Add support for dynamic `<slot>` names

[nolanlawson]

Developers would like to be able to have dynamic <slot> names:

<!-- x-slottable -->
<template>
  <slot name={dynamic}></slot>
</template>

<!-- x-slotter -->
<template>
  <x-slottable>
    <div slot={dynamic}></slot>
  </x-slottable>
</template>

This is currently disallowed by our compiler (Error: LWC1076: Name attribute on slot tag can't be an expression.). But there's nothing in native shadow DOM that inherently disallows this; we could trivially require it there.

In fact, you can already achieve this in native shadow DOM by doing manual DOM manipulation, or cheating by using lwc:spread.

One question is whether we'd want to support this in synthetic shadow DOM (probably not) or light DOM (maybe).

[cardoso]

I tried implementing this and to my surprise it seems to work by simply removing the check:

if (ast.isExpression(nameAttribute.value)) {
    ctx.throwOnNode(ParserDiagnostics.NAME_ON_SLOT_CANNOT_BE_EXPRESSION, nameAttribute);
}

Am I missing something as I suspect? Should I submit a PR?

[nolanlawson]

Does it work if you run yarn && yarn test && yarn test:karma && DISABLE_SYNTHETIC=1 yarn test:karma? Curious whether it actually works across both synthetic and native shadow.

[cardoso]

@nolanlawson yes, it only fails the test for the NAME_ON_SLOT_CANNOT_BE_EXPRESSION error, which I believe would be updated / removed.

packages/@lwc/template-compiler/src/tests/fixtures/slots/error-definition-dynamic-name/expected.js

- Expected
+ Received

+ import { registerTemplate } from "lwc";
+ const stc0 = [];
+ function tmpl($api, $cmp, $slotset, $ctx) {
+   const { s: api_slot } = $api;
+   return [
+     api_slot(
+       "",
+       {
+         attrs: {
+           name: $cmp.slotName,
+         },
+         key: 0,
+       },
+       stc0,
+       $slotset
+     ),
+   ];
+   /*LWC compiler vX.X.X*/
+ }
+ export default registerTemplate(tmpl);
+ tmpl.slots = [""];
+ tmpl.stylesheets = [];

I'm guessing we would need a test for the actual feature and perhaps that would fail for non-native dom.