Due to these tests happening in the wrong order, removing an object from
the end of a sequence of objects would previously cause a bounds-check
panic.
Rather than a more severe rework of the logic here, for now we'll just
introduce an extra precondition to prevent the panic. The code that
follows already handles the case where there _is_ no new object (i.e. the
"old" object is being deleted) as long as we're able to pass through this
type-checking logic.
The new "JSON list of objects - removing item" test covers this problem
by rendering a diff for an object being removed from the end of a list
of objects within a JSON value.
Our initial prototype of new-style diff rendering excluded this because
the old SDK has no support for this construct. However, we want to be able
to introduce this construct in the new SDK without breaking compatibility
with existing versions of Terraform Core, so we need to implement it now
so it's ready to be used once the SDK implements it.
The key associated with each block allows us to properly correlate the
items to recognize the difference between an in-place update of an
existing block and the addition/deletion of a block.
We use cty a little differently when a nested list block contains a
dynamically-typed attribute: it appears as a tuple value instead of a
list value so that we can retain the individual types of each element.
Here we introduce a test for that case, but doing so required also making
the runTestCases function handle types in a stricter way so that it will
produce planned values that match how Terraform Core would do it,
including the necessary late-bound type information for the
dynamically-typed attribute.
Martin Atkins