Annotate marker protocols with @_marker

[mjburghard]

Apple introduced the @_marker attribute to enforce a property at compile time without adding any runtime information.
Swift-protobuf contains two protocols, _MessageBase and ProtobufAPIVersion_3 , that fulfill the required restrictions and can be annotated with the attribute.
The main motivation for this change is the reduction in code size. In our case this result in a 80 kB decrease.

@_marker is prefixed with an underscore, so usage is discouraged in general. I still see a benefit adding the attribute to the two protocols. In the worst case, it needs to be removed when a future Swift version is released and the code size regresses.

More information about the semantics can be found here and here. The attribute was introduced as part of Swift 5.7.

[thomasvl]

Why not also in DEBUG builds?

[mjburghard]

They cannot inherit from non-marker protocols.

CustomDebugStringConvertible is not a marker protocol.

[FranzBusch]

I don't think we should add this for the general case since it uses an underscored attribute. This can change at any point in time and is not a stable feature.

[tbkka]

_MessageBase and the similar _AnyExtensionField protocol were added in #1246 to provide a way to control whether the main Message and AnyExtensionField protocols conform to CustomDebugStringConvertible. Is there another way to achieve this without adding those new intermediate protocols? (Not having those protocols at all would be even better than making them @_marker.)

[mjburghard]

I think I found a way that does not require @_marker, but want to verify that it achieves the same size reduction tomorrow morning.

[mjburghard]

I found a way that achieves the same without using @_marker. I close this PR. Thanks for your suggestions.

[tbkka]

Can you provide any details? I wonder if whatever you found is something we should be recommending to other people?

[thomasvl]

Can you provide any details? I wonder if whatever you found is something we should be recommending to other people?

I believe the answer is #1498.

[tbkka]

Ah. I see #1498 now.