Make AST mutations during a walk safer

[romainmenke]

PostCSS provides strong guarantees that modifying an AST during a walk is safe by storing the index of each active iteration and updating them as necessary when replacements are made. It would be really nice if this tool did so as well.

Originally posted by @nex3 in #1202 (comment)

[romainmenke]

Context :

PostCSS and PostCSS-like libs have a very user friendly API but to achieve this they have a bunch of internal complexity.

This is outside my area of expertise and I also don't care that much about the UX of the parser libs. The focus and priority has always been on correctness of tokenizing and parsing and being able to keep up with the specifications.

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We don't currently test for heavy mutations during AST walks.
We should at least record the current behavior and compare that to other tools.

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PostCSS :

What I see in the code is that they provide methods for insert and remove operations.
In these methods they adjust the indices of all current iterators.

So adding nodes before the current item increments the index.
And removing nodes after the current index decrements the index.

I don't want to do that.
The implementation in PostCSS is brittle and has caused many bugs.

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Todo :

• gather issues as failing tests
• define possible ways forward

[romainmenke]

define possible ways forward

I think I found a way to always determine a next good index even when a list is mutated, without having to provide convenience methods for all mutations.

I keep a reference to the previous state of the list and use that to look for a candidate of a next element.

This seems to work and it makes mutations safe.

[romainmenke]

@nex3 Patches for this issue have been released as part of 2.4.0 : https://github.com/csstools/postcss-plugins/blob/main/packages/css-parser-algorithms/CHANGELOG.md#240

Can you let us know if this resolves the bugs you encountered?

[romainmenke]

@nex3 going to close this for now.
Please let me know if you are still facing issues with this.