HCL diagnostics relating to expression evaluation include these, so this
is really just to pass that information through our Terraform-specific
diagnostics interface.
cty doesn't have a native string representation of a cty.Path because it
is the layer below any particular syntax, but up here in Terraform land
we know that we use HCL native syntax and so we can format a string in
a HCL-ish way for familiarity to the user.
We'll use this form automatically when such an error is used directly as a
diagnostic, but we also expose the function publicly so that other code
that incorporates errors into diagnostic detail strings can apply the
same formatting.
Currently we lean heavily on the Go error type as our primary means of
describing errors, and along with that use several more specialized
implementations of it in different spots for additional capabilities such
as multiple errors in one object, source code range references, etc.
We also have a rather ad-hoc approach of returning an array of warnings
from certain functions along with one or multiple errors.
This rather-disorganized approach makes it hard for us to present
user-facing error messages consistently. As a step towards mitigating
this, package tfdiags provides a model for user-facing error and warning
messages and helper functions for creating them from various other
error and warning types used elsewhere in Terraform.
This mechanism is intended to be used to report errors and warnings where
the audience is the Terraform user, and so it may go a few layers deep
down the call stack into codepaths like config parsing, interpolation, etc
but is primarily a UX concern. The deepest reaches of Terraform core will
continue using "error" as normal, with higher layers preparing error
messages for presentation to the user.
To avoid needing to change the interface of every function that might
generate error diagnostics, the Diagnostics type can be "smuggled" via
an error value through other APIs and then unwrapped at the other end,
though it will lose any naked warnings (without at least one error) along
the way, and so codepaths that are expected to generate warnings
(validation, primarily) should use the concrete Diagnostics type
throughout the call chain.
Martin Atkins