[proposal] Auto magically inject services

[samber]

Linked to #9

This contribution adds support for a service declaration in struct tags. Such as:

type Car struct {
    Engine *Engine `do:""`
    Brand  string  `do:"awesome-brand"`
    Other  int
}

In this proposal, I suggest 3 new helpers:

• do.InjectTag(do.Injector, Service) (Service, error)
• do.MustInjectTag(do.Injector, Service) (Service)
• do.TagProvider[T any](t T) Provider[T]

This implementation is obviously unsafe since it uses reflect and unsafe std libs.

Example:

type Engine struct {
    Watt int
}

func NewEngine(i *do.Injector) (*Engine, error) {
    return &Engine{42}, nil
}

do.Provide(injector, NewEngine)

do.ProvideNamedValue(injector, "awesome-brand", "tesla")

type Car struct {
    Engine *Engine `do:""`
    Brand  string  `do:"awesome-brand"`
    Other  int
}

func NewCar(i *do.Injector) (*Car, error) {
    return do.InjectTag(i, &Car{})
}

do.Provide(injector, NewCar)
// could be replaced by do.Provide(injector, do.TagProvider(&Car{}))

IMO, we should avoid calling this helper automatically on service invocation. It would be error-prone and costly. Being declarative also allows the developer to execute do.InjectTag at the right time in the provider.

do.TagProvider would be a shortcut for very simple providers:

do.Provide(injector, do.TagProvider(&Car{}))

// is equivalent to:

do.Provide(injector, NewCar(i *do.Injector) (*Car, error) {
    return do.InjectTag(i, &Car{})
})

I would be very happy to receive feedback about this!

[codecov-commenter]

Codecov Report

Base: 89.56% // Head: 87.85% // Decreases project coverage by -1.71% :warning:

Coverage data is based on head (7a95bf7) compared to base (16a48d1). Patch coverage: 76.47% of modified lines in pull request are covered.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #28 +/- ## ========================================== - Coverage 89.56% 87.85% -1.71% ========================================== Files 6 7 +1 Lines 460 527 +67 ========================================== + Hits 412 463 +51 - Misses 40 52 +12 - Partials 8 12 +4 ``` | Flag | Coverage Δ | | |---|---|---| | unittests | `87.85% <76.47%> (-1.71%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/samber/do/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe) | Coverage Δ | | |---|---|---| | [tag.go](https://codecov.io/gh/samber/do/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#diff-dGFnLmdv) | `72.88% <72.88%> (ø)` | | | [service.go](https://codecov.io/gh/samber/do/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#diff-c2VydmljZS5nbw==) | `92.30% <100.00%> (+3.41%)` | :arrow_up: | | [service\_eager.go](https://codecov.io/gh/samber/do/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#diff-c2VydmljZV9lYWdlci5nbw==) | `76.00% <100.00%> (+2.08%)` | :arrow_up: | | [service\_lazy.go](https://codecov.io/gh/samber/do/pull/28?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe#diff-c2VydmljZV9sYXp5Lmdv) | `94.52% <100.00%> (+0.15%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Samuel+Berthe)

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[counterposition]

If I wanted to use Uber Fx, I'd just use Uber Fx. DI without reflection is one of the biggest advantages do has over Fx.

[NoGambiNoBugs]

@counterposition without reflection? Ok, very good, but uses assertion, and assertion is not safe type like this package says in readme.MD ("type safe").

https://github.com/samber/do/blob/master/injector.go#L57 https://github.com/samber/do/blob/master/injector.go#L177 https://github.com/samber/do/blob/master/injector.go#L201 https://github.com/samber/do/blob/master/di.go#L128

What is the problem to use reflect? The generics here is a fake, is just to generate names for a map[string]any.