Handle case where user accidentally uses Process() with a Config when…

envconfig.go

@@ -264,13 +264,18 @@ type Config struct {
 	// default value is false.
 	DefaultRequired bool
 
-	// Mutators is an optiona list of mutators to apply to lookups.
+	// Mutators is an optional list of mutators to apply to lookups.
 	Mutators []Mutator
 }
 
 // Process decodes the struct using values from environment variables. See
-// [ProcessWith] for a more customizable version.
+// [ProcessWith] for a more customizable version. If *Config is provided for i,
+// [ProcessWith] is called using the *Config with mus appended.
 func Process(ctx context.Context, i any, mus ...Mutator) error {
+	if v, ok := i.(*Config); ok {
+		v.Mutators = append(v.Mutators, mus...)
+		return ProcessWith(ctx, v)
+	}
 	return ProcessWith(ctx, &Config{
 		Target:   i,
 		Mutators: mus,
@@ -529,7 +534,7 @@ func processWith(ctx context.Context, c *Config) error {
 
 // SplitString splits the given string on the provided rune, unless the rune is
 // escaped by the escape character.
-func splitString(s string, on string, esc string) []string {
+func splitString(s, on, esc string) []string {
 	a := strings.Split(s, on)
 
 	for i := len(a) - 2; i >= 0; i-- {

envconfig_test.go

@@ -229,6 +229,45 @@ type MapStruct struct {
 
 type Base64ByteSlice []Base64Bytes
 
+func TestProcessWithConfigArgument(t *testing.T) {
+	t.Parallel()
+
+	ctx := context.Background()
+	lookuper := MapLookuper(map[string]string{
+		"MEAT_TYPE": "chicken",
+	})
+
+	firstMutator := MutatorFunc(func(ctx context.Context, originalKey, resolvedKey, originalValue, currentValue string) (newValue string, stop bool, err error) {
+		if originalKey == "MEAT_TYPE" && currentValue == "chicken" {
+			return "pork", false, nil
+		}
+		return currentValue, false, nil
+	})
+
+	secondMutator := MutatorFunc(func(ctx context.Context, originalKey, resolvedKey, originalValue, currentValue string) (newValue string, stop bool, err error) {
+		if originalKey == "MEAT_TYPE" && currentValue == "pork" {
+			return "fish", false, nil
+		}
+		return currentValue, false, nil
+	})
+
+	var meatConfig Meat
+	cfg := &Config{
+		Target:   &meatConfig,
+		Lookuper: lookuper,
+		Mutators: []Mutator{firstMutator},
+	}
+
+	err := Process(ctx, cfg, secondMutator)
+	if err != nil {
+		t.Errorf("unexpected error from Process: %s", err.Error())
+	}
+
+	if diff := cmp.Diff(meatConfig.Type, "fish"); diff != "" {
+		t.Errorf("wrong value in meatConfig. Diff (-got +want): %s", diff)
+	}
+}
+
 func TestProcessWith(t *testing.T) {
 	t.Parallel()