Support constant folding for `s`, `f` and `raw` interpolators.

[nicolasstucki]

We should be able to constant fold s, f and raw interpolator with constant inputs.

  val r: "abc" = s"ab${"c"}"

This would be useful when combined with inlining

inline val x = "abc"
inline val y = s"${y}dce"

A realistic use of this feature would be with the scala.compiletime.error method.

inline def f(x: Int) = 
  compiletime.error(s"Error code: $x")

f(3) // error: `Error code: 3`

This would need to happen when typing, probably in Inliner.

[soronpo]

@nicolasstucki Say I want to tackle this, I have some questions regarding the implementation.

1. Where should I start? Where are these implemented in the compiler?
2. Does changing s, f, etc. to inline affect binary compatibility?
3. Theoretically we can special case this only within the context of compiletime.error(), since we can read the tree as a raw StringContext and constant fold it accordingly. Is this better, or should we treat interpolation just like arithmetic operations that are inlined anywhere?

[nicolasstucki]

• We need to change the logic in typer to handle those methods as scala 3 macros (not scala 3)
  • https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/typer/Typer.scala#L3500-L3504
  • https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/typer/Typer.scala#L3494-L3497
• Make those intrinsic macros
  • https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/typer/Inliner.scala#L90-L95
  • https://github.com/lampepfl/dotty/blob/master/compiler/src/dotty/tools/dotc/typer/Inliner.scala#L308-L382

[SethTisue]

@soronpo shall we un-assign you at this point...?

[soronpo]

@soronpo shall we un-assign you at this point...?

Thank you Seth, just did.