Inline traits for specialization in Scala 3 (v2)

[nicolasstucki]

An alternative implementation of #17329. This implementation uses a two-step approach to inlining the definitions. The basic design is discussed in section 5.3.2 of https://infoscience.epfl.ch/record/303591?ln=en.

The idea is to inline a trait

inline trait A:
  def f(x1: T1, ...): R = ...

into

class B extends A
//  def f(x1: T1, ...): R = super[A].f(x1, ...)

The TraitInlining transform will generate the definition that are in A but not in B. The super[A].f(x1, ...) will be treated as an inline call in the Inlining phase.