rule: no v-if on template root

[privatenumber]

Please describe what the rule should do:

Warn on using v-if on the template root node, because:

1. Whether a component should be rendered or not is a usage concern. Specifically, it's confusing when a declared component is not rendered:

   /* Usage */
   <template>
       <div>
           ...
           <custom-component /> /* Readers expect this to be rendered */
       </div>
   </template>
   
   
   /* CustomComponent.vue */
   <template>
       <div v-if="false">
           ...
       </div>
   </template>

2. There are performance benefits:

   1. Instantiating a new stateful component only for it to not be rendered via root v-if="false" is wasteful. If possible, it should happen in the parent usage life-cycle.
   2. When moving the v-if to the parent, the component can be asynchronously loaded when needed, so not only will it save on component registration, it will save on module declaration and asset size.

      <template>
          <div>
              ...
              <custom-component v-if="shouldShow" /> /* Won't be loaded till true */
          </div>
      </template>
      
      <script>
      export default {
      	components: {
              CustomComponent: () => import('./CustomComponent.vue')
          }
      }
      </script>

What category should the rule belong to?

[ ] Enforces code style (layout)
[ ] Warns about a potential error (problem)
[x] Suggests an alternate way of doing something (suggestion)
[ ] Other (please specify:)

Provide 2-3 code examples that this rule should warn about:

Bad

/* Usage */
<template>
    <div>
        <custom-component />
    </div>
</template>


/* CustomComponent.vue */
<template>
    <div v-if="shouldShow">
        ...
    </div>
</template>

Good

/* Usage */
<template>
    <div>
        <custom-component v-if="shouldShow" />
    </div>
</template>


/* CustomComponent.vue */
<template>
    <div>
        ...
    </div>
</template>

Additional context

• There are likely cases where it's not possible to do this. I think this would be a warning in "recommended", or not even a pre-enabled rule.

[ota-meshi]

Thank you for rule proposal!
I think it's good to make it rule available to users.

I think the following template needs to be valid.

<template>
    <div v-if="mode === 'a'">
        ...
    </div>
    <div v-if="mode === 'b'">
        ...
    </div>
</template>

<template>
    <div v-if="loading">
        ...
    </div>
    <div v-else>
        ...
    </div>
</template>

[privatenumber]

Yeah that's a great point. Good catch!

[chelorope]

I think that this is already covered by this rule vue/valid-template-root

[privatenumber]

How so?

[songpengyuan]

I would like to work on this @ota-meshi

[ota-meshi]

Yeah, thank you!