[pull] main from hashicorp:main #18
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This new concept allows constraining the range of an unknown value beyond what can be captured in a type constraint. We'll make more use of this in subsequent commits.
If we encounter an interpolated unknown value during template rendering, we can report the partial buffer we've completed so far as the refined prefix of the resulting unknown value, which can then potentially allow downstream comparisons to produce a known false result instead of unknown if the prefix is sufficient to satisfy them.
When ConditionalExpr has an unknown predicate it can still often infer some refinement to the range of its result by noticing characteristics that the two results have in common. In all cases we can test if either result could be null and return a definitely-not-null unknown value if not. For two known numbers we can constrain the range to be between those two numbers. This is primarily aimed at the common case where the two possible results are zero and one, which significantly constrains the range. For two known collections of the same kind we can constrain the length to be between the two collection lengths. In these last two cases we can also sometimes collapse the unknown into a known value if the range gets reduced enough. For example, if choosing between two collections of the same length we might return a known collection of that length containing unknown elements, rather than an unknown collection.
If either the given value is refined non-null or if the default value is refined non-null then the final attribute value after defaults processing is also guaranteed non-null even if we don't yet know exactly what the value will be. This rule is pretty marginal on its own, but refining some types of value as non-null creates opportunities to deduce further information when the value is used under other operations later, such as collapsing an unknown but definitely not null list of a known length into a known list of that length containing unknown values.
We know that a splat expression can never produce a null result, and also in many cases we can use length refinements from the source collection to also refine the destination collection because we know that a splat expression produces exactly one result for each input element. This also allows us to be a little more precise in the case where the splat operator is projecting a non-list/set value into a zero or one element list and we know the source value isn't null. This refinement is a bit more marginal since it would be weird to apply the splat operator to a value already known to be non-null anyway, but the refinement might come from far away from the splat expression and so could still have useful downstream effects in some cases.
This new spec type allows adding value refinements to the results of some other spec, as long as the wrapped spec does indeed enforce the constraints described by the refinements.
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