fix(bindnode): more helpful error message for enum value footgun

Fixes: #348

node/bindnode/api_test.go

@@ -0,0 +1,38 @@
+package bindnode_test
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+	"github.com/ipld/go-ipld-prime"
+	"github.com/ipld/go-ipld-prime/codec/dagcbor"
+	"github.com/ipld/go-ipld-prime/node/bindnode"
+)
+
+func TestEnumError(t *testing.T) {
+	type Action string
+	const (
+		ActionPresent = Action("p")
+		ActionMissing = Action("m")
+	)
+	type S struct{ Action Action }
+
+	schema := `
+		type S struct {
+			Action Action
+		} representation tuple
+		type Action enum {
+			| Present             ("p")
+			| Missing             ("m")
+		} representation string
+ 	`
+
+	typeSystem, err := ipld.LoadSchemaBytes([]byte(schema))
+	qt.Assert(t, err, qt.IsNil)
+	schemaType := typeSystem.TypeByName("S")
+
+	node := bindnode.Wrap(&S{Action: ActionPresent}, schemaType).Representation()
+	_, err = ipld.Encode(node, dagcbor.Encode)
+	qt.Assert(t, err, qt.IsNotNil)
+	qt.Assert(t, err.Error(), qt.Equals, `AsString: "p" is not a valid member of enum Action (bindnode works at the type level; did you mean "Present"?)`)
+}

node/bindnode/repr.go

@@ -488,6 +488,12 @@ func (w *_nodeRepr) AsString() (string, error) {
 				return s, nil
 			}
 		}
+		for k, v := range stg {
+			// a programming error? we may have the enum string value rather than the type
+			if v == s {
+				return "", fmt.Errorf("AsString: %q is not a valid member of enum %s (bindnode works at the type level; did you mean %q?)", s, w.schemaType.Name(), k)
+			}
+		}
 		return "", fmt.Errorf("AsString: %q is not a valid member of enum %s", s, w.schemaType.Name())
 	default:
 		return (*_node)(w).AsString()