For esbuild: We know other teams that have attempted to improve code splitting based on esbuild and found it very difficult. A big part of it is that in order to be fast, esbuild applies multiple features (bundle, treeshaking, transforms) in as few AST visits possible, but that comes at the cost of the logic of different features not being layered / decoupled nicely. It is difficult for anyone other than Evan Wallace himself to add non-trivial new mechanisms to esbuild, and although we didn't directly talk to Evan about this, we felt it would be too much to ask for a substantial refactor of esbuild in order to get what we need. In addition, the people interested in making Rolldown happen has much more experience in Rust than in Go - and there is a lot more to leverage (e.g. napi-rs & Oxc) in the Rust-for-JS ecosystem.For Rollup: the Rollup team itself has been trying to incrementally improve Rollup's performance, e.g. by swapping acorn with a Rust-based parser. But there's only so much you can gain starting from a pure JavaScript base, especially considering multicore utilization. Another aspect of the performance is in the back-and-forth between Rollup (on the JS side) and native transforms (swc, esbuild) - there is a lot of overhead repeatedly parsing / serializing ASTs and then passing strings across JS/native. By building on top of Oxc (which will ship transforms in the future) we hope to be able to stay on the native-side as much as possible to avoid such overhead.
catlifeonmars|2 years ago