provider functions can only return an error

[jbardin]

The call site for language functions doesn't currently have a way to handle complex diagnostics, so rather than appear to support them in the protocol we remove the concept of diagnostics for now. We do however retain the argument index field, which we can wrap in a function.ArgError to get a little more precise hcl diagnostic from the expression.

[apparentlymart]

It seems to me that this is essentially a one-way door because if we did want to add back support for arbitrary diagnostics later then we'd need to find some way to stuff at least the error ones into this error field for backward-compatibility with older Terraform Core versions that wouldn't know to look for the diagnostics. We'd also not be able to rely on everyone seeing any warnings that a function returns, which would limit the situations where it would be useful to return them.

Given that, I think we should just go all-in on this thing being shaped like a decision between plain error and cty/function.ArgError, and thus reduce this to just a single message and optional function_argument, rather than having it be sorta-kinda-diagnostic-shaped. 🤔

I would then expect the plugin framework to represent functions in a similar way to how cty/function does, where they return error and then it type-asserts for a specific error type to decide whether to populate that function_argument field.

[jbardin]

I don't think this is a one-way door per say, but I do think changing the function call site for all existing functions to handle diagnostics is much harder than re-extending this particular function implementation on its own. I agree that having this be a half-diagnostic doesn't really benefit us, so we might as well make it a simple error with optional function_argument.

[bflad]

We'll be pushing out the provider side changes to match this 👍

[github-actions]

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[github-actions]

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