perf(ast): re-order struct fields to reduce padding

[overlookmotel]

All our AST structs are #[repr(C)], which gives us guaranteed stable memory layouts. This is essential for raw transfer, and helpful for other purposes too.

However, the downside is that we lose out on the automatic field-reordering that default #[repr(Rust)] provides, where structs are packed to minimize padding bytes.

This PR implements a similar field-reordering scheme, to get the same effect.

• oxc_ast_tools determines optimal field order for compact memory layout.
• oxc_ast_tools generates a file (structs.rs) which details the desired field order for each type.
• #[ast] proc macro follows the instructions in structs.rs and re-orders the fields.

This reduces the size of various AST types by 8 bytes. No type now has more than 7 padding bytes (previously e.g. Function had 15 bytes of padding).

Possible future improvements:

1. Get Rust Analyser to display original field order.
2. Remove syn dependency from oxc_ast_macros crate, to improve compile times. It should be possible to do all the AST manipulation using just proc_macro crate, by generating more code in oxc_ast_tools, so the macro's logic can be simpler.

Note: This PR doesn't add syn dependency, it was already there. But it makes it easier to remove it.

[overlookmotel]

• perf(ast): re-order struct fields to reduce padding #11056 👈 (View in Graphite)
• main

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[codspeed-hq]

CodSpeed Instrumentation Performance Report

Merging #11056 will not alter performance

_{Comparing 05-15-perf_ast_re-order_struct_fields_to_reduce_padding (2b0a69f) with main (5dcd0f1)}

Summary

✅ 36 untouched benchmarks

[overlookmotel]

Perf impact is much less than I'd hoped. Only sub-% movement on parser benchmarks, and 1% on a couple of transformer/minifier benchmarks. Meh!

Still, why not? It's crazy to be using memory for padding bytes. And now we don't risk perf regressions if we rearrange fields or add more fields in future.

There is one quite large annoyance. Rustdoc seems to work from the types after proc macros have run, so now the fields in docs for oxc_ast crate are no longer listed in original order, but in memory layout order. I'll see if I can fix that by injecting #[cfg(doc)] attributes.

[overlookmotel]

The Rustdoc issue is now fixed. Rustdoc now shows struct fields in original source order.

[Boshen]

Merge activity

• May 15, 11:42 PM EDT: The merge label '0-merge' was detected. This PR will be added to the Graphite merge queue once it meets the requirements.
• May 15, 11:42 PM EDT: Boshen added this pull request to the Graphite merge queue.
• May 15, 11:50 PM EDT: Merged by the Graphite merge queue.