tfsdk: Clarify DeprecationMessage Field Usage

[bflad]

Module version

0.10.0

Use-cases

Currently, the tfsdk.Attribute type DeprecationMessage field is a message that gets translated into a warning diagnostic during Terraform's validation phase:

Warning: Attribute Deprecated

{DeprecationMessage field string}

This handling however, due to limitations in the protocol, only occurs for Required or Optional attributes when they have a configuration value (known or unknown in the case of the framework). There is currently no effect when the attribute is Computed-only (read-only). The framework already enables the deprecated boolean field available in the protocol, so if/when Terraform core does something with that protocol field in a future version, it'll have an effect on read-only attributes then.

Proposal

Update the Go documentation and website documentation to ensure it denotes this limitation. We could go further and begin failing unit testing for attributes with DeprecationMessage and only Computed via #113, however that feature can be treated separately, if desired.

References

• core: Warn about deprecated fields when they get referenced terraform#7569

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