better exception message when rendering parent with an already rendered child about xhp-lib HOT 8 CLOSED

In my example I thought we'd just mention the double-rendered element's name (Can't render because a child (<br>) was already rendered) but the full path is nicer, I like that.

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I thought I had fixed that confusing error message in ac61647, but I guess not for nested cases. I'll see if there is a fix possible which generates such an error message, which doesn't have a heavy runtime cost.

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This may also require addressing in the migration guide, it seems that "Can't ... after render" errors are much more common in v4.

And yes, Can't X after render is new in XHP-4.

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I've also noticed that these errors often pop up when you try to do something sketchy during rendering:

use XHPHTMLHelpers;
protected async function renderAsync(): Awaitable<x\node> {
  $id = $this->getID();
  // ...
}

will give you "Can't addAttribute after render" if ID wasn't set previously.

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I've also noticed that these errors often pop up when you try to do something sketchy during rendering:

use XHPHTMLHelpers;
protected async function renderAsync(): Awaitable<x\node> {
  $id = $this->getID();
  // ...
}

will give you "Can't addAttribute after render" if ID wasn't set previously.

That is because ->__isRendered = true happens before ->renderAsync().

  protected async function __renderAndProcess(): Awaitable<node> {
    if ($this->__isRendered) {
      throw new UseAfterRenderException(
        'Attempted to render XHP element twice',
      );
    }
    $this->__isRendered = true; // < needs to be moved from here
    if (\Facebook\XHP\ChildValidation\is_enabled()) {
      $this->validateChildren();
    }

    $composed = await $this->renderAsync();
    // to here (1)
    $composed->__transferContext($this->getAllContexts());
    if ($this is HTML\HasXHPAttributeClobbering_DEPRECATED) {
      $this->transferAttributesToRenderedRoot($composed);
    }
    // or here (2)
    return $composed;
  }

@fredemmott We want to allow modification during ->renderAsync(), right?

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It would be better if it said something like Can't render because a child (<li>) was already rendered instead of Can't replaceChildren after render.

I can't quite match that, but this is already worlds better. #281

<div><ui:div><ui:div><span>{$br}</span></ui:div></ui:div></div>
//                          ^^^ rendered already

Attempted to render XHP element twice
Via XHPPath: HTML\\div -> ui\\div -> HTML\\div -> ui\\div -> HTML\\div -> HTML\\span -> HTML\\br.

XHPPath will only be included when a subnode throws during render.
If you are doing something to a node directly, the exception messages are unchanged.

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Fixed by #281

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Thanks!

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