Experimenting with adding typestate constraints in the compiler.
Added a constraint to GEP_tag that says the variant index is in
bounds. Added necessary checks.
Upvars are now marked with def_upvar throughout, not just when going
through freevars::lookup_def. This makes things less error-prone. One
thing to watch out for is that def_upvar is used in `for each` bodies
too, when they refer to a local outside the body.
Having it in the alias pass was slightly more efficient (finding
expression roots has to be done in both passes), but further muddled
up the already complex alias checker.
Also factors out some duplication in the mutability-checking code.
Closes#868. Unfortunately, this causes certain invalid programs to
fail type-checking instead of failing type-state when a type-state
error message would probably be more intuitive. (Although, by any
reasonable interpretation of the static semantics, it technically
ought to be a type error.)
The uniformity doesn't seem to be worth the extra noise and pointless
code being generated. If something doesn't produce a value, don't make
it return one. (For now, trans_[exprtype] things are left in the result-
returning form, even when they never return anything useful, since in
that case uniformity is arguably helpful.)
Vectors are now similar to our old, pre-internal vectors, except that
they are uniquely owned, not refcounted.
Their name should probably change too, then. I've renamed them to vec
in the runtime, will do so throughout the compiler later.
The glue-calling will spill the values again anyway. This should
prevent a lot of load/spill junk in the output. It is also necessary
to be able to have unique vecs be immediate values (take must know the
actual address to be able to duplicate).
trans_be now has a precondition that its expression argument
is a call expr. Obviously this code may be going away soon, but
I wanted to exercise typestate somehow and this was an easy one :-)
