Added UnmarshalKeyExact() as well as non-method UnmarshalExact().

viper.go

@@ -768,6 +768,7 @@ func decode(input interface{}, config *mapstructure.DecoderConfig) error {
 
 // UnmarshalExact unmarshals the config into a Struct, erroring if a field is nonexistent
 // in the destination struct.
+func UnmarshalExact(rawVal interface{}) error { return v.UnmarshalExact(rawVal) }
 func (v *Viper) UnmarshalExact(rawVal interface{}) error {
 	config := defaultDecoderConfig(rawVal)
 	config.ErrorUnused = true
@@ -782,6 +783,23 @@ func (v *Viper) UnmarshalExact(rawVal interface{}) error {
 
 	return nil
 }
+// UnmarshalKeyExact takes a single key and unmarshals it into a Struct, erroring if a field
+// is nonexistent in the destination struct.
+func UnmarshalKeyExact(key string, rawVal interface{}) error { return v.UnmarshalKeyExact(key, rawVal) }
+func (v *Viper) UnmarshalKeyExact(key string, rawVal interface{}) error {
+	config := defaultDecoderConfig(rawVal)
+	config.ErrorUnused = true
+
+	err := decode(v.Get(key), config)
+
+	if err != nil {
+		return err
+	}
+
+	v.insensitiviseMaps()
+
+	return nil
+}
 
 // BindPFlags binds a full flag set to the configuration, using each flag's long
 // name as the config key.