gopls/internal: add code action "move to a new file"

This code action moves selected code sections to a newly created file
within the same package. The created filename is chosen as the first
{function, type, const, var} name encountered. In addition, import
declarations are added or removed as needed.

Fixes golang/go#65707

gopls/internal/cache/snapshot.go

@@ -1658,6 +1658,22 @@ func inVendor(uri protocol.DocumentURI) bool {
 	return found && strings.Contains(after, "/")
 }
 
+// Sandbox clones the reciever, applying given file modfications as overlays.
+func (s *Snapshot) Sandbox(ctx, bgCtx context.Context, modifications []file.Modification) *Snapshot {
+	updatedFiles := make(map[protocol.DocumentURI]file.Handle)
+	for _, m := range modifications {
+		updatedFiles[m.URI] = &overlay{
+			uri:     m.URI,
+			content: m.Text,
+		}
+	}
+	cloned, _ := s.clone(ctx, bgCtx, StateChange{
+		Modifications: modifications,
+		Files:         updatedFiles,
+	}, func() {})
+	return cloned
+}
+
 // clone copies state from the receiver into a new Snapshot, applying the given
 // state changes.
 //

gopls/internal/golang/codeaction.go

@@ -86,6 +86,11 @@ func CodeActions(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle,
 				return nil, err
 			}
 			actions = append(actions, extractions...)
+			moves, err := getMoveToNewFileCodeAction(pgf, rng, snapshot.Options())
+			if err != nil {
+				return nil, err
+			}
+			actions = append(actions, moves)
 		}
 	}
 

gopls/internal/golang/format.go

@@ -92,6 +92,38 @@ func Format(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle) ([]pr
 	return computeTextEdits(ctx, pgf, formatted)
 }
 
+func formatImportsBytes(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle) ([]byte, error) {
+	_, done := event.Start(ctx, "golang.formatImportsBytes")
+	defer done()
+
+	errorPrefix := "formatImportsBytes"
+
+	text, err := fh.Content()
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", errorPrefix, err)
+	}
+
+	var out []byte
+	if err := snapshot.RunProcessEnvFunc(ctx, func(ctx context.Context, opts *imports.Options) error {
+		fixes, err := imports.FixImports(ctx, fh.URI().Path(), text, opts)
+		if err != nil {
+			return fmt.Errorf("%s: %w", errorPrefix, err)
+		}
+		out, err = imports.ApplyFixes(fixes, fh.URI().Path(), text, opts, parsego.Full)
+		if err != nil {
+			return fmt.Errorf("%s: %w", errorPrefix, err)
+		}
+		return nil
+	}); err != nil {
+		return nil, fmt.Errorf("%s: %w", errorPrefix, err)
+	}
+	out, err = format.Source(out)
+	if err != nil {
+		return nil, fmt.Errorf("%s: %w", errorPrefix, err)
+	}
+	return out, nil
+}
+
 func formatSource(ctx context.Context, fh file.Handle) ([]byte, error) {
 	_, done := event.Start(ctx, "golang.formatSource")
 	defer done()