When HCL encounters an error during expression evaluation, it annotates
its diagnostics with information about the expression that was being
evaluated and the EvalContext it was evaluated in.
This gives us enough information to show helpful hints to the user about
the final values of any reference expressions that are present in the
expression, which is very useful extra context for expressions that get
evaluated multiple times, such as:
- Any expression in a block with "count" or "for_each" set
- The sub-expressions within a "for" expression
If we get a diagnostic message that references a source range, and if the
source code for the referenced file is available, we'll show a snippet of
the source code with the source range highlighted.
At the moment we have no cache of source code, so in practice this
codepath can never be visited. Callers to format.Diagnostic will be
gradually updated in subsequent commits.
This new method showDiagnostics takes any value that would be accepted by
tfdiags.Append and renders it to the UI.
This is intended to encourage consistent handling of the different kinds
of errors and diagnostics that can be produced, and allow richer error
objects like the HCL2 diagnostics to be easily unwrapped and shown in
their full-fidelity.
Martin Atkins