internal/lsp/source: don't format generated files

Fixes golang/go#49555

Change-Id: I53e3c0d34be6a545386d6766371fbeda8a2b2ffb
GitHub-Last-Rev: 516168765671c8135523fcf801f50455b352bdab
GitHub-Pull-Request: golang/tools#349
Reviewed-on: https://go-review.googlesource.com/c/tools/+/365295
Reviewed-by: Robert Findley <rfindley@google.com>
Trust: Robert Findley <rfindley@google.com>
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Robert Findley <rfindley@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

gopls/internal/regtest/misc/formatting_test.go

@@ -268,3 +268,36 @@ func main() {
 		})
 	}
 }
+
+func TestFormattingOfGeneratedFile_Issue49555(t *testing.T) {
+	const input = `
+-- main.go --
+// Code generated by generator.go. DO NOT EDIT.
+
+package main
+
+import "fmt"
+
+func main() {
+
+
+
+
+	fmt.Print("hello")
+}
+`
+
+	Run(t, input, func(t *testing.T, env *Env) {
+		wantErrSuffix := "file is generated"
+
+		env.OpenFile("main.go")
+		err := env.Editor.FormatBuffer(env.Ctx, "main.go")
+		if err == nil {
+			t.Fatal("expected error, got nil")
+		}
+		// Check only the suffix because an error contains a dynamic path to main.go
+		if !strings.HasSuffix(err.Error(), wantErrSuffix) {
+			t.Fatalf("unexpected error %q, want suffix %q", err.Error(), wantErrSuffix)
+		}
+	})
+}

internal/lsp/source/format.go

@@ -29,6 +29,11 @@ func Format(ctx context.Context, snapshot Snapshot, fh FileHandle) ([]protocol.T
 	ctx, done := event.Start(ctx, "source.Format")
 	defer done()
 
+	// Generated files shouldn't be edited. So, don't format them
+	if IsGenerated(ctx, snapshot, fh.URI()) {
+		return nil, fmt.Errorf("can't format %q: file is generated", fh.URI().Filename())
+	}
+
 	pgf, err := snapshot.ParseGo(ctx, fh, ParseFull)
 	if err != nil {
 		return nil, err