I've tried AST memory layout work in the Zig style, look for "shrinkage" in these two posts:

- https://nnethercote.github.io/2022/10/27/how-to-speed-up-the...

- https://nnethercote.github.io/2023/03/24/how-to-speed-up-the...

It's hard work. Small AST changes often require hundreds of changes to the code. The required changes usually make the AST less ergonomic to work with. And the perf benefits I obtained were very small. Even after shrinking `ast::Expr` (by far the most common AST node kind) from over 100 bytes to 64 bytes on 64-bit.

The linked Zig PR has very impressive reductions in walltime/cycles/etc. but if you read closely it's restricted just to parsing, which is usually a very small slice of compile time, at least for rustc. My experience with these kinds of changes was disappointing. I concluded "I’d love to be proven wrong, but it doesn’t feel like this work is taking place in a good part of the effort/benefit curve."