gopls/internal/lsp: only diagnose one package per file

The purpose of diagnosing changed files is to compute local diagnostics
with low latency. We only need one package for that. Don't slow
everything down by doing redundant and unnecessary work checking test
variants.

For golang/go#60926

Change-Id: I3d77305d5732fb0036e4865bbe7311d1561f2fd5
Reviewed-on: https://go-review.googlesource.com/c/tools/+/509876
Auto-Submit: Robert Findley <rfindley@google.com>
Run-TryBot: Robert Findley <rfindley@google.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

Author: findleyr

gopls/internal/lsp/cache/check.go

@@ -193,7 +193,7 @@ func (s *snapshot) resolveImportGraph() (*importGraph, error) {
 	ctx, done := event.Start(event.Detach(ctx), "cache.resolveImportGraph")
 	defer done()
 
-	if err := s.awaitLoaded(ctx); err != nil {
+	if err := s.awaitLoaded(ctx); err != nil { // TODO(rfindley): this is problematic, and slows down diagnostics
 		return nil, err
 	}
 

gopls/internal/lsp/diagnostics.go

@@ -230,7 +230,7 @@ func (s *Server) diagnoseChangedFiles(ctx context.Context, snapshot source.Snaps
 		}
 
 		// Find all packages that include this file and diagnose them in parallel.
-		metas, err := snapshot.MetadataForFile(ctx, uri)
+		meta, err := source.NarrowestMetadataForFile(ctx, snapshot, uri)
 		if err != nil {
 			if ctx.Err() != nil {
 				return
@@ -240,10 +240,7 @@ func (s *Server) diagnoseChangedFiles(ctx context.Context, snapshot source.Snaps
 			// noisy to log (and we'll handle things later in the slow pass).
 			continue
 		}
-		source.RemoveIntermediateTestVariants(&metas)
-		for _, m := range metas {
-			toDiagnose[m.ID] = m
-		}
+		toDiagnose[meta.ID] = meta
 	}
 	s.diagnosePkgs(ctx, snapshot, toDiagnose, nil)
 }