gotohelm: improve special case detection in zeroOf

Prior to this commit gotohelm would incorrectly attempt to compute the zero
value of types that implemented `json.{Unm,M}arshaller`. In most cases, this
would lead to a transpiler crash all together. In some corner cases, it was
possible to silently generate incorrect values.

This commit improves `zeroOf` to detect and reject implementers of
`json.{Unm,M}arshaller` with a helpful error message and adds a special case
for Kubernetes' `resource.Quantity` type.

pkg/gotohelm/testdata/src/example/k8s/k8s.go

@@ -181,9 +181,16 @@ func quantity(dot *helmette.Dot) map[string]any {
 		value = append(value, q.Value())
 	}
 
+	// Intentionally generate zero values of resource.Quantity to assert that
+	// zeroOf handles it.
+	var varZero resource.Quantity
+	resources := corev1.ResourceList{}
+
 	return map[string]any{
 		"MustParse": quantities,
 		"Value":     value,
 		"String":    strs,
+		"dictZero":  resources[corev1.ResourceCPU],
+		"varZero":   varZero,
 	}
 }

pkg/gotohelm/testdata/src/example/k8s/k8s.rewritten.go

@@ -182,9 +182,16 @@ func quantity(dot *helmette.Dot) map[string]any {
 		value = append(value, q.Value())
 	}
 
+	// Intentionally generate zero values of resource.Quantity to assert that
+	// zeroOf handles it.
+	var varZero resource.Quantity
+	resources := corev1.ResourceList{}
+
 	return map[string]any{
 		"MustParse": quantities,
 		"Value":     value,
 		"String":    strs,
+		"dictZero":  resources[corev1.ResourceCPU],
+		"varZero":   varZero,
 	}
 }

pkg/gotohelm/testdata/src/example/k8s/k8s.yaml

@@ -90,8 +90,10 @@
 {{- if $_is_returning -}}
 {{- break -}}
 {{- end -}}
+{{- $varZero := "0" -}}
+{{- $resources := (dict ) -}}
 {{- $_is_returning = true -}}
-{{- (dict "r" (dict "MustParse" $quantities "Value" $value "String" $strs )) | toJson -}}
+{{- (dict "r" (dict "MustParse" $quantities "Value" $value "String" $strs "dictZero" (ternary (index $resources "cpu") "0" (hasKey $resources "cpu")) "varZero" $varZero )) | toJson -}}
 {{- break -}}
 {{- end -}}
 {{- end -}}

pkg/gotohelm/transpiler.go

@@ -1459,9 +1459,6 @@ func (t *Transpiler) typeOf(expr ast.Expr) types.Type {
 }
 
 func (t *Transpiler) zeroOf(typ types.Type) Node {
-	// TODO need to detect and reject or special case implementors of
-	// json.Marshaler. Getting a handle to a that interface is... difficult.
-
 	// Special cases.
 	switch typ.String() {
 	case "k8s.io/apimachinery/pkg/apis/meta/v1.Time":
@@ -1470,6 +1467,30 @@ func (t *Transpiler) zeroOf(typ types.Type) Node {
 		// IntOrString's zero value appears to marshal to a 0 though it's
 		// unclear how correct this is.
 		return &Literal{Value: "0"}
+	case "k8s.io/apimachinery/pkg/api/resource.Quantity":
+		return &Literal{Value: `"0"`}
+	}
+
+	// If encoding/json is in the dependency chain for this package, we'll
+	// enable some additional checks (because it's other wise very difficult to
+	// check for implementation of {M,Unm}arshaller...)
+	if json, ok := t.packages["encoding/json"]; ok {
+		marshaller := json.Types.Scope().Lookup("Marshaler").Type().Underlying().(*types.Interface)
+		unmarshaller := json.Types.Scope().Lookup("Unmarshaler").Type().Underlying().(*types.Interface)
+
+		ptr := types.NewPointer(typ)
+
+		// If we can get a handle to Marshaler and Unmarshaler, we'll error out
+		// on any types that implement either and aren't special cased as their
+		// JSON representation likely won't match what we can infer from the go
+		// type.
+		switch {
+		case types.Implements(typ, unmarshaller),
+			types.Implements(typ, marshaller),
+			types.Implements(ptr, marshaller),
+			types.Implements(ptr, unmarshaller):
+			panic(fmt.Sprintf("unsupported type %q implemented json.Unmarshaler or json.Marshaller and needs to be special cased but isn't currently", typ.String()))
+		}
 	}
 
 	switch underlying := typ.Underlying().(type) {