List(...) builder missing Scala 2 optimization to avoid intermediate array

[armanbilge]

Compiler version

3.3.1-RC1-bin-20230301-0df5ae2-NIGHTLY

Minimized code

class Test {
  def foo = List("foo", "bar", "baz")
}

Output

/*
 * Decompiled with CFR 0.151.
 */
import scala.collection.immutable.List;
import scala.collection.immutable.Seq;
import scala.package$;
import scala.runtime.ScalaRunTime$;

public class Test {
    public List<String> foo() {
        return (List)package$.MODULE$.List().apply((Seq)ScalaRunTime$.MODULE$.wrapRefArray((Object[])new String[]{"foo", "bar", "baz"}));
    }
}

Expectation

The List is built directly, without going through an intermediate varargs array.

Scala 2.13.10 decompiles to this:

/*
 * Decompiled with CFR 0.151.
 */
import scala.collection.immutable.;
import scala.collection.immutable.List;
import scala.collection.immutable.Nil$;

public class Test {
    public List<String> foo() {
        return new .colon.colon((Object)"foo", (List)new .colon.colon((Object)"bar", (List)new .colon.colon((Object)"baz", (List)Nil$.MODULE$)));
    }
}

[smarter]

For anyone interested in giving this a try, this should probably be done done as a mini-phase similar to https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/transform/TupleOptimizations.scala.

[nicolasstucki]

It is better to do this one after erasure. The Array.apply transform seems to be more closely related to this optimization https://github.com/lampepfl/dotty/blob/main/compiler/src/dotty/tools/dotc/transform/ArrayApply.scala.

[smarter]

For reference, the Scala 2 implementation is at https://github.com/scala/scala/blob/5ecdcf817d5084060734e5cb2f1b505472e7bda0/src/compiler/scala/tools/nsc/transform/CleanUp.scala#L555-L565
and https://github.com/scala/scala/blob/5ecdcf817d5084060734e5cb2f1b505472e7bda0/src/compiler/scala/tools/nsc/transform/CleanUp.scala#L630-L633

[scala-center-bot]

This issue was picked for the Issue Spree No. 28 of 28 March 2023 which takes place in a week from now. @smarter, @Decel, @KuceraMartin will be working on it. If you have any insight into the issue or guidance on how to fix it, please leave it here.