Add explicit keyword for some constructors in diplomat_runtime.hpp

[makotokato]

Mozilla has a static analysis for C++ code by own's clang-plugin. ExplicitImplicitChecker is a checker for implicit type conversion in constructor.

The constructors of diplomat::Ok, diplomat::Err, diplomat::result and diplomat::span has no explicit keyword. So it means that it can cause implicit type conversion in constructor.

So I would like to request to add explicit keyword for some class's constructors.

I guess that https://google.github.io/styleguide/cppguide.html#Explicit_Constructors will be same things.

[Manishearth]

I'm not opposed to some of these. Ok/Err should be explicit. result and span ... prbably not; it's an intentional choice there. result is supposed to be implicitly constructable from Ok/Err and span is supposed to also be implicitly constructible just like std::span is. The Google style guide you link to talks about interchangeablility as an example.

It would be nice if we could have a CI job that matched the settings Mozilla uses to build so we don't discover these things during integration.