gopls: add opt-in eager diagnostic clearing for agentic workflows

[alliprice]

Summary

Adds an opt-in eagerDiagnosticsClear setting that immediately publishes empty diagnostics for a file when its content changes (via textDocument/didChange), before reanalysis completes.

Motivation

The current debounce behavior is correct for GUI editors - it prevents diagnostic flicker while a human is typing. This default should remain unchanged.

However, LSP is increasingly used by headless agentic tools (AI coding assistants, automated refactoring systems) that consume diagnostics programmatically. For these clients, stale diagnostics between an edit and reanalysis are incorrect input to decision loops. This can cause:

• Re-fixing already fixed errors
• Missing newly introduced errors
• Looping on diagnostics that no longer apply

This is not about servers being buggy - it is about different client requirements. Interactive editors need stable UI. Programmatic clients need immediate staleness signals.

Changes

• New setting: eagerDiagnosticsClear (default: false, status: "advanced")

  • When enabled, textDocument/didChange triggers an immediate publish of empty diagnostics for the changed file
  • Server continues normal analysis pipeline
  • Fresh diagnostics arrive once reanalysis completes

• Implementation:

  • Added EagerDiagnosticsClear to DiagnosticOptions in settings.go
  • Hooked into didModifyFiles in text_synchronization.go for FromDidChange cause
  • Reuses existing version-filtered publishFileDiagnosticsLocked pipeline
  • Because file version just changed, no existing view diagnostics match new version, so published set is naturally empty
  • Only processes files that already have diagnostics tracked

• Tests: New integration test verifies:

  • With setting enabled: first notification after edit is an empty clear
  • With setting disabled: first notification after edit contains diagnostics (no eager clear)

• Documentation: Updated gopls/doc/settings.md with new setting

Configuration

{
  "gopls": {
    "eagerDiagnosticsClear": false
  }
}

Setting this to true is intended for non-UI rapid-edit workflows such as agentic coding tools.

Test Plan

1. Run existing test suite to verify no regressions in default behavior: go test ./...
2. Run new integration test to verify eager clearing: go test -run TestEagerDiagnosticInvalidation ./gopls/internal/test/integration/diagnostics
3. Manual verification in VS Code or other LSP client:
   • With setting disabled: observe normal debounced diagnostic updates
   • With setting enabled: observe immediate diagnostic clearing on edit

Related Work

This is part of a coordinated effort across multiple LSP server ecosystems to support agentic workflows. See: https://github.com/lsp-eager-invalidation

Similar PRs:

• typescript-language-server (TypeScript)
• pyright (Python)
• rust-analyzer (Rust)

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