fix(removeHiddenElems): handle defs better

[KTibow]

Relocate logic to run in node.exit instead of checking in svg's exit:

• remove broken uses
  Changed from checking each use to making a list of uses, to fix Regression in 3.0.4: incorrectly removes reusable elements, but leaves references to it #1851
• remove unused defs
  Fixed a typo so it actually removes them, and add some safeguards
• remove empty defs
  Changed from checking each def to making a list of defs

[KTibow]

Okay so turns out that the unused definitions logic was just always broken, so I'm having to fix the existing implementation

[SethFalco]

Hey! Thanks for submitting a PR.

I've left a few comments, but the proposed fix makes sense. There are just a few issues with the implementation.

Can you please also add a test that reproduces the issue you're fixing.

[KTibow]

Now I'm running into the problem where it has a different behavior in multipass than singlepass, so it fails no matter what.
Context: in this

<svg xmlns="http://www.w3.org/2000/svg">
    <defs>
        <linearGradient id="a">
        </linearGradient>
    </defs>
</svg>

singlepass converts it to this, because the empty defs remover runs before the final pass

<svg xmlns="http://www.w3.org/2000/svg">
    <defs />
</svg>

but multipass converts it to this

<svg xmlns="http://www.w3.org/2000/svg" />

What's the most reasonable solution to this? Is there an elegant way to run the empty defs remover after the final pass?

[SethFalco]

Is there an elegant way to run the empty defs remover after the final pass?

Perhaps the best approach would be to take from here:

svgo/plugins/convertOneStopGradients.js

Line 36 in 3dc2f6f

const effectedDefs = new Set();

So whenever we remove a node from a <defs>, store the effected <defs> node in a Set.
Then in root you can iterate these and check if they're empty there.

(And drop the behavior from element#exit.)

You can put the logic to store a Set of <defs> here:

• https://github.com/svg/svgo/pull/1879/files#diff-49d33e65cfa6a5a38ff7d8248656645a92ea72527bd1ae3841555bd5cc69d823R92

[KTibow]

While there's more to address I'm off to lunch, I'll fix this soon

[SethFalco]

This looks good now! And thanks for resolving the other issues you found.
Just left a few very petty notes, but happy to merge overall.

[SethFalco]

Great! Thanks for resolving the issue, and improving the plugin in general.