Error messages for preconditions, postconditions, and custom variable
validations have until now been string literals. This commit changes
this to treat the field as an HCL expression, which must evaluate to a
string. Most commonly this will either be a string literal or a template
expression.
When the check rule condition is evaluated, we also evaluate the error
message. This means that the error message should always evaluate to a
string value, even if the condition passes. If it does not, this will
result in an error diagnostic.
If the condition fails, and the error message also fails to evaluate, we
fall back to a default error message. This means that the check rule
failure will still be reported, alongside diagnostics explaining why the
custom error message failed to render.
As part of this change, we also necessarily remove the heuristic about
the error message format. This guidance can be readded in future as part
of a configuration hint system.
If a resource or output value has a precondition or postcondition rule
then anything the condition depends on is a dependency of the object,
because the condition rules will be evaluated as part of visiting the
relevant graph node.
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.
Alisdair McDiarmid