fix: align toToml error behavior with toYaml and toJson (closes #31430)

toToml was returning err.Error() on marshal failure, while toYaml and
toJson return an empty string. This caused confusing template output
when marshaling fails (e.g. a struct with cyclic references).

Changes:
- toTOML now returns "" on error, consistent with toYAML and toJSON
- Added mustToTOML which panics on error, consistent with mustToYAML
  and mustToJSON (allows templates to opt-in to strict error handling)
- Registered mustToToml in funcMap
- Added test cases for toToml error swallowing and mustToToml panic

Signed-off-by: Aliipou <Alipourrahim.ap@gmail.com>

pkg/engine/engine_test.go

@@ -83,7 +83,7 @@ func TestFuncMap(t *testing.T) {
 	}
 
 	// Test for Engine-specific template functions.
-	expect := []string{"include", "required", "tpl", "toYaml", "fromYaml", "toToml", "fromToml", "toJson", "fromJson", "lookup"}
+	expect := []string{"include", "required", "tpl", "toYaml", "fromYaml", "toToml", "mustToToml", "fromToml", "toJson", "fromJson", "lookup"}
 	for _, f := range expect {
 		if _, ok := fns[f]; !ok {
 			t.Errorf("Expected add-on function %q", f)

pkg/engine/funcs.go

@@ -50,6 +50,7 @@ func funcMap() template.FuncMap {
 	// Add some extra functionality
 	extra := template.FuncMap{
 		"toToml":        toTOML,
+		"mustToToml":    mustToTOML,
 		"fromToml":      fromTOML,
 		"toYaml":        toYAML,
 		"mustToYaml":    mustToYAML,
@@ -157,7 +158,23 @@ func toTOML(v any) string {
 	e := toml.NewEncoder(b)
 	err := e.Encode(v)
 	if err != nil {
-		return err.Error()
+		// Swallow errors inside of a template.
+		return ""
+	}
+	return b.String()
+}
+
+// mustToTOML takes an interface, marshals it to toml, and returns a string.
+// It will panic if there is an error.
+//
+// This is designed to be called from a template when need to ensure that the
+// output TOML is valid.
+func mustToTOML(v any) string {
+	b := bytes.NewBuffer(nil)
+	e := toml.NewEncoder(b)
+	err := e.Encode(v)
+	if err != nil {
+		panic(err)
 	}
 	return b.String()
 }

pkg/engine/funcs_test.go

@@ -151,6 +151,9 @@ keyInElement1 = "valueInElement1"`,
 	}, {
 		tpl:  `{{ mustToJson . }}`,
 		vars: loopMap,
+	}, {
+		tpl:  `{{ mustToToml . }}`,
+		vars: loopMap,
 	}, {
 		tpl:    `{{ toYaml . }}`,
 		expect: "", // should return empty string and swallow error
@@ -159,6 +162,10 @@ keyInElement1 = "valueInElement1"`,
 		tpl:    `{{ toJson . }}`,
 		expect: "", // should return empty string and swallow error
 		vars:   loopMap,
+	}, {
+		tpl:    `{{ toToml . }}`,
+		expect: "", // should return empty string and swallow error
+		vars:   loopMap,
 	},
 	}