These three types represent the three different address representations we
need to represent different stages of analysis for "moved" blocks in the
configuration.
The goal here is to encapsulate all of the static address wrangling inside
these types so that users of these types elsewhere would have to work
pretty hard to use them incorrectly.
In particular, the MovableEndpoint type intentionally fully encapsulates
the weird relative addresses we use in configuration so that code
elsewhere in Terraform can never end up holding an address of a type that
suggests absolute when it's actually relative. That situation only occurs
in the internals of MoveableEndpoint where we use not-really-absolute
AbsMoveable address types to represent the not-yet-resolved relative
addresses.
This only takes care of the static address wrangling. There's lots of
other rules for what makes a "moved" block valid which will need to be
checked elsewhere because they require more context than just the content
of the address itself.
Our documentation for ModuleCall originally asserted that we didn't need
AbsModuleCall because ModuleInstance captured the same information, but
when we added count and for_each for modules we introduced
ModuleCallInstance to represent a reference to an instance of a local
module call, and now _that_ is the type whose absolute equivalent is
ModuleInstance.
We previously had no absolute representation of the call itself, without
any particular instance. That's what AbsModuleCall now represents,
allowing us to be explicit about when we're talking about the module block
vs. instances it declares, which is the same distinction represented by
AbsResource vs. AbsResourceInstance.
Just like with AbsResource and AbsResourceInstance though, there is
syntactic ambiguity between a no-key call instance and a whole module call,
and so some codepaths might accept both to start and then use other
context to dynamically choose a particular interpretation, in which case
this distinction becomes meaningful in representing the result of that
decision.
The previous name didn't fit with the naming scheme for addrs types:
The "Abs" prefix typically means that it's an addrs.ModuleInstance
combined with whatever type name appears after "Abs", but this is instead
a ModuleCallOutput combined with an InstanceKey, albeit structured the
other way around for convenience, and so the expected name for this would
be the suffix "Instance".
We don't have an "Abs" type corresponding with this one because it would
represent no additional information than AbsOutputValue.
This is part of a general effort to move all of Terraform's non-library
package surface under internal in order to reinforce that these are for
internal use within Terraform only.
If you were previously importing packages under this prefix into an
external codebase, you could pin to an earlier release tag as an interim
solution until you've make a plan to achieve the same functionality some
other way.
Martin Atkins