Allow val defined as a const to be used as NES

[Kordyjan]

This change allows references to values defined as non-empty constants to be used as NonEmptyStrings.
Works when the -Yretain-trees flag is enabled.

It can be taken further by applying the checks recursively, e.g., we can check make things like

val x = s"abc${"xyz"}"
val nes: NonEmptyString = x

or

val x = "ABC"
val y = "def"
val nes: NonEmptyString = s"$x$y"

work.

[lbialy]

@prolativ can you please add -YretainTrees to default setup of Scala-cli launcher, templates and a test?

[prolativ]

@lbialy is this something we need for our initial release? I would opt for holding this a bit to do some experiments and see how this pattern might influence users' code

[lbialy]

Ok, sure.

…

On Thu 21. Sep 2023 at 10:22, Michał Pałka ***@***.***> wrote: @lbialy <https://github.com/lbialy> is this something we need for our initial release? I would opt for holding this a bit to do some experiments and see how this pattern might influence users' code — Reply to this email directly, view it on GitHub <#137 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/ACBVNUROWOWREA6N34DY2ZTX3P2NJANCNFSM6AAAAAA5AQNV5U> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[lbialy]

This is subsumed by #375. Thank you @Kordyjan for all the help with this!