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Sep 2, 2020
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enum values array is constructed from field references #9677

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enum values array is constructed from field references
bishabosha Aug 28, 2020
bdea97a
add comment about unchecked annotation
bishabosha Sep 2, 2020
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15 changes: 8 additions & 7 deletions compiler/src/dotty/tools/dotc/ast/DesugarEnums.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -104,7 +104,7 @@ object DesugarEnums {

/** The following lists of definitions for an enum type E and known value cases e_0, ..., e_n:
*
* private val $values = Array[E](e_0,...,e_n)(ClassTag[E](classOf[E]))
* private val $values = Array[E](e_0,...,e_n)(ClassTag[E](classOf[E])): @unchecked
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Please add a comment explaining the interaction with the init checker. Is this a known limitation of the init checker? Is it something that can be fixed and if so is there an issue for it?

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@bishabosha bishabosha Sep 2, 2020 •
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The issue is that the init checker will not try to prove that new E {...} is safe for performance reasons, so all enum cases that call $new creator method can't be checked. The init checker however knows that selecting from E companion object must be safe because it is already constructed, so we can either select from this which means $values does not have to be lazy, or we have to select from E which is under construction so it has to be lazy.

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cc @liufengyun

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we also have this comment #9665 (comment)

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OK, as long as this is all properly documented and referenced in the code, I'm happy :).

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In the long run, we might introduce type annotations and/or eagerly check small classes, both could be helpful to avoid @unchecked in such cases. Initially, we want to keep the checker stupid to see how it works in the wild and make sure the core algorithm is reliable.

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Maybe an issue should be opened for that (either here or on dotty-feature-requests) so that Jamie can link to it in the code?

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I opened https://github.com/lampepfl/dotty-feature-requests/issues/135

* def values = $values.clone
* def valueOf($name: String) = $name match {
* case "e_0" => e_0
Expand All @@ -117,9 +117,11 @@ object DesugarEnums {
val rawEnumClassRef = rawRef(enumClass.typeRef)
extension (tpe: NamedType) def ofRawEnum = AppliedTypeTree(ref(tpe), rawEnumClassRef)

val lazyFlagOpt = if enumCompanion.owner.isStatic then EmptyFlags else Lazy
val privateValuesDef = ValDef(nme.DOLLAR_VALUES, TypeTree(), ArrayLiteral(enumValues, rawEnumClassRef))
.withFlags(Private | Synthetic | lazyFlagOpt)
val privateValuesDef =
val uncheckedValues =
Annotated(ArrayLiteral(enumValues, rawEnumClassRef), New(ref(defn.UncheckedAnnot.typeRef)))
ValDef(nme.DOLLAR_VALUES, TypeTree(), uncheckedValues)
.withFlags(Private | Synthetic)
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The lazyFlagOpt disappeared. Was this a mistake during conflict resolution?

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@bishabosha bishabosha Aug 29, 2020 •
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no, it was removed because it was only required when the enum values were explicitly selected from the companion, and the companion was a local lazy val. Now they are selected from this which doesn't force the companion to be initialised.

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Ah, that makes sense!


val valuesDef =
DefDef(nme.values, Nil, Nil, defn.ArrayType.ofRawEnum, valuesDot(nme.clone_))
Expand Down Expand Up @@ -170,7 +172,6 @@ object DesugarEnums {
* def ordinal = _$ordinal // if `E` does not derive from `java.lang.Enum`
* def enumLabel = $name // if `E` does not derive from `java.lang.Enum`
* def enumLabel = this.name // if `E` derives from `java.lang.Enum`
* $values.register(this)
* }
*/
private def enumValueCreator(using Context) = {
Expand Down Expand Up @@ -307,8 +308,8 @@ object DesugarEnums {
case name: TermName => (ordinal, name) :: seenCases
case _ => seenCases
if definesLookups then
val companionRef = ref(enumCompanion.termRef)
val cachedValues = cases.reverse.map((i, name) => (i, Select(companionRef, name)))
val thisRef = This(EmptyTypeIdent)
val cachedValues = cases.reverse.map((i, name) => (i, Select(thisRef, name)))
(ordinal, enumLookupMethods(EnumConstraints(minKind, maxKind, cachedValues)))
else
ctx.tree.pushAttachment(EnumCaseCount, (ordinal + 1, minKind, maxKind, cases))
Expand Down
14 changes: 14 additions & 0 deletions tests/run/enums-thunk.scala
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Expand Up @@ -20,8 +20,22 @@ object Outer2 {
}
}

object Outer3 {
def thunk() = {
enum E { case C1 }
E.C1
}
def thunk2() = {
enum E { case C2 }
E.values
}
}


@main def Test =
assert(Outer().thunk().toString == "A1")
assert(Outer().thunk2()(0).toString == "A2")
assert(Outer2.thunk().toString == "B1")
assert(Outer2.thunk2()(0).toString == "B2")
assert(Outer3.thunk().toString == "C1")
assert(Outer3.thunk2()(0).toString == "C2")