is therefore needed to carry `$tpe`'s type information into the quoted code.
+        |Without an implicit `${witness}`, the type `$tpe` cannot be properly referenced within the expression.
         |To resolve this, ensure that a `${witness}` is available, either through a context-bound or explicitly.
         |"""
 
@@ -3408,3 +3408,26 @@ final class EnumMayNotBeValueClasses(sym: Symbol)(using Context) extends SyntaxM
 
     def explain(using Context) = ""
 end EnumMayNotBeValueClasses
+
+class IllegalUnrollPlacement(origin: Option[Symbol])(using Context)
+extends DeclarationMsg(IllegalUnrollPlacementID):
+  def msg(using Context) = origin match
+    case None => "@unroll is only allowed on a method parameter"
+    case Some(method) =>
+      val isCtor = method.isConstructor
+      def what = if isCtor then i"a ${if method.owner.is(Trait) then "trait" else "class"} constructor" else i"method ${method.name}"
+      val prefix = s"Cannot unroll parameters of $what"
+      if method.is(Deferred) then
+        i"$prefix: it must not be abstract"
+      else if isCtor && method.owner.is(Trait) then
+        i"implementation restriction: $prefix"
+      else if !(isCtor || method.is(Final) || method.owner.is(ModuleClass)) then
+        i"$prefix: it is not final"
+      else if method.owner.companionClass.is(CaseClass) then
+        i"$prefix of a case class companion object: please annotate the class constructor instead"
+      else
+        assert(method.owner.is(CaseClass))
+        i"$prefix of a case class: please annotate the class constructor instead"
+
+  def explain(using Context) = ""
+end IllegalUnrollPlacement

compiler/src/dotty/tools/dotc/transform/PostTyper.scala

@@ -21,6 +21,7 @@ import reporting.*
 import NameKinds.WildcardParamName
 import cc.*
 import dotty.tools.dotc.transform.MacroAnnotations.hasMacroAnnotation
+import dotty.tools.dotc.core.NameKinds.DefaultGetterName
 
 object PostTyper {
   val name: String = "posttyper"
@@ -119,8 +120,31 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
 
     private var inJavaAnnot: Boolean = false
 
+    private val seenUnrolledMethods: util.EqHashMap[Symbol, Boolean] = new util.EqHashMap[Symbol, Boolean]
+
     private var noCheckNews: Set[New] = Set()
 
+    def isValidUnrolledMethod(method: Symbol, origin: SrcPos)(using Context): Boolean =
+      seenUnrolledMethods.getOrElseUpdate(method, {
+        val isCtor = method.isConstructor
+        if
+          method.name.is(DefaultGetterName)
+        then
+          false // not an error, but not an expandable unrolled method
+        else if
+          method.is(Deferred)
+          || isCtor && method.owner.is(Trait)
+          || !(isCtor || method.is(Final) || method.owner.is(ModuleClass))
+          || method.owner.companionClass.is(CaseClass)
+            && (method.name == nme.apply || method.name == nme.fromProduct)
+          || method.owner.is(CaseClass) && method.name == nme.copy
+        then
+          report.error(IllegalUnrollPlacement(Some(method)), origin)
+          false
+        else
+          true
+      })
+
     def withNoCheckNews[T](ts: List[New])(op: => T): T = {
       val saved = noCheckNews
       noCheckNews ++= ts
@@ -199,6 +223,12 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
       tree
     }
 
+    private def registerIfUnrolledParam(sym: Symbol)(using Context): Unit =
+      if sym.hasAnnotation(defn.UnrollAnnot) && isValidUnrolledMethod(sym.owner, sym.sourcePos) then
+        val cls = sym.enclosingClass
+        val additions = Array(cls, cls.linkedClass).filter(_ != NoSymbol)
+        ctx.compilationUnit.unrolledClasses ++= additions
+
     private def processValOrDefDef(tree: Tree)(using Context): tree.type =
       val sym = tree.symbol
       tree match
@@ -215,6 +245,7 @@ class PostTyper extends MacroTransform with InfoTransformer { thisPhase =>
                     ++ sym.annotations)
           else
             if sym.is(Param) then
+              registerIfUnrolledParam(sym)
               sym.keepAnnotationsCarrying(thisPhase, Set(defn.ParamMetaAnnot), orNoneOf = defn.NonBeanMetaAnnots)
             else if sym.is(ParamAccessor) then
               // @publicInBinary is not a meta-annotation and therefore not kept by `keepAnnotationsCarrying`