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.
This function returns the type of value that should be returned when
decoding the given spec. As well as being generally useful to the caller
for book-keeping purposes, this also allows us to return correct type
information when we are returning null and empty values, where before we
were leaning a little too much on cty.DynamicPseudoType.
A BlockLabelSpec can be placed in the nested spec structure of one of the
block specs to require and obtain labels on that block.
This is a more generic methodology than BlockMapSpec since it allows the
result to be a list or set with the labels inside the values, rather than
forcing all the label tuples to be unique and losing the ordering by
collapsing into a map structure.
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.
