Fix #9970: Reconcile how Scala 3 and 2 decide whether a trait has $init$.

Scala 2 uses a very simple mechanism to detect traits that have or
don't have a `$init$` method:

* At parsing time, a `$init$` method is created if and only if
  there at least one member in the body that "requires" it.
* `$init$` are otherwise never created nor removed.
* A call to `T.$init$` in a subclass `C` is generated if and only
  if `T.$init$` exists.

Scala 3 has a flags-based approach to the above:

* At parsing time, a `<init>` method is always created for traits.
* The `NoInits` flag is added if the body does not contain any
  member that "requires" an initialization.
* In addition, the constructor receives the `StableRealizable` flag
  if the trait *and all its base classes/traits* have the `NoInits`
  flag. This is then used for purity analysis in the inliner.
* The `Constructors` phase creates a `$init$` method for traits
  that do not have the `NoInits` flag, and generates calls based on
  the same criteria.

Now, one might think that this is all fine, except it isn't when we
mix Scala 2 and 3, and in particular when a Scala 2 class extend a
Scala 3 trait.

Indeed, since Scala 3 *always* defines a `<init>` method in traits,
which Scala 2 translates as `$init$`, Scala 2 would think that it
always needs to emit a call to `$init$` (even for traits where
Scala 3 does not, in fact, emit a `$init$` method in the end).
This was mitigated in the TASTy reader of Scala 2 by removing
`$init$` if it has the `STABLE` flag (coming from
`StableRealizable`).

This would have been fine if `StableRealizable` was present if and
only if the owner trait has `NoInits`. But until this commit, that
was not the case: a trait without initialization in itself, but
extending a trait with initialization code, would have the flag
`NoInits` but its constructor would not have the `StableRealizable`
flag.

Therefore, this commit basically reconciles the `NoInits` and
`StableRealizable` flags, so that Scala 2 understands whether or
not a Scala 3 trait has a `$init$` method. We also align those
flags when reading from Scala 2 traits, so that Scala 3 also
understands whether or not a Scala 2 trait has a `$init$` trait.

This solves the compatibility issue between Scala 2 and Scala 3.

One detail remains. The attentive reader may have noticed the
quotes in 'an element of the body that "requires" initialization'.
Scala 2 and Scala 3 do not agree on what requires initialization:
notably, Scala 2 thinks that a concrete `def` requires
initialization, whereas Scala 3 knows that this is not the case.
This is not an issue for synchronous interoperability between Scala
2 and 3, since each decides on its own which of its traits has a
`$init$` method, and communicates that fact to the other version.

However, this still poses an issue for "diachronic" compatibility:
if a library defines a trait with a concrete `def` and is compiled
by Scala 2 in version v1, that trait will have a `$init$`. When the
library upgrades to Scala 3 in version v2, the trait will *lose*
the `$init$`, which can break third-party subclasses.

This commit does not address this issue. There are two ways we
could do so:

* Precisely align the detection of whether a trait should have a
  `$init$` method with Scala 2 (notably, adding one when there is a
  concrete `def`), or
* *Always* emit an empty static `$init$` method, even for traits
  that have the `NoInits` flag (but do not generate calls to them,
  nor have that affect whether or not subclasses are considered
  pure).

Author: sjrd

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

@@ -316,7 +316,7 @@ trait UntypedTreeInfo extends TreeInfo[Untyped] { self: Trees.Instance[Untyped]
   /**  The largest subset of {NoInits, PureInterface} that a
    *   trait or class enclosing this statement can have as flags.
    */
-  def defKind(tree: Tree)(using Context): FlagSet = unsplice(tree) match {
+  private def defKind(tree: Tree)(using Context): FlagSet = unsplice(tree) match {
     case EmptyTree | _: Import => NoInitsInterface
     case tree: TypeDef => if (tree.isClassDef) NoInits else NoInitsInterface
     case tree: DefDef =>
@@ -402,8 +402,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
         || sym.owner == defn.StringClass
         || defn.pureMethods.contains(sym)
       if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
-          || (fn.symbol.isStableMember && !fn.symbol.is(Lazy))
-          || fn.symbol.isPrimaryConstructor && fn.symbol.owner.isNoInitsClass) // TODO: include in isStable?
+          || (fn.symbol.isStableMember && !fn.symbol.is(Lazy))) // constructors of no-inits classes are stable
         minOf(exprPurity(fn), args.map(exprPurity)) `min` Pure
       else if (fn.symbol.is(Erased)) Pure
       else if (fn.symbol.isStableMember) /* && fn.symbol.is(Lazy) */
@@ -459,7 +458,7 @@ trait TypedTreeInfo extends TreeInfo[Type] { self: Trees.Instance[Type] =>
     else if tree.tpe.isInstanceOf[ConstantType] then PurePath
     else if (!sym.isStableMember) Impure
     else if (sym.is(Module))
-      if (sym.moduleClass.isNoInitsClass) PurePath else IdempotentPath
+      if (sym.moduleClass.isNoInitsRealClass) PurePath else IdempotentPath
     else if (sym.is(Lazy)) IdempotentPath
     else if sym.isAllOf(Inline | Param) then Impure
     else PurePath

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

@@ -181,7 +181,7 @@ class Definitions {
   }
 
   private def completeClass(cls: ClassSymbol, ensureCtor: Boolean = true): ClassSymbol = {
-    if (ensureCtor) ensureConstructor(cls, EmptyScope)
+    if (ensureCtor) ensureConstructor(cls, cls.denot.asClass, EmptyScope)
     if (cls.linkedClass.exists) cls.linkedClass.markAbsent()
     cls
   }

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

@@ -293,7 +293,14 @@ object Flags {
    */
   val (Abstract @ _, _, _) = newFlags(23, "abstract")
 
-  /** Lazy val or method is known or assumed to be stable and realizable */
+  /** Lazy val or method is known or assumed to be stable and realizable.
+   *
+   *  For a trait constructor, this is set if and only if owner.is(NoInits),
+   *  including for Java interfaces and for Scala 2 traits. It will be used by
+   *
+   *  - the purity analysis used by the inliner, and
+   *  - the TASTy reader of Scala 2.13, to determine whether there is a $init$ method.
+   */
   val (_, StableRealizable @ _, _) = newFlags(24, "<stable>")
 
   /** A case parameter accessor */
@@ -319,7 +326,9 @@ object Flags {
 
   /** Variable is accessed from nested function
    *    /
-   *  Trait does not have fields or initialization code.
+   *  Trait does not have own fields or initialization code or class does not
+   *  have own or inherited initialization code.
+   *
    *  Warning: NoInits is set during regular typer pass, should be tested only after typer.
    */
   val (_, Captured @ _, NoInits @ _) = newFlags(32, "<captured>", "<noinits>")
@@ -536,7 +545,6 @@ object Flags {
   val DeferredOrTermParamOrAccessor: FlagSet = Deferred | ParamAccessor | TermParam           // term symbols without right-hand sides
   val DeferredOrTypeParam: FlagSet           = Deferred | TypeParam                           // type symbols without right-hand sides
   val EnumValue: FlagSet                     = Enum | StableRealizable                        // A Scala enum value
-  val StableOrErased: FlagSet                = Erased | StableRealizable                      // Assumed to be pure
   val FinalOrInline: FlagSet                 = Final | Inline
   val FinalOrModuleClass: FlagSet            = Final | ModuleClass                            // A module class or a final class
   val EffectivelyFinalFlags: FlagSet         = Final | Private

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

@@ -725,11 +725,11 @@ object SymDenotations {
       isType || is(StableRealizable) || exists && !isUnstableValue
     }
 
-    /** Is this a denotation of a class that does not have - either direct or inherited -
+    /** Is this a denotation of a real class that does not have - either direct or inherited -
      *  initialization code?
      */
-    def isNoInitsClass(using Context): Boolean =
-      isClass &&
+    def isNoInitsRealClass(using Context): Boolean =
+      isRealClass &&
       (asClass.baseClasses.forall(_.is(NoInits)) || defn.isAssuredNoInits(symbol))
 
     /** Is this a "real" method? A real method is a method which is:

compiler/src/dotty/tools/dotc/core/unpickleScala2/Scala2Unpickler.scala

@@ -59,9 +59,14 @@ object Scala2Unpickler {
     denot.info = PolyType.fromParams(denot.owner.typeParams, denot.info)
   }
 
-  def ensureConstructor(cls: ClassSymbol, scope: Scope)(using Context): Unit = {
+  def ensureConstructor(cls: ClassSymbol, clsDenot: ClassDenotation, scope: Scope)(using Context): Unit = {
     if (scope.lookup(nme.CONSTRUCTOR) == NoSymbol) {
       val constr = newDefaultConstructor(cls)
+      // Scala 2 traits have a constructor iff they have initialization code
+      // In dotc we represent that as !StableRealizable, which is also owner.is(NoInits)
+      if clsDenot.flagsUNSAFE.is(Trait) then
+        constr.setFlag(StableRealizable)
+        clsDenot.setFlag(NoInits)
       addConstructorTypeParams(constr)
       cls.enter(constr, scope)
     }
@@ -95,7 +100,7 @@ object Scala2Unpickler {
       if (tsym.exists) tsym.setFlag(TypeParam)
       else denot.enter(tparam, decls)
     }
-    if (!denot.flagsUNSAFE.isAllOf(JavaModule)) ensureConstructor(denot.symbol.asClass, decls)
+    if (!denot.flagsUNSAFE.isAllOf(JavaModule)) ensureConstructor(cls, denot, decls)
 
     val scalacCompanion = denot.classSymbol.scalacLinkedClass
 
@@ -465,11 +470,13 @@ class Scala2Unpickler(bytes: Array[Byte], classRoot: ClassDenotation, moduleClas
       denot.setFlag(flags)
       denot.resetFlag(Touched) // allow one more completion
 
-      // Temporary measure, as long as we do not read these classes from Tasty.
-      // Scala-2 classes don't have NoInits set even if they are pure. We override this
-      // for Product and Serializable so that case classes can be pure. A full solution
-      // requires that we read all Scala code from Tasty.
-      if (owner == defn.ScalaPackageClass && ((name eq tpnme.Serializable) || (name eq tpnme.Product)))
+      // Temporary measure, as long as we do not recompile these traits with Scala 3.
+      // Scala 2 is more aggressive when it comes to defining a $init$ method than Scala 3.
+      // Any concrete definition, even a `def`, causes a trait to receive a $init$ method
+      // to be created, even if it does not do anything, and hence causes not have the NoInits flag.
+      // We override this for Product so that cases classes can be pure.
+      // A full solution requires that we compile Product with Scala 3 in the future.
+      if (owner == defn.ScalaPackageClass && (name eq tpnme.Product))
         denot.setFlag(NoInits)
 
       denot.setPrivateWithin(privateWithin)

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

@@ -540,11 +540,10 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
           meth.name == nme.apply &&
           meth.flags.is(Synthetic) &&
           meth.owner.linkedClass.is(Case) &&
-          cls.isNoInitsClass
+          cls.isNoInitsRealClass
         }
         if (tree.tpe.isInstanceOf[ConstantType] && isKnownPureOp(tree.symbol) // A constant expression with pure arguments is pure.
-            || (fn.symbol.isStableMember && !fn.symbol.is(Lazy))
-            || fn.symbol.isPrimaryConstructor && fn.symbol.owner.isNoInitsClass) // TODO: include in isStable?
+            || (fn.symbol.isStableMember && !fn.symbol.is(Lazy))) // constructors of no-inits classes are stable
           apply(fn) && args.forall(apply)
         else if (isCaseClassApply)
           args.forall(apply)
@@ -849,7 +848,7 @@ class Inliner(call: tpd.Tree, rhsToInline: tpd.Tree)(using Context) {
     def reduceProjection(tree: Tree)(using Context): Tree = {
       if (ctx.debug) inlining.println(i"try reduce projection $tree")
       tree match {
-        case Select(NewInstance(cls, args, prefix, precomputed), field) if cls.isNoInitsClass =>
+        case Select(NewInstance(cls, args, prefix, precomputed), field) if cls.isNoInitsRealClass =>
           def matches(param: Symbol, selection: Symbol): Boolean =
             param == selection || {
               selection.name match {

compiler/src/dotty/tools/dotc/typer/Namer.scala

@@ -1197,7 +1197,10 @@ class Namer { typer: Typer =>
       cls.info = avoidPrivateLeaks(cls)
       cls.baseClasses.foreach(_.invalidateBaseTypeCache()) // we might have looked before and found nothing
       cls.setNoInitsFlags(parentsKind(parents), untpd.bodyKind(rest))
-      if (cls.isNoInitsClass) cls.primaryConstructor.setFlag(StableRealizable)
+      val ctorStable =
+        if cls.is(Trait) then cls.is(NoInits)
+        else cls.isNoInitsRealClass
+      if ctorStable then cls.primaryConstructor.setFlag(StableRealizable)
       processExports(using localCtx)
       defn.patchStdLibClass(denot.asClass)
     }