gopls/internal/golang: Format generated files

This CL reverts the effect of CL 365295 (to address issue golang/go#49555),
which caused Format to fail on generated files. We believe this
was a mistake: Go developers have plenty of reasons to temporarily
edit generated files, for example to quickly experiment with changes
without having to rewrite the code generator, or to add a log.Print
statement while debugging. If a client asks gopls to format a file
or organize its imports, gopls should do that, even if the file
contains a "generated..." comment.

+ test

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

Change-Id: I2d02d5b2611b1599068df8fc267453773c66e027
Reviewed-on: https://go-review.googlesource.com/c/tools/+/678815
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Alan Donovan <adonovan@google.com>
Reviewed-by: Robert Findley <rfindley@google.com>

Author: adonovan

gopls/internal/golang/format.go

@@ -40,11 +40,6 @@ func Format(ctx context.Context, snapshot *cache.Snapshot, fh file.Handle) ([]pr
 		return nil, err
 	}
 
-	// Generated files shouldn't be edited. So, don't format them.
-	if ast.IsGenerated(pgf.File) {
-		return nil, fmt.Errorf("can't format %q: file is generated", fh.URI().Path())
-	}
-
 	// Even if this file has parse errors, it might still be possible to format it.
 	// Using format.Node on an AST with errors may result in code being modified.
 	// Attempt to format the source of this file instead.

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

@@ -268,39 +268,6 @@ 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)
-		}
-	})
-}
-
 func TestGofumptFormatting(t *testing.T) {
 	// Exercise some gofumpt formatting rules:
 	//  - No empty lines following an assignment operator

gopls/internal/test/marker/testdata/format/generated.txt

@@ -0,0 +1,27 @@
+This test checks that formatting includes generated files too
+(reversing https://go.dev/cl/365295 to address issue #49555).
+
+See https://github.com/golang/go/issues/73959.
+
+-- flags --
+-ignore_extra_diags
+
+-- go.mod --
+module example.com
+go 1.21
+
+-- a/a.go --
+// Code generated by me. DO NOT EDIT.
+
+package a; func main() { fmt.Println("hello") }
+
+//@format(out)
+
+-- @out --
+// Code generated by me. DO NOT EDIT.
+
+package a
+
+func main() { fmt.Println("hello") }
+
+//@format(out)