By collecting information about the input variables during analysis, we
can return approximate type information for any references to those
variables in expressions.
Since Terraform 0.11 allowed maps of maps and lists of lists in certain
circumstances even though this was documented as forbidden, we
conservatively return collection types whose element types are unknown
here, which allows us to do shallow inference on them but will cause
us to get an incomplete result if any operations are performed on
elements of the list or map value.
Although we can't do fully-precise type inference with access only to a
single module's configuration, we can do some approximate inference using
some clues within the module along with our resource type schemas.
This depends on HCL's ability to pass through type information even if the
input values are unknown, mapping our partial input type information into
partial output type information by evaluating the same expressions.
This will allow us to do some upgrades that require dynamic analysis to
fully decide, by giving us three outcomes: needed, not needed, or unknown.
If it's unknown then that'll be our prompt to emit a warning for the user
to make a decision.
Martin Atkins