Change how Injected structs are tagged

[Hiram Chirino]

Currently injected structs resolve using tag constraint like so:

type Parameters struct {
	di.Inject
	
	// use tag for the container to know that field need to be injected.
	Leader   *DatabaseConfig `type:"leader"`
	Follower *DatabaseConfig `type:"follower"`
}

But things start to break down if that same struct is used in other ways, for example marshalling:

type Parameters struct {
	di.Inject
	
	// use tag for the container to know that field need to be injected.
	Leader   *DatabaseConfig `type:"leader"   json:"leader"`
	Follower *DatabaseConfig `type:"follower" json:"follower"`
}

To avoid conflicting with other libs that also use tags to provide features on structs, I propose that we change how goava/di looks for struct tags that look like this instead:

type Parameters struct {
	di.Inject
	
	// use tag for the container to know that field need to be injected.
	Leader   *DatabaseConfig `di:"type=leader"`
	Follower *DatabaseConfig `di:"type=follower"`
}

[Dmitrii Makeev]

Hi @chirino! This change looks rational.
But since we use this library in most of our services, we don't want to lose backward compatibility.
Maybe it can be implemented like this: first we look for a di: tag, and if we find it, we will use comma-separated key=value pairs from it as our di tags (e.g.: di:"type=leader,foo=bar"). If no di: tag is present on the field, we will parse struct tags like before.

[Hiram Chirino]

Sounds good to me. Should we print warning messages when the old style is found to encourage users to convert?

[Dmitrii Makeev]

I think we should, using global tracer for messages.
Also there are skip and optional tags, do you think we should move them also inside di: tag or leave them where they are?

[Hiram Chirino]

Yes, and perhaps to avoid mixing them up with user defined tags (which have a = in them) it should just be:
di:"skip" or di:"optional"