skip over testdata dirs like we do with vendor

And, just like vendor, one can opt into formatting them by explicitly
passing them as arguments.

Updates #250.
Fixes #260.

README.md

@@ -19,7 +19,7 @@ The project includes copies of `go/printer` and `go/doc/comment` as of Go 1.20
 to ensure consistent formatting independent of what Go version is being used.
 The [added formatting rules](#Added-rules) are implemented in the `format` package.
 
-Note that vendor directories are skipped unless given as explicit arguments.
+`vendor` and `testdata` directories are skipped unless given as explicit arguments.
 Similarly, the added rules do not apply to generated Go files unless they are
 given as explicit arguments.
 

gofmt.go

@@ -84,9 +84,6 @@ var fdSem = make(chan bool, 200)
 var (
 	fileSet    = token.NewFileSet() // per process FileSet
 	parserMode parser.Mode
-
-	// walkingVendorDir is true if we are explicitly walking a vendor directory.
-	walkingVendorDir bool
 )
 
 func usage() {
@@ -512,11 +509,14 @@ func gofmtMain(s *sequencer) {
 			})
 		default:
 			// Directories are walked, ignoring non-Go files.
-			walkingVendorDir = filepath.Base(arg) == "vendor"
 			err := filepath.WalkDir(arg, func(path string, f fs.DirEntry, err error) error {
-				if !walkingVendorDir && filepath.Base(path) == "vendor" {
+				// vendor and testdata directories are skipped,
+				// unless they are explicitly passed as an argument.
+				base := filepath.Base(path)
+				if path != arg && (base == "vendor" || base == "testdata") {
 					return filepath.SkipDir
 				}
+
 				if err != nil || !isGoFile(f) {
 					return err
 				}

testdata/script/ignore-dirs.txtar

@@ -0,0 +1,55 @@
+exec gofumpt orig.go.golden
+cp stdout formatted.go.golden
+mkdir -p vendor/foo testdata/foo
+cp orig.go.golden vendor/foo/foo.go
+cp orig.go.golden testdata/foo/foo.go
+
+# format explicit dirs
+exec gofumpt -l vendor testdata
+stdout -count=1 'vendor[/\\]foo[/\\]foo.go'
+stdout -count=1 'testdata[/\\]foo[/\\]foo.go'
+! stderr .
+
+# format explicit files
+exec gofumpt -l vendor/foo/foo.go testdata/foo/foo.go
+stdout -count=1 'vendor[/\\]foo[/\\]foo.go'
+stdout -count=1 'testdata[/\\]foo[/\\]foo.go'
+! stderr .
+
+# ignore implicit dirs via fs walking
+exec gofumpt -l .
+! stdout .
+! stderr .
+
+# format explicit pkg while ignoring rest
+mkdir vendor/ignore testdata/ignore
+cp orig.go.golden vendor/ignore/ignore.go
+cp orig.go.golden testdata/ignore/ignore.go
+exec gofumpt -l vendor/foo testdata/foo .
+stdout -count=1 'vendor[/\\]foo[/\\]foo.go'
+stdout -count=1 'testdata[/\\]foo[/\\]foo.go'
+! stderr .
+
+# format explicit dirs without clean paths
+exec gofumpt -l $WORK//vendor ./testdata/./
+stdout -count=1 'vendor[/\\]foo[/\\]foo.go'
+stdout -count=1 'testdata[/\\]foo[/\\]foo.go'
+! stderr .
+
+-- orig.go.golden --
+package p
+
+func f() {
+	if true {
+		// lone comment
+	}
+	{
+
+	}
+
+	{
+
+		// lone comment
+
+	}
+}