helper/schema: Clarify Deprecated Field Usage

[bflad]

SDK version

2.19.0

Use-cases

Currently, the helper/schema.Schema type Deprecated field is a message that gets translated into a warning diagnostic during Terraform's validation phase:

Warning: Argument is deprecated

{Deprecated field string}

This handling however, due to limitations in the protocol, only occurs for Required or Optional attributes when they have a known configuration value. There is currently no effect when the attribute is Computed-only (read-only). The SDK 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 Deprecated and only Computed via schema.InternalValidate(), 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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