mty lsp

Run the Mighty Language Server over stdio. Used by editor plugins (VS Code, Neovim, Helix, Emacs, etc.) to provide live diagnostics, hover, go-to-definition, formatting, completion, semantic tokens, rename, inlay hints, code actions, and signature help for .mty files.

Synopsis

mty lsp

No flags; no arguments. The process reads JSON-RPC framed messages from stdin and writes responses + notifications to stdout, per the LSP 3.17 specification. Trace / log output goes to the LSP window/logMessage channel (not stderr) so it is editor-visible.

Capabilities advertised

The server reports the following capabilities in its initialize response:

Capability	Value	Notes
textDocumentSync	Incremental	Per-edit range patches; full sync also accepted.
hoverProvider	true	Markdown body.
definitionProvider	true	Top-level item names only (deferred).
completionProvider	{ triggerCharacters: [".", ":"] }	Keywords + def names + locals + receiver-aware methods + built-in methods.
documentFormattingProvider	true	Whole-document via mty fmt.
semanticTokensProvider	full + range, legend below	v0.5: 14 types × 3 modifiers.
renameProvider	{ prepareProvider: true }	Single-file scope (v0.5).
codeActionProvider	{ codeActionKinds: ["quickfix"] }	MT2021 / MT2002 / MT3001 / MT4001 fixes.
signatureHelpProvider	{ triggerCharacters: ["(", ","] }	Call + method-call sites.
inlayHintProvider	{ resolveProvider: false }	let + fn-param type hints.
workspace.workspaceFolders	{ supported: true, changeNotifications: true }	Event surface; per-file analysis.

Semantic token legend (v0.5)

index	type	index	type	index	type
0	keyword	5	string	10	enumMember
1	type	6	number	11	typeParameter
2	function	7	comment	12	macro
3	variable	8	operator	13	property
4	parameter	9	namespace

Modifier bits: 0 = declaration, 1 = readonly, 2 = defaultLibrary.

In-scope methods (v0.5)

• initialize / initialized / shutdown / exit
• textDocument/didOpen
• textDocument/didChange (incremental)
• textDocument/didClose
• textDocument/publishDiagnostics (server → client, on every change)
• textDocument/hover
• textDocument/definition
• textDocument/formatting
• textDocument/completion
• textDocument/semanticTokens/full
• textDocument/semanticTokens/range
• textDocument/rename
• textDocument/prepareRename
• textDocument/inlayHint
• textDocument/codeAction
• textDocument/signatureHelp
• workspace/didChangeWorkspaceFolders
• workspace/didChangeWatchedFiles

Out-of-scope (still deferred)

• Cross-file rename / cross-file go-to-def (workspace folders are observed but each file is still analyzed independently).
• Borrow-check diagnostics in the editor — mty check from the CLI still runs the full borrow checker.
• Call hierarchy / type hierarchy.
• Signature-help overload disambiguation by receiver type (we list every candidate today).
• Closure parameter inlay hints, argument-name inlay hints.
• inlayHint/resolve (we don't yet attach lazy data).

Editor setup

VS Code

Install the editor/vscode extension from the Mighty repo:

cd editor/vscode
npm install
npm run compile
npx vsce package
code --install-extension mighty-0.5.0.vsix

By default the extension launches whatever mty is on PATH. Override via the mighty.server.path setting. Inlay hints are off by default (mighty.inlayHints.enable = false); enable them in your settings if you want them.

Neovim (nvim-lspconfig)

local lspconfig = require('lspconfig')
local configs = require('lspconfig.configs')

if not configs.mighty then
  configs.mighty = {
    default_config = {
      cmd = { "mty", "lsp" },
      filetypes = { "mighty" },
      root_dir = lspconfig.util.root_pattern("mighty.toml", ".git"),
      settings = {},
    },
  }
end

lspconfig.mighty.setup({})

Plus a filetype mapping:

autocmd BufNewFile,BufRead *.mty set filetype=mighty

Helix

# ~/.config/helix/languages.toml
[[language]]
name = "mighty"
scope = "source.mighty"
file-types = ["mty"]
comment-token = "//"
language-servers = ["mty"]
indent = { tab-width = 4, unit = "    " }

[language-server.mty]
command = "mty"
args = ["lsp"]

Exit codes

Code	Meaning
0	Server shut down cleanly.
1	Failed to start (e.g. tokio runtime build failed).

Examples

Run the server directly (for transport debugging) and feed it a hand-rolled initialize:

echo 'Content-Length: 99\r\n\r\n{"jsonrpc":"2.0","id":1,"method":"initialize","params":{"capabilities":{}}}' \
  | mty lsp

You should see initialize reply with the capability set above.

Implementation notes

The server lives in crates/mty-lsp and is built on tower-lsp 0.20. Per-document state (source text, line index, parsed CST, lowered HIR, type-check side tables) is cached in a DashMap<Url, Arc<DocAnalysis>> and re-run on every change. See docs/internals/lsp.md for the full architecture.