sui-lsp — a Nix language server built on sui's own parser and lowering.

Why this is not another nil / nixd

Both existing servers re-implement a Nix front end in order to answer editor questions, which means the editor's idea of a file and the evaluator's idea of the same file are two independent implementations that agree only by effort. sui already owns a parser (rnix), a resolver (sui-resolve), a lowering pass (sui-ir) and an evaluator — so the interesting move is not writing a third front end, it is exposing the one that already evaluates the file. A diagnostic here is not a lookalike of what sui would say; it is what sui says.

Shape

[diagnostics] is pure — &str in, Vec<Diagnostic> out, no async, no LSP types, no IO — and holds every decision worth testing. [server] is the thin tower-lsp shell that owns document state and does the protocol. The split is the fleet's mockable-seam default: the logic is provable without a client, and the part that needs a client has almost no logic in it.

Positions come from [zahyou], shared with escriba-lsp-client — the same UTF-16 arithmetic on both ends of the wire, which is the only way the two agree about where a squiggle goes.

M0 scope, stated so it is not mistaken for more

Diagnostics on open and on change. No hover, goto-definition, completion or workspace symbols yet — those need the resolver's scope chain surfaced, which is the next slice. initialize advertises exactly what is implemented.