ban instantiation of protobuf enum variant: `Unspecified`

[kwannoel]

So far, in our codebase, we don't use Unspecified variant of protobuf enums. However It is required by protobuf format: https://cloud.google.com/apis/design/design_patterns#enum_default_value.

It can be a source of bugs as seen in #8360.

We will have occurrences of it, since we need to handle protobuf messages. However should ban its instantiation, e.g.

let x = <MESSAGE>::Unspecified

We can do so via clippy.

[kwannoel]

Does not seem to have a straightforward way to ban in clippy. Have to probably write a rule for it. Please suggest other ideas.

Mark as low priority since it does not seem to be a big issue, if we see Unspecified in error message it will be easy to debug.

[xxchan]

For custom clippy lint you can check https://github.com/trailofbits/dylint

[github-actions]

This issue has been open for 60 days with no activity. Could you please update the status? Feel free to continue discussion or close as not planned.