LSP Phase 1: Diagnostics — compile errors + lint warnings in editor

[gregwinn]

Summary

Build a minimal LSP server that pushes compiler diagnostics and lint warnings to the editor in real-time. This is the foundation that Phase 2 (navigation) and Phase 3 (completion) build on.

Architecture

VS Code extension (language-winn-vscode)
  │ stdin/stdout JSON-RPC
  ▼
winn_lsp.erl (gen_server)
  ├── winn_lsp_transport.erl    # JSON-RPC framing over stdio
  ├── winn_lsp_handler.erl      # LSP method dispatch
  ├── winn_lsp_documents.erl    # In-memory document buffer (ETS)
  └── winn_lsp_diagnostic.erl   # #diag → LSP Diagnostic conversion
          │
          ▼
    Existing compiler pipeline
    (winn_lexer → winn_parser → winn_semantic → winn_lint)

LSP Methods to Implement

Method	Direction	Description
initialize	request	Return server capabilities
initialized	notification	No-op acknowledgment
shutdown / exit	request/notif	Clean shutdown
textDocument/didOpen	notification	Store document, run diagnostics
textDocument/didChange	notification	Update buffer, re-run diagnostics
textDocument/didSave	notification	Re-run diagnostics
textDocument/didClose	notification	Remove from buffer
textDocument/publishDiagnostics	server→client	Push diagnostics

Diagnostic Pipeline

On every didOpen/didChange/didSave:

1. Get source from document buffer
2. winn_lexer:string(Source) → lex errors OR tokens
3. winn_newline_filter:filter(Tokens)
4. winn_parser:parse(Filtered) → parse errors OR AST
5. winn_lint:check_string(Source) → lint warnings
6. Convert all #diag records → LSP Diagnostic JSON
7. Publish via textDocument/publishDiagnostics

If lexing/parsing fails, still publish those errors (don't require a valid AST for diagnostics).

#diag → LSP Diagnostic Mapping

%% Winn #diag record:
#diag{severity, title, file, line, col, span_len, message, hint}

%% LSP Diagnostic:
#{
  <<"range">> => #{
    <<"start">> => #{<<"line">> => Line - 1, <<"character">> => Col - 1},
    <<"end">>   => #{<<"line">> => Line - 1, <<"character">> => Col - 1 + SpanLen}
  },
  <<"severity">> => 1 (error) | 2 (warning),
  <<"source">>   => <<"winn">>,
  <<"message">>  => Message,
  <<"code">>     => RuleName  %% for lint rules
}

New Files

File	Purpose	~Lines
apps/winn/src/winn_lsp.erl	gen_server entry point, process lifecycle	80
apps/winn/src/winn_lsp_transport.erl	JSON-RPC read/write over stdio	120
apps/winn/src/winn_lsp_handler.erl	Method dispatch + request handlers	150
apps/winn/src/winn_lsp_documents.erl	ETS-backed document buffer	60
apps/winn/src/winn_lsp_diagnostic.erl	#diag → LSP conversion	80
apps/winn/test/winn_lsp_tests.erl	Unit tests for handler + diagnostics	150

CLI Integration

winn lsp        # Start LSP server on stdio (for editor use)

Add parse_args(["lsp"]) -> lsp and handler in winn_cli.erl.

VS Code Extension Changes

Update language-winn-vscode:

• Add vscode-languageclient dependency
• Configure LanguageClient to spawn winn lsp via stdio
• Remove existing compile-on-save shell spawning (LSP replaces it)

Acceptance Criteria

• winn lsp starts and responds to initialize
• Syntax errors appear as red squiggles on save
• Lint warnings appear as yellow squiggles on save
• Fixing an error clears the diagnostic
• Opening/closing files doesn't leak memory
• Clean shutdown on editor close

Part of v0.9.0 — Developer Tooling

[gregwinn]

Shipped on develop in PR #140 — LSP Phase 1: diagnostics (lint + compile errors) and didClose cleanup. Will reach main in v0.10.0.