feat!: return unit type instead of wkt::Empty

[coryan]

This is the normal mapping for wkt::Empty (or google.protobuf.Empty)
when used as a return type for a unary RPC.

Fixes #1509

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.72%. Comparing base (345cd34) to head (d3c8122).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1635   +/-   ##
=======================================
  Coverage   95.72%   95.72%           
=======================================
  Files          43       43           
  Lines        1825     1825           
=======================================
  Hits         1747     1747           
  Misses         78       78           

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[dbolduc]

Approved, but consider my suggestions :)

[dbolduc]

Maybe too cute, but consider:

        self.inner.execute(
            builder,
            {{#PathInfo.Codec.HasBody}}Some(req{{Codec.BodyAccessor}}){{/PathInfo.Codec.HasBody}}
            {{^PathInfo.Codec.HasBody}}None::<gaxi::http::NoBody>{{/PathInfo.Codec.HasBody}},
            options,
        ).await
        {{#ReturnsEmpty}}
        .map(|_| ())
        {{/ReturnsEmpty}}

[coryan]

Hmm... because the execute function is generic, we have to say .map(|_: wkt::Empty| ()) It is a bit weird. If I was typing the code that is not how I would do it, but it does express what we are doing.

[dbolduc]

optional: we could avoid all of the ReturnsEmpty blocks (except for the one in transport.rs) if we add a ReturnType to the message¹ codec, that is QualifiedName == "wkt::Empty" ? "()" : QualifiedName

up to you if you think that would be nicer.

Footnotes

1. It probably makes more sense on the method codec, though. ↩

[coryan]

Done.