An earlier commit today reworked this to handle non-fatal errors, which
are returned "smuggled" as a special type of error to avoid changing the
EvalNode interface.
Unfortunately, that change then broke the _other_ special thing we smuggle
through the error return path: early exit.
Now we'll handle them both. This is not perfect because the early-exit
path causes us to discard any warnings we've already collected, but it's
more important that we bail early than retain warnings.
To avoid a massively-disruptive change to how EvalNode works, we're now
"smuggling" warnings through the error return value for these, but this
depends on all of the Eval machinery correctly handling this special case
and continuing evaluation when only warnings are returned.
Previous changes missed EvalSequence as a place where execution halts on
error. Now it will accumulate diagnostics itself, aborting if any of
them are error diagnostics, and then wrap its own result up in an error
to be returned by the main Eval function, which already treats non-fatal
errors as a special case, though now produces an explicit log message
about that situation to make it easier to spot in trace logs.
This also includes a more detailed warning message for the warning about
provider input being disabled. While this warning should be removed before
we release anyway, having this additional detail is helpful in debugging
tests where it's being returned.
This disables the computed value check for `count` during the validation
pass. This enables partial support for #3888 or #1497: as long as the
value is non-computed during the plan, complex values will work in
counts.
**Notably, this allows data source values to be present in counts!**
The "count" value can be disabled during validation safely because we
can treat it as if any field that uses `count.index` is computed for
validation. We then validate a single instance (as if `count = 1`) just
to make sure all required fields are set.
