internal/lsp: handle deletion of file content

We had previously been treating file changes with no content in the same
way as the deletion of content. Now, we distinguish between the two.

Fixes golang/go#38424.

Change-Id: I44b338006af0c13a8a3f842844521789ea378470
Reviewed-on: https://go-review.googlesource.com/c/tools/+/243577
Run-TryBot: Rebecca Stambler <[email protected]>
Reviewed-by: Robert Findley <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>

Author: stamblerre

internal/lsp/cache/session.go

@@ -407,9 +407,10 @@ func (s *Session) updateOverlays(ctx context.Context, changes []source.FileModif
 			continue
 		}
 
-		// If the file is on disk, check if its content is the same as the overlay.
+		// If the file is on disk, check if its content is the same as in the
+		// overlay. Saves don't necessarily come with the file's content.
 		text := c.Text
-		if text == nil {
+		if text == nil && c.Action == source.Save {
 			text = o.text
 		}
 		hash := hashContents(text)

internal/lsp/regtest/diagnostics_test.go

@@ -1039,3 +1039,51 @@ func main() {}
 		)
 	})
 }
+
+// Reproduces golang/go#38424.
+func TestCutAndPaste(t *testing.T) {
+	const basic = `
+-- go.mod --
+module mod.com
+
+go 1.14
+-- main2.go --
+package main
+`
+	runner.Run(t, basic, func(t *testing.T, env *Env) {
+		env.CreateBuffer("main.go", "")
+		env.Await(CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidOpen), 1))
+
+		env.Editor.SaveBufferWithoutActions(env.Ctx, "main.go")
+		env.Await(
+			CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidSave), 1),
+			CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidChangeWatchedFiles), 1),
+		)
+
+		env.EditBuffer("main.go", fake.NewEdit(0, 0, 0, 0, `package main
+
+func main() {
+}
+`))
+		env.Await(CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidChange), 1))
+
+		env.SaveBuffer("main.go")
+		env.Await(
+			CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidSave), 2),
+			CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidChangeWatchedFiles), 2),
+		)
+
+		env.EditBuffer("main.go", fake.NewEdit(0, 0, 4, 0, ""))
+		env.Await(CompletedWork(lsp.DiagnosticWorkTitle(lsp.FromDidChange), 2))
+
+		env.EditBuffer("main.go", fake.NewEdit(0, 0, 0, 0, `package main
+
+func main() {
+	var x int
+}
+`))
+		env.Await(
+			env.DiagnosticAtRegexp("main.go", "x"),
+		)
+	})
+}