Place staged type captures in Quote AST (#17424)

This change makes it easier to handle instances `Type[T]` that are
inserted into a level 0 quote. These used to be encoded in the staging
phase as type aliases `@TypeSplice type A = t.Underlying`. This makes it
hard to debug and check invariants. Instead, we can collect the list of
type tags needed and store them in the quote. At the same time, we can
convert references to those types into `t.Underlying`. We can generate
the `@TypeSplice type A = t.Underlying` just before pickling and replace
all references to `t.Underlying` with `A` at that point.

This is only an internal representation change while transforming the
quote. It will not affect TASTy.

Author: nicolasstucki

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

@@ -157,7 +157,7 @@ object CompilationUnit {
       if tree.symbol.is(Flags.Inline) then
         containsInline = true
       tree match
-        case tpd.Quote(_) =>
+        case _: tpd.Quote =>
           containsQuote = true
         case tree: tpd.Apply if tree.symbol == defn.QuotedTypeModule_of =>
           containsQuote = true

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

@@ -1976,7 +1976,7 @@ object desugar {
         trees foreach collect
       case Block(Nil, expr) =>
         collect(expr)
-      case Quote(body) =>
+      case Quote(body, _) =>
         new UntypedTreeTraverser {
           def traverse(tree: untpd.Tree)(using Context): Unit = tree match {
             case Splice(expr) => collect(expr)

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

@@ -690,9 +690,14 @@ object Trees {
    *  when type checking. TASTy files will not contain type quotes. Type quotes are used again
    *  in the `staging` phase to represent the reification of `Type.of[T]]`.
    *
+   *  Type tags `tags` are always empty before the `staging` phase. Tags for stage inconsistent
+   *  types are added in the `staging` phase to level 0 quotes. Tags for types that refer to
+   *  definitions in an outer quote are added in the `splicing` phase
+   *
    *  @param  body  The tree that was quoted
+   *  @param  tags  Term references to instances of `Type[T]` for `T`s that are used in the quote
    */
-  case class Quote[+T <: Untyped] private[ast] (body: Tree[T])(implicit @constructorOnly src: SourceFile)
+  case class Quote[+T <: Untyped] private[ast] (body: Tree[T], tags: List[Tree[T]])(implicit @constructorOnly src: SourceFile)
     extends TermTree[T] {
     type ThisTree[+T <: Untyped] = Quote[T]
 
@@ -1313,9 +1318,9 @@ object Trees {
         case tree: Inlined if (call eq tree.call) && (bindings eq tree.bindings) && (expansion eq tree.expansion) => tree
         case _ => finalize(tree, untpd.Inlined(call, bindings, expansion)(sourceFile(tree)))
       }
-      def Quote(tree: Tree)(body: Tree)(using Context): Quote = tree match {
-        case tree: Quote if (body eq tree.body) => tree
-        case _ => finalize(tree, untpd.Quote(body)(sourceFile(tree)))
+      def Quote(tree: Tree)(body: Tree, tags: List[Tree])(using Context): Quote = tree match {
+        case tree: Quote if (body eq tree.body) && (tags eq tree.tags) => tree
+        case _ => finalize(tree, untpd.Quote(body, tags)(sourceFile(tree)))
       }
       def Splice(tree: Tree)(expr: Tree)(using Context): Splice = tree match {
         case tree: Splice if (expr eq tree.expr) => tree
@@ -1558,8 +1563,8 @@ object Trees {
             case Thicket(trees) =>
               val trees1 = transform(trees)
               if (trees1 eq trees) tree else Thicket(trees1)
-            case tree @ Quote(body) =>
-              cpy.Quote(tree)(transform(body)(using quoteContext))
+            case Quote(body, tags) =>
+              cpy.Quote(tree)(transform(body)(using quoteContext), transform(tags))
             case tree @ Splice(expr) =>
               cpy.Splice(tree)(transform(expr)(using spliceContext))
             case tree @ Hole(isTerm, idx, args, content, tpt) =>
@@ -1703,8 +1708,8 @@ object Trees {
               this(this(x, arg), annot)
             case Thicket(ts) =>
               this(x, ts)
-            case Quote(body) =>
-              this(x, body)(using quoteContext)
+            case Quote(body, tags) =>
+              this(this(x, body)(using quoteContext), tags)
             case Splice(expr) =>
               this(x, expr)(using spliceContext)
             case Hole(_, _, args, content, tpt) =>

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

@@ -170,8 +170,8 @@ object tpd extends Trees.Instance[Type] with TypedTreeInfo {
   def Inlined(call: Tree, bindings: List[MemberDef], expansion: Tree)(using Context): Inlined =
     ta.assignType(untpd.Inlined(call, bindings, expansion), bindings, expansion)
 
-  def Quote(body: Tree)(using Context): Quote =
-    untpd.Quote(body).withBodyType(body.tpe)
+  def Quote(body: Tree, tags: List[Tree])(using Context): Quote =
+    untpd.Quote(body, tags).withBodyType(body.tpe)
 
   def Splice(expr: Tree, tpe: Type)(using Context): Splice =
     untpd.Splice(expr).withType(tpe)

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

@@ -397,7 +397,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
   def SeqLiteral(elems: List[Tree], elemtpt: Tree)(implicit src: SourceFile): SeqLiteral = new SeqLiteral(elems, elemtpt)
   def JavaSeqLiteral(elems: List[Tree], elemtpt: Tree)(implicit src: SourceFile): JavaSeqLiteral = new JavaSeqLiteral(elems, elemtpt)
   def Inlined(call: tpd.Tree, bindings: List[MemberDef], expansion: Tree)(implicit src: SourceFile): Inlined = new Inlined(call, bindings, expansion)
-  def Quote(body: Tree)(implicit src: SourceFile): Quote = new Quote(body)
+  def Quote(body: Tree, tags: List[Tree])(implicit src: SourceFile): Quote = new Quote(body, tags)
   def Splice(expr: Tree)(implicit src: SourceFile): Splice = new Splice(expr)
   def TypeTree()(implicit src: SourceFile): TypeTree = new TypeTree()
   def InferredTypeTree()(implicit src: SourceFile): TypeTree = new InferredTypeTree()

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

@@ -211,6 +211,7 @@ object Phases {
     private var mySbtExtractDependenciesPhase: Phase = _
     private var myPicklerPhase: Phase = _
     private var myInliningPhase: Phase = _
+    private var myStagingPhase: Phase = _
     private var mySplicingPhase: Phase = _
     private var myFirstTransformPhase: Phase = _
     private var myCollectNullableFieldsPhase: Phase = _
@@ -235,6 +236,7 @@ object Phases {
     final def sbtExtractDependenciesPhase: Phase = mySbtExtractDependenciesPhase
     final def picklerPhase: Phase = myPicklerPhase
     final def inliningPhase: Phase = myInliningPhase
+    final def stagingPhase: Phase = myStagingPhase
     final def splicingPhase: Phase = mySplicingPhase
     final def firstTransformPhase: Phase = myFirstTransformPhase
     final def collectNullableFieldsPhase: Phase = myCollectNullableFieldsPhase
@@ -262,6 +264,7 @@ object Phases {
       mySbtExtractDependenciesPhase = phaseOfClass(classOf[sbt.ExtractDependencies])
       myPicklerPhase = phaseOfClass(classOf[Pickler])
       myInliningPhase = phaseOfClass(classOf[Inlining])
+      myStagingPhase = phaseOfClass(classOf[Staging])
       mySplicingPhase = phaseOfClass(classOf[Splicing])
       myFirstTransformPhase = phaseOfClass(classOf[FirstTransform])
       myCollectNullableFieldsPhase = phaseOfClass(classOf[CollectNullableFields])
@@ -449,6 +452,7 @@ object Phases {
   def sbtExtractDependenciesPhase(using Context): Phase = ctx.base.sbtExtractDependenciesPhase
   def picklerPhase(using Context): Phase                = ctx.base.picklerPhase
   def inliningPhase(using Context): Phase               = ctx.base.inliningPhase
+  def stagingPhase(using Context): Phase               = ctx.base.stagingPhase
   def splicingPhase(using Context): Phase               = ctx.base.splicingPhase
   def firstTransformPhase(using Context): Phase         = ctx.base.firstTransformPhase
   def refchecksPhase(using Context): Phase              = ctx.base.refchecksPhase

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

@@ -665,7 +665,7 @@ class TreePickler(pickler: TastyPickler) {
               pickleTree(hi)
               pickleTree(alias)
           }
-        case tree @ Quote(body) =>
+        case tree @ Quote(body, Nil) =>
           // TODO: Add QUOTE tag to TASTy
           assert(body.isTerm,
             """Quote with type should not be pickled.

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

@@ -1269,7 +1269,7 @@ class TreeUnpickler(reader: TastyReader,
 
       def quotedExpr(fn: Tree, args: List[Tree]): Tree =
         val TypeApply(_, targs) = fn: @unchecked
-        untpd.Quote(args.head).withBodyType(targs.head.tpe)
+        untpd.Quote(args.head, Nil).withBodyType(targs.head.tpe)
 
       def splicedExpr(fn: Tree, args: List[Tree]): Tree =
         val TypeApply(_, targs) = fn: @unchecked

compiler/src/dotty/tools/dotc/inlines/Inliner.scala

@@ -827,7 +827,7 @@ class Inliner(val call: tpd.Tree)(using Context):
 
     override def typedQuote(tree: untpd.Quote, pt: Type)(using Context): Tree =
       super.typedQuote(tree, pt) match
-        case Quote(Splice(inner)) => inner
+        case Quote(Splice(inner), _) => inner
         case tree1 =>
           ctx.compilationUnit.needsStaging = true
           tree1

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

@@ -1243,7 +1243,7 @@ object Parsers {
               }
             }
             in.nextToken()
-            Quote(t)
+            Quote(t, Nil)
           }
           else
             if !in.featureEnabled(Feature.symbolLiterals) then
@@ -2480,7 +2480,7 @@ object Parsers {
               val body =
                 if (in.token == LBRACKET) inBrackets(typ())
                 else stagedBlock()
-              Quote(body)
+              Quote(body, Nil)
             }
           }
         case NEW =>