tree-sitter-rust VS rust.vim

Yes. It's typically a fair bit slower than a hand-coded parser. The idea sounds great until you're working on a codebase with 2 million LOC. At that point, the speed of a cold parse is most important, whereas TS is designed for fast re-parsing of a single file. These aren't great numbers but the Rust TS parser is reportedly 2x slower than rustc's https://github.com/tree-sitter/tree-sitter-rust. It's no surprise that you just use the faster option if it's convenient.

Notes for anyone interested, there is this PR https://github.com/tree-sitter/tree-sitter-rust/pull/128, which merges successive doc comments.

Tree-sitter is a fantastic parser generation and incremental parsing library, that supports Rust language bindings and has an available parser Rust tree-sitter-rust.

You can see a much more in-depth version at https://github.com/tree-sitter/tree-sitter-typescript or https://github.com/tree-sitter/tree-sitter-rust

I'll be really interested to see what folks end up preferring once nvim 0.5 ships with treesitter support. Do folks have any experience with the Rust grammar?

Check out my fork of rust.vim, which has type highlighting. It isn't LSP-based so it isn't what you're asking for, but it'll hold you off.