Improving Scala 3 forward compatibility

[eed3si9n]

@nafg

Why did --scala-target become -Yscala-release? The latter name is a lot less self-documenting. There’s no way to know what it does without having learned about it, while --scala-target is self-explanatory.

I also find the term -Yscala-release confusing, even though I think I am relatively familiar with JDK tooling. In JDK, the idea of release is (and has been) a thing – JDK Releases for example lists all JDK releases and their associated release type, release support timeline.

JEP 238: Multi-Release JAR Files and JEP 247: Compile for Older Platform Versions extended Java compiler toolchain to support multi-release JAR file that contains multiple A.class against the Java class A, but targeting JDK 8 and JDK 11 etc. In that context the flag --release, kind of makes sense because

javac --release 8 -d classes src\main\java\A.java
javac --release 11 -d classes-11 src\main\java11\A.java

are denoting JDK releases, and often compiled against different source code with one using Unsafe etc.

Since 2018 (Scala 2.12.5) Scala 2.x has supported --release 8 in the same semantics (https://github.com/scala/scala/pull/6362), which allows downgrading of JDK target and generating multiple *.class.

-Yscala-release 3.0 I don’t think translates into Scala version context, chiefly because we don’t use the term “release” to describe minor versions within a binary compatible series, and it might also be confusing especially because the idea of forward compatibility was something Scala 2.x library authors took for granted.

I am commenting here because there’s now a proposal that translates -Yscala-release name into an sbt setting (Add support for Scala 3 -scala-output-version flag by prolativ · Pull Request #6814 · sbt/sbt · GitHub):

ThisBuild / scalaVersion := "3.1.2-RC1"
ThisBuild / scalaReleaseVersion := "3.0.2"

@prolativ

-target is more like YOLO mode, hence it was renamed to -Xtarget in scala 3 and it’s usage is discouraged - -release should be used instead whenever possible.

This notion that the word release implies better checking of API compatibility but target doesn’t is an implementation detail I don’t most people appreciate or care since most libraries probably just use older JDK to build and not create multi-release JAR.

As alternative, I think something like

ThisBuild / scalaVersion := "3.1.2-RC1"
ThisBuild / targetScalaVersion := "3.0.2"

or

ThisBuild / scalaVersion := "3.1.2-RC1"
ThisBuild / compatibleScalaVersion := "3.0.2"

would be more intuitive. Alternatively:

ThisBuild / compileTimeScalaVersion := "3.1.2-RC1"
ThisBuild / scalaVersion := "3.0.2"

I think also should be considered. Primary objection (Add support for Scala 3 -scala-output-version flag by prolativ · Pull Request #6814 · sbt/sbt · GitHub) to that is that now we’d have to rewrite builds that uses scalaVersion to pick compiler options, which is fair.