The only contextual information returned by the plugins in a diagnostic
is an attribute path. In order to test the conversions, we need to be
able to extract the cty.Path from a diagnostic if it exists.
The usual usage of diagnostics requires us to pass around source location
information to everywhere that might generate a diagnostic, and that is
always the best way to get the most precise diagnostic source locations.
However, it's impractical to require source location information to be
retained in every Terraform subsystem, and so this new idea of "contextual
diagnostics" allows us to separate the generation of a diagnostic from
the resolution of its source location, instead resolving the location
information as a post-processing step once the call stack unwinds to a
place where there is enough context to find it.
This is necessarily a less precise approach than reading the source ranges
directly from the configuration AST, but gives us an alternative to no
diagnostics at all in portions of Terraform where full location
information is not available.
This is a best-effort sort of thing which will get as precise as it can
but may return a range in a parent block if the precise location of a
particular attribute cannot be found. Diagnostics that rely on this
mechanism should include some other contextual information in the detail
message to make up for any loss of precision that results.
Kristin Laemmert