JuliaLang / julia / #37776

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Move nargs/isva to CodeInfo (#54341)

This changes the canonical source of truth for va handling from `Method`
to `CodeInfo`. There are multiple goals for this change:

1. This addresses a longstanding complaint about the way that
CodeInfo-returning generated functions work. Previously, the va-ness or
not of the returned CodeInfo always had to match that of the generator.
For Cassette-like transforms that generally have one big generator
function that is varargs (while then looking up lowered code that is not
varargs), this could become quite annoying. It's possible to workaround,
but there is really no good reason to tie the two together. As we
observed when we implemented OpaqueClosures, the vararg-ness of the
signature and the `vararg arguments`->`tuple` transformation are mostly
independent concepts. With this PR, generated functions can return
CodeInfos with whatever combination of nargs/isva is convenient.

2. This change requires clarifying where the va processing boundary is
in inference. #54076 was already moving in that direction for irinterp,
and this essentially does much of the same for regular inference. As a
consequence the constprop cache is now using non-va-cooked signatures,
which I think is preferable.

3. This further decouples codegen from the presence of a `Method` (which
is already not assumed, since the code being generated could be a
toplevel thunk, but some codegen features are only available to things
that come from Methods). There are a number of upcoming features that
will require codegen of things that are not quite method specializations
(See design doc linked in #52797 and things like #50641). This helps
pave the road for that.

4. I've previously considered expanding the kinds of vararg signatures
that can be described (see e.g. #53851), which also requires a
decoupling of the signature and ast notions of vararg. This again lays
the groundwork for that, although I have no immediate plans... (continued)

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1301 existing lines in 49 files now uncovered.

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