https://blog.rust-lang.org/2022/07/01/RLS-deprecation.html was the announcement of the replacement, and talks about it a little bit:

> LS was introduced by RFC 1317 and development was very active from 2016 through 2019. However, the architecture of RLS has several limitations that can make it difficult to provide low-latency and high-quality responses needed for an interactive environment.

> rust-analyzer uses a fundamentally different approach that does not rely on using rustc. In RFC 2912 rust-analyzer was adopted as the official replacement for RLS.

These RFCs provide some more color.

The long and short of it is, the RLS relied on a process where rustc would sort of pre-process your codebase, and spit out a "save analysis" file. RLS would read this file, and do its thing. rust-analyzer, on the other hand, doesn't use rustc at all(*), and instead analyzes your code on-demand, in an incremental style. The latter has many advantages in an IDE context.

The * is there because part of the original idea is that rustc would end up producing some libraries that rust-analyzer would also use, allowing them to share code. Given that rustc has also been moving towards an incremental, on-demand architecture, this would be easier. I am out of the loop these days, but from the relative outside, this effort seemed to stall out. I don't really know why. Maybe it has been happening and just isn't really visible.

Regardless of those details, rust-analyzer is very good. It's a shame the transition took so long, in retrospect!

Very cool. Thanks for the links & write up!

I can't find the original link to his proposal, but I did send him a message on reddit asking if still had a link to it.

The gist of it was, he felt the first attempt to make a Rust LSP (RLS) had the wrong architecture. It was basically running the Rust compiler in check mode and relaying the errors back. In his opinion an IDE needed a different, incremental architecture that stored state. It also needed to be much more tolerant of errors.

He had already made a Rust IDE plugin in Java for JetBrain's IntelliJ-Rust plugin, and wanted to make a similar one in Rust.

So he started the rust-analyzer project and it succeeded. Last year it became the official LSP of the Rust project. (https://blog.rust-lang.org/2022/02/21/rust-analyzer-joins-ru...)