Ivy: DI decorators tree-shaking when they are used to configure useFactory deps

[AndrewKushnir]

Angular DI allows providing DI tokens using useFactory function, which can accepts arguments configured via deps array.

Here is a quick example:

@Component({
    selector: 'comp',
    template: '...',
    viewProviders: [{
        provide: 'B',
        deps: [[new Inject('A'), new SkipSelf()]],
        useFactory: (token: string) => token,
    }]
})
class Comp {}

The underlying logic that constructs these args refers to DI decorators (such as SkipSelf, Self, etc) explicitly, thus making them non-tree-shakable:

angular/packages/core/src/di/injector_compatibility.ts

Lines 145 to 159 in 2e7727c

	for (let j = 0; j < arg.length; j++) {
	const meta = arg[j];
	if (meta instanceof Optional || meta.ngMetadataName === 'Optional' || meta === Optional) {
	flags |= InjectFlags.Optional;
	} else if (
	meta instanceof SkipSelf || meta.ngMetadataName === 'SkipSelf' || meta === SkipSelf) {
	flags |= InjectFlags.SkipSelf;
	} else if (meta instanceof Self || meta.ngMetadataName === 'Self' || meta === Self) {
	flags |= InjectFlags.Self;
	} else if (meta instanceof Inject || meta === Inject) {
	type = meta.token;
	} else {
	type = meta;
	}
	}

Instead, we should consider using an approach of patching a field (like __NG_DI_FLAG___) onto a class and update the mentioned logic to check for this flag instead.

      for (let j = 0; j < arg.length; j++) {
        const meta = arg[j];
        const metaFlags = meta.__NG_DI_FLAG__;
        if (flags === undefined) {
          type = meta;
        } else if (typeof metaFlags === 'number') {
          flags |= metaFlags;
        } else {
          // `meta instanceof Inject || meta === Inject` case:
          type = meta.token;
        }
      }

Important:

• this should not cause and runtime perf issues, since we already have a megamorphic calls (meta.ngMetadataName), the new approach will reduce the number of calls from 4 to 1.
• we should verify that skipping the meta.ngMetadataName check here is critical and break any existing use-cases.
• the __NG_DI_FLAG__ flag should be present on both class definition and class instance:

Inject.__NG_DI_FLAG__ = ...;
(new Inject()).__NG_DI_FAG__ = ...;

As a result, DI decorators should only be present in the bundle when they are used explicitly in the app code.

[AndrewKushnir]

Related PR that triggered investigation - #40122.

[angular-automatic-lock-bot]

This issue has been automatically locked due to inactivity.
Please file a new issue if you are encountering a similar or related problem.

Read more about our automatic conversation locking policy.

_{This action has been performed automatically by a bot.}