perf: use walk_readonly for analysis-phase AST walks

[MathiasWP]

Summary

• Switch all read-only AST walks in the compiler's analysis phase from zimmerframe's walk() to walk_readonly()
• Update type aliases in types.d.ts to use ReadonlyContext/ReadonlyVisitors, propagating correct types to all 63 visitor files without touching them
• Update direct zimmerframe type imports in files that don't use the shared type aliases (css-analyze.js, css-warn.js, scope.js, migrate/index.js)

Motivation

walk() creates per-node mutation tracking overhead (a mutations object, Object.keys checks, apply_mutations calls, and an object spread for the universal visitor) that is unnecessary for analysis-phase walks where visitors never return replacement nodes. walk_readonly() eliminates all of this.

Benchmark results

Compiled 7 diverse test components (ranging from 32 to 371 lines, including CSS-heavy, a11y, and template-heavy components) x 200 iterations each, trimmed mean of 4 runs:

Branch	Median compile time (7 components)
main	23.54 ms
perf/use-walk-readonly	22.37 ms
Improvement	~5% faster

The raw walk_readonly vs walk speedup in isolation is ~20% (benchmarked in sveltejs/zimmerframe#33), but the end-to-end compiler improvement is ~5% because AST walking is one part of the total compile pipeline (which includes parsing via acorn, transformation, and code generation via esrap).

At scale (2000 components): A project compiling 2000 components that takes ~47s would save ~2.3s. The savings grow linearly with project size.

Benchmark methodology

• 50 warmup iterations to stabilize V8 JIT
• 200 measured iterations per run
• Top/bottom 10% trimmed for stability
• Median of 4 independent runs reported
• Components used: a11y-role-supports-aria-props (371 lines), css/has (179 lines), siblings-combinator-each-nested (113 lines), select-with-rich-content (157 lines), form-default-value (197 lines), skip-static-subtree (49 lines), App.svelte (32 lines)

Files changed (9 files, 24 walk calls)

File	walk() calls switched
phases/2-analyze/index.js	7
phases/2-analyze/css/css-prune.js	6
phases/2-analyze/css/css-analyze.js	3
phases/scope.js	2
compiler/utils/ast.js	2
compiler/migrate/index.js	2
phases/2-analyze/css/css-warn.js	1
phases/2-analyze/visitors/shared/a11y/index.js	1
phases/2-analyze/types.d.ts	Type alias update

Dependencies

Depends on sveltejs/zimmerframe#33 which adds the walk_readonly export to zimmerframe.

Test plan

• All 7329 existing tests pass — the full test suite validates compiler output end-to-end, covering every code path touched by this change
• No behavioral change — walk_readonly has identical traversal semantics to walk for read-only visitors

🤖 Generated with Claude Code

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Latest commit: 45ea32b

The changes in this PR will be included in the next version bump.

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[vercel]

PR imports non-existent walk_readonly, ReadonlyContext, and ReadonlyVisitors from zimmerframe, causing build failure

[MathiasWP]

I know this isn’t the biggest impact, i’m still exploring and trying to understand how to attack perfomance improvements. Not sure if it’s best to to a lot of small improvements, or try to find fewer large improvements that may be more difficult to implement. I’ll keep investigating.

[gterras]

It's kind of scary to see LLM generated code of this magnitude making its way into the codebase lately like it's nothing. It's a common flaw of these tools to make changes that looks good in isolation before slowly turning into nonsense when accumulated.

[MathiasWP]

It's kind of scary to see LLM generated code of this magnitude making its way into the codebase lately like it's nothing. It's a common flaw of these tools to make changes that looks good in isolation before slowly turning into nonsense when accumulated.

But is it though? I have been supervising the code Claude has written in my PR's, and i cannot really see anything with this code that will turn into nonsense. Making large, unstructured changes just "because" is one thing, but improving isolated parts of the code in a way that improves it isn't scary. That's simply writing code. Especially when the tests back it up.

[gterras]

Note that this comment isn't specifically towards you or this PR but more general to the repo, this is a subject that probably would benefit from an official statement from the team.

But is it though? I have been supervising the code Claude has written in my PR's, and i cannot really see anything with this code that will turn into nonsense.

Yes that's exactly my point. It looks good, then it looks good again, then you generally trust the output, then you start reviewing like it's been written by a experienced coworker, then there's these 5 lines that looks slightly off but hey that might just be you, then after a hundred PRs something is truly wrong. That's a pattern so many people are finding these days.

That's simply writing code.

No this is reviewing code, written by something that works differently than a human brain in a radical manner (it never gets the full picture and is about efficiency before everything else). Eventually you forget this and start seeing it as a human, because it's more convenient this way. In review stance your brain is also more passive than in write code stance so it's really not the same thing as "writing code". Humans make mistake too but at least they can explain why.

[MathiasWP]

Note that this comment isn't specifically towards you or this PR but more general to the repo, this is a subject that probably would benefit from an official statement from the team.

But is it though? I have been supervising the code Claude has written in my PR's, and i cannot really see anything with this code that will turn into nonsense.

Yes that's exactly my point. It looks good, then it looks good again, then you generally trust the output, then you start reviewing like it's been written by a experienced coworker, then there's these 5 lines that looks slightly off but hey that might just be you, then after a hundred PRs something is truly wrong. That's a pattern so many people are finding these days.

That's simply writing code.

No this is reviewing code, written by something that works differently than a human brain in a radical manner (it never gets the full picture and is about efficiency before everything else). Eventually you forget this and start seeing it as a human, because it's more convenient this way. In review stance your brain is also more passive than in write code stance so it's really not the same thing as "writing code". Humans make mistake too but at least they can explain why.

IMO most open source code is the manifestation of then there's these 5 lines that looks slightly off but hey that might just be you, then after a hundred PRs something is truly wrong. I've yet to find an amazingly clean, SOLID open source code-base, and even if it exists, you'll find a hundred people arguing it's not - because code is personal.

So my point here is, i don't think the issue you're addressing here purely stems from LLM's writing code. This stuff happened a lot pre-LLM as well. So why be scared? And also, it's bold to claim that code written with LLM's is making its way into the codebase lately like it's nothing. I believe the maintainers are doing a good job accepting code they believe is valid.

And also a personal note: for me it's the outcome that truly matters, and tests are the guardrails that allow fast momentum while keeping focus on making stuff that works, even if the code isn't always the prettiest, which is something that happened frequently before LLM's became a part of our toolkit. But, i still agree that code should be kept clean, but the best AI models are pretty goddamn good at writing great code.

[gterras]

So my point here is, i don't think the issue you're addressing here purely stems from LLM's writing code. This stuff happened a lot pre-LLM as well. So why be scared? And also, it's bold to claim that code written with LLM's is making its way into the codebase lately like it's nothing. I believe the maintainers are doing a good job accepting code they believe is valid.

LLM generated code was introduced in the repo like it's nothing, and by nothing I mean like it was a contributor like the others, which is not the case hence my point. I don't doubt the maintainers review skills, I just point out a subtle but painful drawback that can manifest itself overtime.

I ask specifically in this repo because it's complex enough that no LLMs are able to understand it as an experienced maintainer would AND it came to a point where even maintainers have trouble guaranteeing that even a small change won't break something in a distant place.

So what I'm scared about (if you will) is a slight decrease in reliability while it already needs the opposite at this time, that minor updating Svelte is scary today might and get scarier over time.

and tests are the guardrails

Tests only test what they test, and don't test what they don't test. LLMs by nature also tend to accommodate them so they work, and they are usually less accurately reviewed than code so it may create a false sensation of security (cf my point). LLMs generated tests need extra reviewing focus that most people (somewhat rightfully) won't give. They do have a crucial role to play in test generation but this requires a much more rigorous approach and a layer of tooling that don't exist yet.

One thing to note is that LLMs just replicate what they've been trained on so they will only output an average of what already exists elsewhere. They will never think outside the box (which is where Svelte comes from) and never replace a skilled maintainer for core tasks in a complex system, but their ability to act like so is something to be aware of.