This is the hcldec interface to Body.JustAttributes, producing a map whose
keys are the child attribute names and whose values are the results of
evaluating those expressions.
We can't just expose a JustAttributes-style spec directly here because
it's not really compatible with how hcldec thinks about things, but we
can expose a spec that decodes a specific child block because that can
then compose properly with other specs at the same level without
interfering with their operation.
The primary use for this is to allow the use of the block syntax to define
a map:
dynamic_stuff {
foo = "bar"
}
JustAttributes is normally used in static analysis situations such as
enumerating the contents of a block to decide what to include in the
final EvalContext. That's not really possible with the hcldec model
because both structural decoding and expression evaluation happen
together. Therefore the use of this is pretty limited: it's useful if you
want to be compatible with an existing format based on legacy HCL where a
map was conventionally defined using block syntax, relying on the fact
that HCL did not make a strong distinction between attribute and block
syntax.
Previously this implementation was doing only one level of recursion in
its walk, which gave the appearance of working until the
transform/container-type specs (DefaultSpec, TransformSpec, ...) were
introduced, creating the possibility of "same body children" being more
than one level away from the initial spec.
It's still correct to only process the schema and content once, because
ImpliedSchema is already collecting all of the requirements from the
"same body children", and so our content object will include everything
that the nested specs should need to analyze needed variables.
This is a super-invasive update since the "zcl" package in particular
is referenced all over.
There are probably still a few zcl references hanging around in comments,
etc but this takes care of most of it.
The main "zcl" package requires a bit more care because of how many
callers it has and because of its two subpackages, so we'll take care
of that one separately.
