Also, the store_borrow work in the previous patch caused some additional issues
to crop up. I fixed them in this PR and added some tests in the process.
This prevents another type of copy of noncopyable value error.
I also as a small change, changed the tuple version to use a formal access
temporary since we are projecting a component out implying that the lifetime of
the temporary must end within the formal access. Otherwise, we cause the
lifetime of the temporary to outlive the access. This can be seen in the change
to read_accessor.swift where we used to extend the lifetime of the destroy_addr
outside of the coroutine access we are performing.
We want these to be borrowed in most cases and to create an appropriate onion
wrapping. Since we are doing this in more cases now, we fix a bunch of cases
where we used to be forced to insert a copy since a coroutine or access would
end too early.
The reason why we are doing this is that this combination of read/set forces the
compiler to emit a copy if we want to emit a modify operation.
The reason why we are forced to emit such a copy is that:
1. _read provides a guaranteed value in memory
2. performing a modify requires an owned value in memory.
This together implies that the only way we can do this is to copy from the _read
into temporary memory. But we have a noncopyable type so we can't do this.
rdar://112915525
The reason why I am doing this is that this was not part of the original
evolution proposal (it was called an extension) and after some discussion it was
realized that partial consumption would benefit from discussion on the forums.
rdar://111353459
The form of the AST changes slightly when a type has a read and a modify.
Specifically, we now have a load on the subscript and an inout_expr on the base.
I dealt with this by making the inout_expr something that when we look for
storage we look through and by tweaking the load lookthrough code.
I also added a bunch of tests that showed the behavior of subscripts/other accessors with the following combinations of semantics:
1. get only.
2. get/set.
3. get/modify.
4. read/set.
5. read/modify.
rdar://109746476
