Add custom tag enc/dec test

_generated/custom_tag.go

@@ -0,0 +1,9 @@
+package _generated
+
+//go:generate msgp
+//msgp:tag mytag
+
+type CustomTag struct {
+	Foo string `mytag:"foo_custom_name"`
+	Bar int    `mytag:"bar1234"`
+}

_generated/custom_tag_test.go

@@ -0,0 +1,64 @@
+package _generated
+
+import (
+	"encoding/json"
+	"fmt"
+	"reflect"
+	"testing"
+
+	"bytes"
+
+	"github.com/tinylib/msgp/msgp"
+)
+
+func TestCustomTag(t *testing.T) {
+	t.Run("File Scope", func(t *testing.T) {
+		ts := CustomTag{
+			Foo: "foostring13579",
+			Bar: 999_999}
+		encDecCustomTag(t, ts, "mytag")
+	})
+}
+
+func encDecCustomTag(t *testing.T, testStruct msgp.Encodable, tag string) {
+	var b bytes.Buffer
+	msgp.Encode(&b, testStruct)
+
+	// Check tag names using JSON as an intermediary layer
+	// TODO: is there a way to avoid the JSON layer? We'd need to directly decode raw msgpack -> map[string]any
+	refJSON, err := json.Marshal(testStruct)
+	if err != nil {
+		t.Error(fmt.Sprintf("error encoding struct as JSON: %v", err))
+	}
+	ref := make(map[string]any)
+	// Encoding and decoding the original struct via JSON is necessary
+	// for field comparisons to work, since JSON -> map[string]any
+	// relies on type inferences such as all numbers being float64s
+	json.Unmarshal(refJSON, &ref)
+
+	var encJSON bytes.Buffer
+	msgp.UnmarshalAsJSON(&encJSON, b.Bytes())
+	encoded := make(map[string]any)
+	json.Unmarshal(encJSON.Bytes(), &encoded)
+
+	tsType := reflect.TypeOf(testStruct)
+	for i := 0; i < tsType.NumField(); i++ {
+		// Check encoded field name
+		field := tsType.Field(i)
+		encodedValue, ok := encoded[field.Tag.Get(tag)]
+		if !ok {
+			t.Error("missing encoded value for field", field.Name)
+			continue
+		}
+		// Check encoded field value (against original value post-JSON enc + dec)
+		jsonName, ok := field.Tag.Lookup("json")
+		if !ok {
+			jsonName = field.Name
+		}
+		refValue := ref[jsonName]
+		if !reflect.DeepEqual(refValue, encodedValue) {
+			t.Error(fmt.Sprintf("incorrect encoded value for field %s. reference: %v, encoded: %v",
+				field.Name, refValue, encodedValue))
+		}
+	}
+}