Make sure you have rust-analyzer installed, and check `hx --health` to see that Helix finds it.

Not sure if this is still necessary, but I needed this workaround in ~/.config/helix/languages.toml:

    language-server = { command = "rustup", args = ["run", "nightly", "rust-analyzer"]}

This should not be necessary when rust-analyzer is stabilized (may be already), but last time I tried it still needed nightly.