Better Support for Varargs in Inlining

[odersky]

Inlining cannot deal with varargs well, which means we are stopped in the starting block when trying to turn string interpolators into whitebox macros. For instance, consider the f interpolator. An interpolated string is expanded to:

    new StringContext(s_0, ..., s_n).f(e_1, ..., e_n)

Currently f is a whitebox macro which produces a specialized interpolator with some specific function type, such as

   (Int, Double, String) => String

How can we get analogous functionality with just inlining? Here's a sequence of action items that could help get us there:

• Turn f into an extension method. I.e.

  inline def f(this sc: StringContext)(args: Any*): String = 

Makes it easier to get at the string context. Requires #5114.

• Make varargs immutable arrays instead of Seqs (this just makes the next step easier). Requires Opaque types - selftype encoding #5300.

• Add rewrite rules that deal with array length and indexing. I.e.

     new Array(x_0, ..., x_n)(i)  -->   x_i
     new Array(x_0, ..., x_n).length = n + 1

This lets us get to the arguments passed in the parts array.

• Implement string parsing using string constant folding (#5406)
• Define a type and implicit instances from strings to the types they represent. Something like

    trait Format[S <: String & Singleton, T]

• Summon an implicit instance of Format for each suffix string.
• Somehow assemble a result type from this, which has the form ((T_1, ..., _T_n)) => String and cast the runtime implementation of f to this type.
• Apply an untuple operation (i.e. the inverse of tupled) to the result, yielding an n-ary method. This last step also needs to be supported with a native rewrite rule.

Or some other mix. In any case, to get off the ground we need to be able to recursively access a sequence of vararg arguments and synthesize a method of arbitrary arity. These require specialized support.

[nicolasstucki]

Now we can do all we can do this using the macro infrastructure.

• xml-interpolator
• f interpolator

This needs no extra language feature to be supported. Quotes and splices with quote pattern matching gets most of the work done.