These will never appear in the source language, but can arise
after substitution when the original type is a tuple type with
a pack expansion type.
Two examples:
- original type: (Int, T...), substitution T := {}
- original type: (T...), substitution T := {Int}
We need to model these correctly to maintain invariants.
Callers that previously used to rely on TupleType::get()
returning a ParenType now explicitly check for the one-element
case instead.
IRGen only implements box lowering for single-field boxes at the moment.
We can represent closure contexts that don't capture type info as just
capturing a tuple of the values, so let's do that for now to allow
for initial end-to-end testing of the pass.
- Add a `[reflection]` bit to `alloc_box` instructions, to indicate that a box
should be allocated with reflection metadata attached.
- Add a `@captures_generics` attribute to SILLayouts, to indicate a type layout
that captures the generic arguments it's substituted with, meaning it can
recreate the generic environment without additional ABI-level arguments, like
a generic partial application can.
This will turn `partial_apply` instructions into explicit box construction and
extraction code sequences. To begin with, recognize when a private function
is only used in partial applications and directly modify the function to be
usable as a closure invocation function. This simplifies the lowering in IRGen
and avoids generating a "partial application forwarder" thunk.
