This allows basic static validation of a traversal against a schema, to
verify that it represents a valid path through the structural parts of
the schema.
The main purpose of this is to produce better error messages (using our
knowledge of the schema) than we'd be able to achieve by just relying
on HCL expression evaluation errors. This is particularly important for
nested blocks because it may not be obvious whether one is represented
internally by a set or a list, and incorrect usage would otherwise produce
a confusing HCL-oriented error message.
We need to make the collection itself be a tuple or object rather than
list or map in this case, since otherwise all of the elements of the
collection are constrained to be of the same type and that isn't the
intent of a provider indicating that it accepts any type.
This produces a "proposed new state", which already has prior computed
values propagated into it (since that behavior is standard for all
resource types) but could be customized further by the provider to make
the "_planned_ new state".
In the process of implementing this it became clear that our configschema
DecoderSpec behavior is incorrect, since it's producing list values for
NestingList and map values for NestingMap. While that seems like it should
be right, we should actually be using tuple and object types respectively
to allow each block to have a different runtime type in situations where
an attribute is given the type cty.DynamicPseudoType. That's not fixed
here, and so without a further fix list and map blocks will panic here.
The DecoderSpec implementation will be fixed in a subsequent commit.
Any value that is not set in the configuration should decode as a Null
value. Unknown should still be returned if a computed value expression
is unknown.
The "config" package is no longer used and will be removed as part
of the 0.12 release cleanup. Since configschema is part of the
"new world" of configuration modelling, it makes more sense for
it to live as a subdirectory of the newer "configs" package.
Martin Atkins