RFC: embedded string interpolations

This issue is a follow-up from https://github.com/http4s/http4s/issues/3433.
I've struggled with this a bit and think it's quite problematic to embed string interpolations to uri"" and path"" of http4s (actually, it's so for every interpolated macros based on literally):

1. At the moment, compilation fails if invalid strings have been passed (underlying validate is calling at compile time).
2. Say

val foo = "/foo/bar"
val someuri = uri"https://example.com$foo"

means that we should check the value of foo at the runtime (generally, foo could be some computation, not a constant value). So bringing embedded string interpolations means we should proceed with runtime values at compile time (because of 1). I'm not that much a Scala macros astronaut, but am feeling it's probably impossible.

UPD: speaking more accurately, compile-time checking is only possible for the literal part of the passed string.

If anyone has insights about that, please share them here.

Start by reading the linked http4s issue and the discussion of uri"", path"", literally, and validate. The issue does not identify files, tests, or a settled implementation; a contribution would first need to establish whether runtime interpolation with compile-time validation of literal parts is feasible and define the intended scope.