I mean, I don't want to speak for the esbuild author, but he did mention that his rust implementation ran into a number of issues[0], one of them being related to making lifetime stuff happy with concurrency. Judging from other recent rust threads, I'm not super convinced that "easy" is necessarily a fair representation of how it feels to do not-quite-bog-standard-functional-stuff w/ rust.

[0] https://news.ycombinator.com/item?id=22336284