Reuse a single TreeWalker in nested templates

[jridgewell]

Creating TreeWalkers isn't very optimized, so reusing a single one in the nested templates can speed it up a bit.

https://jsbench.github.io/#d0363ded1753dec3e060e61e1794896b

[justinfagnani]

I don't care that much about optimizing nested templates, since that should be a pretty rare case and we added support mainly for correctness so lit-html templates didn't blow up with nested templates...

But, if reusing a TreeWalker is that much faster, what if we reused one across the all templates? That could have an impact on real-world code (though I do think we're at the point where DOM mutation operations are dominating costs already).

The Template constructor isn't re-entrant. TemplateResult#_clone shouldn't be either, but I'd want some tests of scenarios like templates with custom elements that synchronously create ShadowRoots under the polyfill, etc.

[jridgewell]

But, if reusing a TreeWalker is that much faster, what if we reused one across the all templates? That could have an impact on real-world code

That was the end-goal. 😉

The Template constructor isn't re-entrant. TemplateResult#_clone shouldn't be either, but I'd want some tests of scenarios like templates with custom elements that synchronously create ShadowRoots under the polyfill, etc.

Unfortunately, we're not there yet. #831 has to come first, then we need to fix the reentrancy behavior in insertNodeIntoTemplate, which can fire upgrade/connectedCallback from the insertBefore.