Fewer braces breaks rewrites

[ckipp01]

Compiler version

3.3.1-RC1-bin-20230503-b8d2966-NIGHTLY

Minimized code

//> using scala "3.nightly"
//> using options "-old-syntax", "-rewrite"


@main def run() = {
  List(1, 2, 3).map: num =>
    num * 2
}

Output

[error] ./main.scala:6:3
[error] not a legal formal parameter for a function literal
[error]   List(1, 2, 3).map: num =>
[error]   ^^^^^^^^^^^^^^^^^^^^^^
[error] ./main.scala:7:5
[error] Not found: num
[error]     num * 2
[error]     ^^^

Expectation

I'd expect to be able to compile this code and get it re-written to the old syntax.

I'm not sure if this was overlooked or just never taken into account, but I consider this a pretty serious regression. With fewer braces entering the real world in 3.3.0, we'll be introducing code that cannot be rewritten by the compiler to the old syntax.

[mbovel]

@adpi2 @ckipp01 how complex would you estimate the fix to be? Do you think this could be appropriate for an issue spree?

Seems like we would need to add appropriate patch calls in Parsers.scala around isColonLambda calls.

[bishabosha]

I think this should instead be for the -no-indent option. similarly the -indent rewrite doesn't remove braces for fewer braces.

[ckipp01]

I think this should instead be for the -no-indent option. similarly the -indent rewrite doesn't remove braces for fewer braces.

just for clarity, note that currently -no-ident on this snippet just does nothing. It compiles fine, but doesn't actually rewrite anything.

[ckipp01]

@adpi2 @ckipp01 how complex would you estimate the fix to be? Do you think this could be appropriate for an issue spree?

To be honest I'm not super familiar with the rewrite stuff, but if I had to guess, I think with some guidance yea this could totally be worked on during a spree.

[adpi2]

@adpi2 @ckipp01 how complex would you estimate the fix to be? Do you think this could be appropriate for an issue spree?

Hard to tell without giving a try. I'll try to look into it some time soon.

I think this should instead be for the -no-indent option. similarly the -indent rewrite doesn't remove braces for fewer braces.

Agreed, I also think the fewer braces should be handled by -no-ident. -old-syntax only applies to the control structures. Not sure if -indent should remove braces for fewer braces though.

[odersky]

An alternative would be to drop the new-to-old syntax rewriting entirely and only allow old-to-new.

[som-snytt]

By coincidence (or necessity), sbt/sbt#8045 (comment)

I wonder if it's feasible for a "merge tool" to rewrite the target to old syntax, apply a patch, then rewrite back. (The old syntax would also require old scalafmt config.)

The use case is sbt "merge forward" is harder once sbt 2 code is rewritten to scala 3.