Mutant LSP + VS Code Extension Low-Level Design (LLD)

This is the implementation-level developer guide for the Mutant language tooling stack.

Audience:

Junior or new engineers who need to understand how Mutant language tooling works end-to-end.
Maintainers adding or changing LSP features, diagnostics, formatting, completion, hover/signature behavior, or extension lifecycle behavior.

Scope:

Go LSP server in lsp/cmd/mlsp/main.go and lsp/internal/server/server.go
Analyzer pipeline in lsp/internal/analyzer
Workspace state/index in lsp/internal/workspace
VS Code extension host/client in vscode-extension/src/extension.ts

1. Architecture Overview

Mermaid diagram

Core separation of concerns:

Extension is process lifecycle + UX shell around the language server.
LSP server is request handling, state, and protocol behavior.
Analyzer is language intelligence over current AST snapshot.
Workspace index enables cross-file symbol/reference behavior.

2. Runtime Components

2.1 LSP binary entrypoint

Entrypoint: lsp/cmd/mlsp/main.go
Behavior:

Parse -debug flag
Build server via server.New(debug)
Run over stdio via server.Run()

2.2 JSON-RPC transport bridge

Adapter: lsp/internal/server/transport.go
Why it exists:
glsp handler methods are wired into a jsonrpc2 stdio connection.
It translates method/params and maps validation failures to JSON-RPC error codes.

Transport flow:

Read request from stdin stream.
Build glsp.Context with method, params, Notify, and Call handlers.
Dispatch with handler.Handle(...).
Reply (unless notification).

2.3 Server state and handler wiring

Main implementation: lsp/internal/server/server.go

Server owns:

LSP method handler table
Document store (documents)
Symbol index (symbols)
Analyzer instance (analyzer)
Per-document snapshots map (snapshots)
Lint configuration
Crash/fallback flags (for semantic token panic fallback warning)

Important constructor behavior:

New(debug bool) binds all supported LSP endpoints to receiver methods.
initialize advertises capabilities like completion, hover, signature help, references, rename, formatting, semantic tokens.

2.4 Document store

Files: lsp/internal/workspace/store.go, lsp/internal/workspace/document.go
Responsibility:
Track opened documents and versions.
Apply incremental text changes from LSP content change events.
Return immutable clone snapshots to avoid accidental shared mutation.

2.5 Analyzer snapshot model

Files: lsp/internal/analyzer/snapshot.go, lsp/internal/analyzer/analyzer.go
Snapshot contains:
raw source
parsed AST program (Program)
parse errors

Analyzer steps:

Lex + parse source into AST.
Preserve node ranges (NodePositions) from parser output.
Serve semantic queries (hover, completion, definitions, references, symbols, semantic tokens, signature help).

2.6 Workspace symbol index

File: lsp/internal/workspace/symbol_index.go
Purpose:
Cache top-level symbols per document for workspace symbol search.
Cache unresolved identifier usages to improve cross-document references.

This enables cross-file behaviors when a symbol is not resolvable only within a single snapshot.

3. LSP Capability Map (Method -> Implementation)

LSP method	Server method	Core implementation dependencies
initialize	`initialize`	Capabilities + semantic legend from analyzer
textDocument/didOpen	`didOpen`	store open -> analyze -> set snapshot -> publish diagnostics
textDocument/didChange	`didChange`	incremental apply -> analyze -> publish diagnostics
textDocument/didClose	`didClose`	delete store/snapshot/index + clear diagnostics
textDocument/hover	`hover`	`Snapshot.HoverText`
textDocument/completion	`completion`	`Snapshot.CompletionItemsAt`
textDocument/signatureHelp	`signatureHelp`	`Snapshot.SignatureHelp`
textDocument/documentSymbol	`documentSymbols`	`Snapshot.DocumentSymbols`
textDocument/definition	`definition`	local definition + workspace fallback
textDocument/typeDefinition	`typeDefinition`	`Snapshot.TypeDefinitionLocation`
textDocument/references	`references`	local refs + workspace refs + dedupe
textDocument/prepareRename	`prepareRename`	local rename target + workspace fallback
textDocument/rename	`rename`	location collection + per-URI sorted text edits
textDocument/codeAction	`codeActions`	diagnostic-driven quick fixes
textDocument/semanticTokens/full	`semanticTokensFull`	semantic token data + panic fallback
textDocument/formatting	`formatting`	AST formatter + safety-preserving fallback
textDocument/onTypeFormatting	`onTypeFormatting`	trigger-based canonicalization while typing (opt-in)
workspace/symbol	`workspaceSymbols`	SymbolIndex query
workspace/didChangeConfiguration	`didChangeConfiguration`	lint config parse + diagnostics republish

4. Key Request Flows

4.1 Open/change diagnostics flow

Mermaid diagram

4.2 Completion flow

Mermaid diagram

Implementation notes:

Completions include keywords, builtins, snippets, visible scope bindings.
Ordering is deterministic and stabilized with explicit sort keys in analyzer.

4.3 Rename flow

Mermaid diagram

5. Diagnostics and Linting

Core file: lsp/internal/analyzer/diagnostics.go

Diagnostics sources:

mutant-parser: parser errors from snapshot parse errors.
mutant-lint: semantic lint rules.

Current lint rules:

duplicate top-level declaration
unused declaration (top-level and local)
undefined identifier
nesting complexity in function bodies (depth > 2)

Config ingestion path:

Extension sends settings changes via workspace/didChangeConfiguration.
Server parses with lsp/internal/server/lint_config.go.
republishAllDiagnostics updates existing open documents.

Supported severities:

error, warning, information, hint, off

6. Formatting Design

Core file: lsp/internal/server/formatter.go

Behavior model:

If parse errors exist (or snapshot invalid), formatter uses whitespace normalization only.
If source contains comments or intentional blank lines, formatter preserves structure by using normalization path.
Otherwise it applies AST-driven formatting for statements/expressions.

Canonical style policy:

Opening braces stay on the same line for supported constructs (if (...) {, for (...) {, fn(...) {, else {).
Source-layout mode still preserves authored comments/blank lines, but style-2 standalone opening braces are canonicalized to style-1.

On-type formatting behavior:

Server advertises textDocument/onTypeFormatting and supports triggers for }, ;, and newline.
Extension keeps on-type formatting disabled by default to avoid intrusive edits, and exposes an opt-in setting.

Important safety guarantees:

String literal quotes are preserved and escaped.
Formatting returns nil edits for noop output.

7. Semantic Tokens and Resilience

Semantic token production is in analyzer logic and requested through textDocument/semanticTokens/full.

Resilience behavior in lsp/internal/server/server.go:

method-level panic recovery in semantic token handler
one-time user warning if fallback path is used
returns empty token list on failure instead of crashing LSP process

This prevents editor crash loops from semantic token panics.

8. Hover, Signature Help, and Teaching Metadata

Teaching metadata source:

lsp/internal/analyzer/language_teach.go

What it provides:

keyword hover docs
builtin hover/signature docs
snippet completion templates

Builtin coverage model:

Rich docs from builtinDocs map when defined.
Prefix-based fallback for builtin families (fs_, db_, bytes_, http_, etc.).
Generic fallback for any builtin registered in builtin/builtin.go.

Result:

Newly added builtins are auto-discoverable in completion and still have baseline hover/signature coverage.

9. VS Code Extension Design

Main file: vscode-extension/src/extension.ts

Responsibilities:

Activate language client and commands.
Resolve language server command path.
Manage restarts and crash-loop backoff.
Offer operational commands (status, logs, restart, smoke checks).

9.1 Binary selection algorithm

If mutant.languageServer.path is configured and non-empty:

use it exactly.

Else:

Gather workspace roots and parent directories.
Search for mlsp* binaries with accepted naming variants.
Pick latest by modification time.
Fallback to mlsp.exe on Windows or mlsp elsewhere.

Code path:

resolveLanguageServerCommandFromInputs
findLatestServerBinary
isLanguageServerBinaryName

9.2 Crash-loop mitigation

The extension tracks crash timestamps in a rolling window:

window: 3 minutes
block threshold: 5 crashes in window

When threshold is reached:

automatic restart is disabled
user is prompted to run manual restart command

Code path:

recordCrashAndGetStatus
language client errorHandler.closed

9.3 Commands exposed

Declared in vscode-extension/package.json, implemented in vscode-extension/src/extension.ts:

Mutant: Open Smoke File
Mutant: Run LSP Smoke Checks
Mutant: Show LSP Status
Mutant: Show LSP Logs
Mutant: Restart LSP
Mutant: Copy LSP Logs

10. Deterministic Behavior Rules

Determinism safeguards already in codebase:

Completion ordering is canonicalized and stable (category + label + kind + explicit SortText).
Node selection tie-breaks use deterministic specificity logic.
Workspace symbol ordering is sorted by name/location.
Rename edits are sorted by range per file before response.

These rules reduce editor flicker and test flakiness.

11. Testing Strategy

11.1 Server tests

File: lsp/internal/server/server_test.go
Covers initialization capabilities, diagnostics, completion, hover, references, formatting, semantic tokens, rename, configuration behavior, and regressions.

11.2 Analyzer tests

File: lsp/internal/analyzer/analyzer_test.go
Includes semantic robustness and builtin teaching coverage regression checks.

11.3 Extension integration tests

File: vscode-extension/src/test/suite/extension.test.ts
Covers activation, command registration, crash backoff, binary selection, and config override behavior.

12. Change Playbooks

12.1 Add a new LSP feature

Add capability advertisement in initialize if needed.
Add server handler in constructor wiring.
Implement method in server layer.
Add analyzer APIs if semantic analysis is needed.
Add tests in server/analyzer test suites.
Add extension-side UX command only if user-facing operation is needed.

12.2 Add a new builtin function

Register builtin in builtin/builtin.go.
Optional but recommended: add rich doc entry in builtinDocs at lsp/internal/analyzer/language_teach.go.
Run analyzer/server tests. Existing regression tests verify baseline completion/hover/signature coverage for all builtins.

12.3 Add a new lint rule

Implement lint function in lsp/internal/analyzer/diagnostics.go.
Add config field to LintConfig and severity parser in lsp/internal/server/lint_config.go.
Expose setting in vscode-extension/package.json.
Add tests for default severity, override, and off behavior.

12.4 Add or change extension setting

Add schema entry under contributes.configuration.properties in vscode-extension/package.json.
Read/consume in vscode-extension/src/extension.ts.
Add integration test in vscode-extension/src/test/suite/extension.test.ts.
Document in vscode-extension/README.md and troubleshooting docs.

13. Build, Run, and Debug

LSP/server tests:

go test ./lsp/internal/server -v
go test ./lsp/internal/analyzer -v
go test ./...

Extension:

cd vscode-extension
npm install
npm run compile
npm test
Launch Extension Development Host with VS Code F5

Operational debugging:

Use Mutant: Show LSP Status
Use Mutant: Show LSP Logs
Use Mutant: Copy LSP Logs
Use Mutant: Restart LSP for manual recovery

14. Known Design Trade-offs

Current formatter chooses preservation for comments/blank-line input rather than forcing full AST rewrite, to avoid destructive edits.
Workspace reference fallback focuses on top-level symbols for predictable and performant cross-document behavior.
Semantic token failures degrade gracefully to empty token data instead of hard-failing language services.

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