Implement tracked members (#21761)

closes #21754 

Allow for the `tracked` modifier to be used for `val` members of classes
and traits. `tracked` members and members inheriting from `tracked`
force the type of the member (or it's overriding member) to be as exact
as
possible. More precisely, it will will assign the `tracked` member the
infered type of the rhs. For instance, consider the following
definition:

```scala 3
trait F:
  tracked val a: Int
  tracked val b: Int

class N extends F:
  val a = 22 // a.type =:= 22
  val b: Int = 22 // b.type =:= Int
  tracked val c = 22 // c.type =:= 22
```

Here, the `tracked` modifier ensures that the type of `a` in `N` is `22`
and not `Int`. But the type of `b` is `N` is `Int` since it's explicitly
declared as `Int`. `tracked` members can also be immediately
initialized, as in the case of `c`.

---------

Co-authored-by: Matt Bovel <[email protected]>
Co-authored-by: odersky <[email protected]>

Author: 3 people

Diff for: compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -1086,12 +1086,13 @@ object desugar {
       if mods.isAllOf(Given | Inline | Transparent) then
         report.error("inline given instances cannot be trasparent", cdef)
       var classMods = if mods.is(Given) then mods &~ (Inline | Transparent) | Synthetic else mods
-      if vparamAccessors.exists(_.mods.is(Tracked)) then
+      val newBody = tparamAccessors ::: vparamAccessors ::: normalizedBody ::: caseClassMeths
+      if newBody.collect { case d: ValOrDefDef => d }.exists(_.mods.is(Tracked)) then
         classMods |= Dependent
       cpy.TypeDef(cdef: TypeDef)(
         name = className,
         rhs = cpy.Template(impl)(constr, parents1, clsDerived, self1,
-          tparamAccessors ::: vparamAccessors ::: normalizedBody ::: caseClassMeths)
+          newBody)
       ).withMods(classMods)
     }
 
@@ -1561,6 +1562,12 @@ object desugar {
       rhsOK(rhs)
   }
 
+  val legalTracked: Context ?=> MemberDefTest = {
+    case valdef @ ValDef(_, _, _) =>
+      val sym = valdef.symbol
+      !ctx.owner.exists || ctx.owner.isClass || ctx.owner.is(Case) || ctx.owner.isConstructor || valdef.mods.is(Param) || valdef.mods.is(ParamAccessor)
+  }
+
   def checkOpaqueAlias(tree: MemberDef)(using Context): MemberDef =
     def check(rhs: Tree): MemberDef = rhs match
       case bounds: TypeBoundsTree if bounds.alias.isEmpty =>
@@ -1586,6 +1593,7 @@ object desugar {
         } else tested
       tested = checkOpaqueAlias(tested)
       tested = checkApplicable(Opaque, legalOpaque)
+      tested = checkApplicable(Tracked, legalTracked)
       tested
     case _ =>
       tree

Diff for: compiler/src/dotty/tools/dotc/core/Flags.scala

@@ -480,7 +480,7 @@ object Flags {
    */
   val AfterLoadFlags: FlagSet = commonFlags(
     FromStartFlags, AccessFlags, Final, AccessorOrSealed,
-    Abstract, LazyOrTrait, SelfName, JavaDefined, JavaAnnotation, Transparent, Tracked)
+    Abstract, LazyOrTrait, SelfName, JavaDefined, JavaAnnotation, Transparent)
 
   /** A value that's unstable unless complemented with a Stable flag */
   val UnstableValueFlags: FlagSet = Mutable | Method

Diff for: compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3519,7 +3519,7 @@ object Parsers {
      *  UsingClsTermParamClause::= ‘(’ ‘using’ [‘erased’] (ClsParams | ContextTypes) ‘)’
      *  ClsParams         ::=  ClsParam {‘,’ ClsParam}
      *  ClsParam          ::=  {Annotation}
-     *                         [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
+     *                         [{Modifier} (‘val’ | ‘var’)] Param
      *  TypelessClause    ::= DefTermParamClause
      *                      | UsingParamClause
      *
@@ -3557,8 +3557,6 @@ object Parsers {
         if isErasedKw then
           mods = addModifier(mods)
         if paramOwner.isClass then
-          if isIdent(nme.tracked) && in.featureEnabled(Feature.modularity) && !in.lookahead.isColon then
-            mods = addModifier(mods)
           mods = addFlag(modifiers(start = mods), ParamAccessor)
           mods =
             if in.token == VAL then

Diff for: compiler/src/dotty/tools/dotc/parsing/Scanners.scala

@@ -212,6 +212,7 @@ object Scanners {
 
     def featureEnabled(name: TermName) = Feature.enabled(name)(using languageImportContext)
     def erasedEnabled = featureEnabled(Feature.erasedDefinitions)
+    def trackedEnabled = featureEnabled(Feature.modularity)
 
     private var postfixOpsEnabledCache = false
     private var postfixOpsEnabledCtx: Context = NoContext
@@ -1195,7 +1196,7 @@ object Scanners {
 
     def isSoftModifier: Boolean =
       token == IDENTIFIER
-      && (softModifierNames.contains(name) || name == nme.erased && erasedEnabled)
+      && (softModifierNames.contains(name) || name == nme.erased && erasedEnabled || name == nme.tracked && trackedEnabled)
 
     def isSoftModifierInModifierPosition: Boolean =
       isSoftModifier && inModifierPosition()

Diff for: compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -878,7 +878,7 @@ class Namer { typer: Typer =>
       case original: untpd.MemberDef =>
         lazy val annotCtx = annotContext(original, sym)
         original.setMods:
-          original.mods.withAnnotations :
+          original.mods.withAnnotations:
             original.mods.annotations.mapConserve: annotTree =>
               val cls = typedAheadAnnotationClass(annotTree)(using annotCtx)
               if (cls eq sym)
@@ -2017,6 +2017,11 @@ class Namer { typer: Typer =>
       paramFn: Type => Type,
       fallbackProto: Type
     )(using Context): Type =
+    /** Is this member tracked? This is true if it is marked as `tracked` or if
+     *  it overrides a `tracked` member. To account for the later, `isTracked`
+     *  is overriden to `true` as a side-effect of computing `inherited`.
+     */
+    var isTracked: Boolean = sym.is(Tracked)
 
     /** A type for this definition that might be inherited from elsewhere:
      *  If this is a setter parameter, the corresponding getter type.
@@ -2052,8 +2057,10 @@ class Namer { typer: Typer =>
               if paramss.isEmpty then info.widenExpr
               else NoType
 
-          val iRawInfo =
-            cls.info.nonPrivateDecl(sym.name).matchingDenotation(site, schema, sym.targetName).info
+          val iDenot = cls.info.nonPrivateDecl(sym.name).matchingDenotation(site, schema, sym.targetName)
+          val iSym = iDenot.symbol
+          if iSym.is(Tracked) then isTracked = true
+          val iRawInfo = iDenot.info
           val iResType = instantiatedResType(iRawInfo, paramss).asSeenFrom(site, cls)
           if (iResType.exists)
             typr.println(i"using inherited type for ${mdef.name}; raw: $iRawInfo, inherited: $iResType")
@@ -2147,6 +2154,7 @@ class Namer { typer: Typer =>
             if defaultTp.exists then TypeOps.SimplifyKeepUnchecked() else null)
         match
           case ctp: ConstantType if sym.isInlineVal => ctp
+          case tp if isTracked => tp
           case tp => TypeComparer.widenInferred(tp, pt, Widen.Unions)
 
     // Replace aliases to Unit by Unit itself. If we leave the alias in
@@ -2157,7 +2165,7 @@ class Namer { typer: Typer =>
     def lhsType = fullyDefinedType(cookedRhsType, "right-hand side", mdef.srcPos)
     //if (sym.name.toString == "y") println(i"rhs = $rhsType, cooked = $cookedRhsType")
     if (inherited.exists)
-      if sym.isInlineVal then lhsType else inherited
+      if sym.isInlineVal || isTracked then lhsType else inherited
     else {
       if (sym.is(Implicit))
         mdef match {

Diff for: compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -2433,7 +2433,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         else if ctx.reporter.errorsReported then UnspecifiedErrorType
         else errorType(em"cannot infer type; expected type $pt is not fully defined", tree.srcPos))
 
-  def typedTypeTree(tree: untpd.TypeTree, pt: Type)(using Context): Tree =
+  def typedTypeTree(tree: untpd.TypeTree, pt: Type)(using Context): Tree = {
     tree match
       case tree: untpd.DerivedTypeTree =>
         tree.ensureCompletions
@@ -2449,6 +2449,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         }
       case _ =>
         completeTypeTree(InferredTypeTree(), pt, tree)
+  }
 
   def typedInLambdaTypeTree(tree: untpd.InLambdaTypeTree, pt: Type)(using Context): Tree =
     val tp =
@@ -2860,7 +2861,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     val nnInfo = rhs1.notNullInfo
     vdef1.withNotNullInfo(if sym.is(Lazy) then nnInfo.retractedInfo else nnInfo)
   }
-
   private def retractDefDef(sym: Symbol)(using Context): Tree =
     // it's a discarded method (synthetic case class method or synthetic java record constructor or overridden member), drop it
     val canBeInvalidated: Boolean =
@@ -3672,7 +3672,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   }
 
   /** Typecheck and adapt tree, returning a typed tree. Parameters as for `typedUnadapted` */
-  def typed(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree =
+  def typed(tree: untpd.Tree, pt: Type, locked: TypeVars)(using Context): Tree = {
     trace(i"typing $tree, pt = $pt", typr, show = true) {
       record(s"typed $getClass")
       record("typed total")
@@ -3684,6 +3684,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         tree.withType(WildcardType)
       else adapt(typedUnadapted(tree, pt, locked), pt, locked)
     }
+  }
 
   def typed(tree: untpd.Tree, pt: Type = WildcardType)(using Context): Tree =
     typed(tree, pt, ctx.typerState.ownedVars)
@@ -3799,7 +3800,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   def typedExpr(tree: untpd.Tree, pt: Type = WildcardType)(using Context): Tree =
     withoutMode(Mode.PatternOrTypeBits)(typed(tree, pt))
 
-  def typedType(tree: untpd.Tree, pt: Type = WildcardType, mapPatternBounds: Boolean = false)(using Context): Tree =
+  def typedType(tree: untpd.Tree, pt: Type = WildcardType, mapPatternBounds: Boolean = false)(using Context): Tree = {
     val tree1 = withMode(Mode.Type) { typed(tree, pt) }
     if mapPatternBounds && ctx.mode.is(Mode.Pattern) && !ctx.isAfterTyper then
       tree1 match
@@ -3815,6 +3816,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
         case tree1 =>
           tree1
     else tree1
+  }
 
   def typedPattern(tree: untpd.Tree, selType: Type = WildcardType)(using Context): Tree =
     withMode(Mode.Pattern)(typed(tree, selType))

Diff for: docs/_docs/internals/syntax.md

@@ -141,7 +141,7 @@ type      val       var       while     with      yield
 ### Soft keywords
 
 ```
-as  derives  end  erased  extension  infix  inline  opaque  open  throws transparent  using  |  *  +  -
+as  derives  end  erased  extension  infix  inline  opaque  open  throws tracked transparent  using  |  *  +  -
 ```
 
 See the [separate section on soft keywords](../reference/soft-modifier.md) for additional
@@ -381,7 +381,7 @@ ClsParamClause    ::=  [nl] ‘(’ ClsParams ‘)’
                     |  [nl] ‘(’ ‘using’ (ClsParams | FunArgTypes) ‘)’
 ClsParams         ::=  ClsParam {‘,’ ClsParam}
 ClsParam          ::=  {Annotation}                                             ValDef(mods, id, tpe, expr) -- point of mods on val/var
-                       [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
+                       [{Modifier} (‘val’ | ‘var’)] Param
 
 DefParamClauses   ::=  DefParamClause { DefParamClause } -- and two DefTypeParamClause cannot be adjacent
 DefParamClause    ::=  DefTypeParamClause
@@ -418,6 +418,7 @@ LocalModifier     ::=  ‘abstract’
                     |  ‘transparent’
                     |  ‘infix’
                     |  ‘erased’
+                    |  ‘tracked’
 
 AccessModifier    ::=  (‘private’ | ‘protected’) [AccessQualifier]
 AccessQualifier   ::=  ‘[’ id ‘]’

Diff for: docs/_docs/reference/experimental/modularity.md

@@ -108,14 +108,6 @@ This works as it should now. Without the addition of `tracked` to the
 parameter of `SetFunctor` typechecking would immediately lose track of
 the element type `T` after an `add`, and would therefore fail.
 
-**Syntax Change**
-
-```
-ClsParam  ::=  {Annotation} [{Modifier | ‘tracked’} (‘val’ | ‘var’)] Param
-```
-
-The (soft) `tracked` modifier is only allowed for `val` parameters of classes.
-
 **Discussion**
 
 Since `tracked` is so useful, why not assume it by default? First, `tracked` makes sense only for `val` parameters. If a class parameter is not also a field declared using `val` then there's nothing to refine in the constructor result type. One could think of at least making all `val` parameters tracked by default, but that would be a backwards incompatible change. For instance, the following code would break:
@@ -134,6 +126,38 @@ only if the class refers to a type member of `x`. But it turns out that this
 scheme is unimplementable since it would quickly lead to cyclic references
 when typechecking recursive class graphs. So an explicit `tracked` looks like the best available option.
 
+## Tracked members
+
+The `tracked` modifier can also be used for `val` members of classes and traits
+to force the type of the member (or it's overriding member) to be as exact as
+possible. More precisely, it will will assign the `tracked` member the infered
+type of the rhs. For instance, consider the following definition:
+
+```scala
+trait F:
+  tracked val a: Int
+  tracked val b: Int
+
+class N extends F:
+  val a = 22 // a.type =:= 22
+  val b: Int = 22 // b.type =:= Int
+  tracked val c = 22 // c.type =:= 22
+```
+
+Here, the `tracked` modifier ensures that the type of `a` in `N` is `22` and not
+`Int`. But the type of `b` is `N` is `Int` since it's explicitly declared as
+`Int`. `tracked` members can also be immediately initialized, as in the case of
+`c`.
+
+## Tracked syntax change
+
+```
+LocalModifier     ::=  ‘tracked’
+```
+
+The (soft) `tracked` modifier is allowed as a local modifier.
+
+
 ## Allow Class Parents to be Refined Types
 
 Since `tracked` parameters create refinements in constructor types,

Diff for: tests/neg/abstract-tracked-1.scala

@@ -0,0 +1,12 @@
+import scala.language.experimental.modularity
+import scala.language.future
+
+trait F:
+  tracked val a: Int
+
+class G:
+  val a: Int = 1
+
+def Test =
+  val g = new G
+  summon[g.a.type <:< 1] // error

Diff for: tests/neg/abstract-tracked.check

@@ -0,0 +1,20 @@
+-- [E156] Syntax Error: tests/neg/abstract-tracked.scala:4:14 ----------------------------------------------------------
+4 |tracked trait F // error
+  |^^^^^^^^^^^^^^^
+  |Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/abstract-tracked.scala:9:15 ----------------------------------------------------------
+9 |tracked object O // error
+  |^^^^^^^^^^^^^^^^
+  |Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/abstract-tracked.scala:11:14 ---------------------------------------------------------
+11 |tracked class C // error
+   |^^^^^^^^^^^^^^^
+   |Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/abstract-tracked.scala:7:14 ----------------------------------------------------------
+7 |  tracked def f: F // error
+  |  ^^^^^^^^^^^^^^^^
+  |  Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/abstract-tracked.scala:14:14 ---------------------------------------------------------
+14 |  tracked val x = 1 // error
+   |  ^^^^^^^^^^^^^^^^^
+   |  Modifier tracked is not allowed for this definition

Diff for: tests/neg/abstract-tracked.scala

@@ -0,0 +1,14 @@
+import scala.language.experimental.modularity
+import scala.language.future
+
+tracked trait F // error
+
+trait G:
+  tracked def f: F // error
+
+tracked object O // error
+
+tracked class C // error
+
+def f =
+  tracked val x = 1 // error

Diff for: tests/neg/tracked.check

@@ -6,22 +6,6 @@
 7 |  def foo(tracked a: Int) = // error
   |                  ^
   |                  ':' expected, but identifier found
--- Error: tests/neg/tracked.scala:8:12 ---------------------------------------------------------------------------------
-8 |    tracked val b: Int = 2 // error
-  |            ^^^
-  |            end of statement expected but 'val' found
--- Error: tests/neg/tracked.scala:11:10 --------------------------------------------------------------------------------
-11 |  tracked object Foo // error // error
-   |          ^^^^^^
-   |          end of statement expected but 'object' found
--- Error: tests/neg/tracked.scala:14:10 --------------------------------------------------------------------------------
-14 |  tracked class D // error // error
-   |          ^^^^^
-   |          end of statement expected but 'class' found
--- Error: tests/neg/tracked.scala:17:10 --------------------------------------------------------------------------------
-17 |  tracked type T = Int // error // error
-   |          ^^^^
-   |          end of statement expected but 'type' found
 -- Error: tests/neg/tracked.scala:20:25 --------------------------------------------------------------------------------
 20 |  given g2: (tracked val x: Int) => C = C(x) // error
    |             ^^^^^^^^^^^^^^^^^^
@@ -30,21 +14,19 @@
 4 |class C2(tracked var x: Int) // error
   |                     ^
   |                     mutable variables may not be `tracked`
--- [E006] Not Found Error: tests/neg/tracked.scala:11:2 ----------------------------------------------------------------
-11 |  tracked object Foo // error // error
-   |  ^^^^^^^
-   |  Not found: tracked
-   |
-   | longer explanation available when compiling with `-explain`
--- [E006] Not Found Error: tests/neg/tracked.scala:14:2 ----------------------------------------------------------------
-14 |  tracked class D // error // error
-   |  ^^^^^^^
-   |  Not found: tracked
-   |
-   | longer explanation available when compiling with `-explain`
--- [E006] Not Found Error: tests/neg/tracked.scala:17:2 ----------------------------------------------------------------
-17 |  tracked type T = Int // error // error
-   |  ^^^^^^^
-   |  Not found: tracked
-   |
-   | longer explanation available when compiling with `-explain`
+-- [E156] Syntax Error: tests/neg/tracked.scala:8:16 -------------------------------------------------------------------
+8 |    tracked val b: Int = 2 // error
+  |    ^^^^^^^^^^^^^^^^^^^^^^
+  |    Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/tracked.scala:11:17 ------------------------------------------------------------------
+11 |  tracked object Foo // error
+   |  ^^^^^^^^^^^^^^^^^^
+   |  Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/tracked.scala:14:16 ------------------------------------------------------------------
+14 |  tracked class D // error
+   |  ^^^^^^^^^^^^^^^
+   |  Modifier tracked is not allowed for this definition
+-- [E156] Syntax Error: tests/neg/tracked.scala:17:15 ------------------------------------------------------------------
+17 |  tracked type T = Int // error
+   |  ^^^^^^^^^^^^^^^^^^^^
+   |  Modifier tracked is not allowed for this definition

Diff for: tests/neg/tracked.scala

@@ -8,13 +8,13 @@ object A:
     tracked val b: Int = 2 // error
 
 object B:
-  tracked object Foo // error // error
+  tracked object Foo // error
 
 object C:
-  tracked class D // error // error
+  tracked class D // error
 
 object D:
-  tracked type T = Int // error // error
+  tracked type T = Int // error
 
 object E:
   given g2: (tracked val x: Int) => C = C(x) // error