Although addrs.Target can in principle capture the information we need to
represent move endpoints, it's semantically confusing because
addrs.Targetable uses addrs.Abs... types which are typically for absolute
addresses, but we were using them for relative addresses here.
We now have specialized address types for representing moves and probably
other things which have similar requirements later on. These types
largely communicate the same information in the end, but aim to do so in
a way that's explicit about which addresses are relative and which are
absolute, to make it less likely that we'd inadvertently misuse these
addresses.
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.
Martin Atkins