support flag.Value interface in flags backend

backend/flags/flags.go

@@ -33,6 +33,16 @@ func (b *Backend) LoadStruct(ctx context.Context, cfg *confita.StructConfig) err
 			continue
 		}
 
+		// Check if value type implements flag.Value interface and process value accordingly
+		valuePtr := f.Value
+		if f.Value.Kind() != reflect.Ptr && f.Value.CanAddr() {
+			valuePtr = f.Value.Addr()
+		}
+		if iface, ok := valuePtr.Interface().(flag.Value); ok {
+			b.flags.Var(iface, f.Key, f.Description)
+			continue
+		}
+
 		// Display all the flags and their default values but override the field only if the user has explicitely
 		// set the flag.
 		k := f.Value.Kind()

backend/flags/flags_test.go

@@ -3,6 +3,7 @@ package flags
 import (
 	"context"
 	"flag"
+	"fmt"
 	"os"
 	"testing"
 	"time"
@@ -12,6 +13,36 @@ import (
 	"github.com/stretchr/testify/require"
 )
 
+type logLevel int
+
+var (
+	logLevelDebug logLevel = 1
+	logLevelInfo  logLevel = 2
+)
+
+func (l *logLevel) Set(val string) error {
+	switch val {
+	case "debug":
+		*l = logLevelDebug
+	case "info":
+		*l = logLevelInfo
+	default:
+		return fmt.Errorf("unknown log level: %s", val)
+	}
+	return nil
+}
+
+func (l logLevel) String() string {
+	switch l {
+	case logLevelDebug:
+		return "debug"
+	case logLevelInfo:
+		return "info"
+	default:
+		return "<unknown>"
+	}
+}
+
 func runHelper(t *testing.T, cfg interface{}, args ...string) {
 	t.Helper()
 
@@ -30,16 +61,18 @@ func TestFlags(t *testing.T) {
 			D int           `config:"d"`
 			E uint          `config:"e"`
 			F float32       `config:"f"`
+			G logLevel      `config:"g"`
 		}
 		var cfg config
-		runHelper(t, &cfg, "-a=hello", "-b=true", "-c=10s", "-d=-100", "-e=1", "-f=100.01")
+		runHelper(t, &cfg, "-a=hello", "-b=true", "-c=10s", "-d=-100", "-e=1", "-f=100.01", "-g=info")
 		require.Equal(t, config{
 			A: "hello",
 			B: true,
 			C: 10 * time.Second,
 			D: -100,
 			E: 1,
 			F: 100.01,
+			G: logLevelInfo,
 		}, cfg)
 	})
 
@@ -51,14 +84,16 @@ func TestFlags(t *testing.T) {
 			Ddef int           `config:"d-def,short=dd"`
 			Edef uint          `config:"e-def,short=ed"`
 			Fdef float32       `config:"f-def,short=fd"`
+			Gdef logLevel      `config:"g-def,short=gd"`
 		}
 		cfg := &config{
 			Adef: "hello",
 			Bdef: true,
 			Cdef: 10 * time.Second,
 			Ddef: -100,
+			Gdef: logLevelInfo,
 		}
-		runHelper(t, cfg, "-a-def=bye", "-b-def=false", "-c-def=15s", "-d-def=-200", "-e-def=400", "-f-def=2.33")
+		runHelper(t, cfg, "-a-def=bye", "-b-def=false", "-c-def=15s", "-d-def=-200", "-e-def=400", "-f-def=2.33", "-g-def=debug")
 
 		require.Equal(t, &config{
 			Adef: "bye",
@@ -67,6 +102,7 @@ func TestFlags(t *testing.T) {
 			Ddef: -200,
 			Edef: 400,
 			Fdef: 2.33,
+			Gdef: logLevelDebug,
 		}, cfg)
 	})
 }