Add a preset for testing.Short

The preset seems to fix the problem where the formatting of the statement had
strange newlines added by the ast writer.

cmd/tool/slowest/ast.go

@@ -61,8 +61,6 @@ func normalizePkgName(name string) string {
 	return strings.TrimSuffix(name, "_test")
 }
 
-// TODO: sometimes this writes the new AST with strange indentation. It appears
-// to be non-deterministic. Given the same input, it only happens sometimes.
 func writeFile(path string, file *ast.File, fset *token.FileSet) error {
 	fh, err := os.Create(path)
 	if err != nil {
@@ -72,9 +70,15 @@ func writeFile(path string, file *ast.File, fset *token.FileSet) error {
 	return format.Node(fh, fset, file)
 }
 
-// TODO: support preset values. Maybe that will help with the problem
-// of strange indentation described on writeFile.
 func parseSkipStatement(text string) (ast.Stmt, error) {
+	switch text {
+	case "default", "testing.Short":
+		text = `
+	if testing.Short() {
+		t.Skip("too slow for testing.Short")
+	}
+`
+	}
 	// Add some required boilerplate around the statement to make it a valid file
 	text = "package stub\nfunc Stub() {\n" + text + "\n}\n"
 	file, err := parser.ParseFile(token.NewFileSet(), "fragment", text, 0)

cmd/tool/slowest/ast_test.go

@@ -0,0 +1,21 @@
+package slowest
+
+import (
+	"bytes"
+	"go/format"
+	"go/token"
+	"testing"
+
+	"gotest.tools/v3/assert"
+)
+
+func TestParseSkipStatement_Preset_testingShort(t *testing.T) {
+	stmt, err := parseSkipStatement("testing.Short")
+	assert.NilError(t, err)
+	expected := `if testing.Short() {
+	t.Skip("too slow for testing.Short")
+}`
+	buf := new(bytes.Buffer)
+	format.Node(buf, token.NewFileSet(), stmt)
+	assert.DeepEqual(t, buf.String(), expected)
+}

cmd/tool/slowest/slowest.go

@@ -36,19 +36,34 @@ func setupFlags(name string) (*pflag.FlagSet, *options) {
     %s [flags]
 
 Read a json file and print or update tests which are slower than threshold.
-The json file can be created by 'gotestsum --jsonfile' or 'go test -json'.
+The json file can be created with 'gotestsum --jsonfile' or 'go test -json'.
 
 By default this command will print the list of tests slower than threshold to stdout.
 If --skip-stmt is set, instead of printing the list of stdout, the AST for the
 Go source code in the working directory tree will be modified. The --skip-stmt
 will be added to Go test files as the first statement in all the test functions
 which are slower than threshold.
 
-Example - use testing.Short():
+The --skip-stmt flag may be set to the name of a predefine statement, or a
+source code which will be parsed as a go/ast.Stmt. Currently there is only one
+predefined statement: testing.Short:
 
-    skip_stmt='if testing.Short() { t.Skip("too slow for short run") }'
+    if testing.Short() {
+        t.Skip("too slow for testing.Short")
+    }
+
+Example - using a custom --skip-stmt:
+
+    skip_stmt='
+        if os.Getenv("TEST_FAST") {
+            t.Skip("too slow for TEST_FAST")
+        }
+    '
     go test -json -short ./... | %s --skip-stmt "$skip_stmt"
 
+Note that this tool does not add imports, so using a custom statement may require
+you to add any necessary imports to the file.
+
 Flags:
 `, name, name)
 		flags.PrintDefaults()