gopls/internal/server: improve "editing generated file" warning

The phrasing should be more informative and less prescriptive.

Also, a test.

Updates golang/go#49555
Fixes golang/go#73959

Change-Id: Ie8206b39d1b03c323690095552400c75bfeb9c5a
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678836
Reviewed-by: Robert Findley <rfindley@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: adonovan

gopls/internal/server/text_synchronization.go

@@ -156,18 +156,20 @@ func (s *server) warnAboutModifyingGeneratedFiles(ctx context.Context, uri proto
 		return nil
 	}
 
-	// Ideally, we should be able to specify that a generated file should
-	// be opened as read-only. Tell the user that they should not be
-	// editing a generated file.
+	// Warn the user that they are editing a generated file, but
+	// don't try to stop them: there are often good reasons to do
+	// so, such as adding temporary logging, or evaluating changes
+	// to the generated code without the trouble of modifying the
+	// generator logic (see #73959).
 	snapshot, release, err := s.session.SnapshotOf(ctx, uri)
 	if err != nil {
 		return err
 	}
 	isGenerated := golang.IsGenerated(ctx, snapshot, uri)
 	release()
-
 	if isGenerated {
-		msg := fmt.Sprintf("Do not edit this file! %s is a generated file.", uri.Path())
+		msg := fmt.Sprintf("Warning: editing %s, a generated file.",
+			filepath.Base(uri.Path()))
 		showMessage(ctx, s.client, protocol.Warning, msg)
 	}
 	return nil

gopls/internal/test/integration/misc/generate_test.go

@@ -103,3 +103,33 @@ package main
 			env.RunGenerate("./")
 		})
 }
+
+func TestEditingGeneratedFileWarning(t *testing.T) {
+	const src = `
+-- go.mod --
+module example.com
+go 1.21
+
+-- a/a.go --
+// Code generated by me. DO NOT EDIT.
+
+package a
+
+var x = 1
+`
+	Run(t, src, func(t *testing.T, env *Env) {
+		env.OpenFile("a/a.go")
+		env.RegexpReplace("a/a.go", "var", "const")
+		collectMessages := env.Awaiter.ListenToShownMessages()
+		env.Await(env.DoneWithChange())
+		messages := collectMessages()
+
+		const want = "Warning: editing a.go, a generated file."
+		if len(messages) != 1 || messages[0].Message != want {
+			for _, message := range messages {
+				t.Errorf("got message %q", message.Message)
+			}
+			t.Errorf("no %q warning", want)
+		}
+	})
+}