Account for class only having template tag in no-empty-glimmer-component-classes rule

[chrisrng]

Fix the no-empty-glimmer-component-classes lint rule to avoid creating empty backing classes for Glimmer template tag only components.

The sample AST of the glimmer template tag is something like:

import Component from '@glimmer/component';
export default class Chris extends Component {
  [__GLIMMER_TEMPLATE(`...`, true)];
}

[chriskrycho]

👍🏼

[lin-ll]

Do you mind updating the docs for this rule as well? Changes look good

[bmish]

Do you think we should consider this a bug fix or a more substantial breaking change?

[chriskrycho]

I'd call it a bug fix – the semantics of a class with <template> in its body are identical to those of an empty class with a colocated .hbs file. And to be clear, I am not exaggerating for effect: they are literally identical.

[bmish]

Thanks!

[chriskrycho]

We noticed that this needs a follow-on: it should be legal to have template-only components in a class body for the specific case where you need the component to be generic over part of its signature. For example:

import Component from '@glimmer/component';

interface ListSignature<T> {
  Args: {
    items: Array<T>;
  };
  Blocks: {
    default: [item: T]
  };
}

export default class List<T> extends Component<ListSignature<T>> {
  <template>
    <ul>
      {{#each @items as |item|}}
        <li>
          {{yield item}}
        </li>
      {{/each}}
    </ul>
  </template>
}

While it is possible to write this without the backing class, it is… not great:

import { ComponentLike } from '@glint/template';

interface ListSignature<T> {
  // snip...
}

const List: abstract new <T>() => ComponentLike<ListSignature<T>> = <template>
  <ul>
    {{#each @items as |item|}}
      <li>
        {{yield item}}
      </li>
    {{/each}}
  </ul>
</template>;

export default List;

Accordingly, we should allow it in this one specific case.

(Unfortunately, you cannot even write a type utility to make that easier: the only place you're allowed to have a free-floating type parameter in the appropriate position for our needs is on a function definition, inclusive of constructor definitions, and for a host of reasons we need a constructor type specifically to make TS do the right things with our component types.)

[chrisrng]

#1876 PR to address ^