During the plan operation we need to retain _somewhere_ the planned
changes for all outputs so we can refer to them during expression
evaluation. For consistency with how we handle resource instance changes,
we'll keep them in the plan so we can properly retain unknown values,
which cannot be written to state.
As with output values in the state, only root output plans are retained
in a round-trip through the on-disk plan file format, but that's okay
because we can trivially re-calculate all of these during apply. We
include the _root_ outputs in the plan file only because they are
externally-visible side effects that ought to be included in any rendering
of the plan made from the plan file for user inspection.
We were previously tracking this as a []cty.Path, but having it turned
into a pathset on creation makes downstream use of it more convenient and
ensures that it'll obey expected invariants like not containing the same
path twice.
This allows a provider to retain arbitrary extra data in the plan and
make use of it during apply. The contents are not used by Terraform and
never shown to the user.
The types here were originally written to allow us to defer decoding of
object values until schemas are available, but it turns out that this was
forcing us to defer decoding longer than necessary and potentially decode
the same value multiple times.
To avoid this, we create pairs of types to represent the encoded and
decoded versions and methods for moving between them. These types are
identical to one another apart from how the dynamic values are
represented.
Kazuki Higashiguchi