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Turns out the TokenStream implementation of rust-analyzer's proc-macro server is super slow when it comes to one of the most common proc-macro operations: concatenation. I noticed that in Zed's codebase, there were ~4 derives that just straight up together took 8 seconds to expand in rust-analyzer.

All that proc-macro was doing though is `quote!`ing a bunch, not much more. So I spent the day rewriting the implementation of TokenStream used by the server to make use of an immutable shared tree instead to get super cheap concatenations.

And sure enough, this changed the 8 second expansion time into a 100ms! A bit more info can be found in the PR description here github.com/rust-lang/ru.... This should improve general speed of rust-analyzer in the majority of projects given how pervasive these kinds of macros are in Rust.