fix(decode): report the smallest unknown field deterministically

Assisted-by: GLM 5.3

interpres_test.go

@@ -5,6 +5,7 @@ package interpres
 
 import (
 	"math"
+	"strings"
 	"testing"
 	"time"
 )
@@ -316,6 +317,26 @@ func TestDisallowUnknownFields(t *testing.T) {
 	}
 }
 
+func TestDisallowUnknownFieldsReportsSmallestKey(t *testing.T) {
+	// Map iteration order is random, so the reported key must be chosen
+	// deterministically: the smallest unknown key, whichever order the map
+	// iterates in.
+	type C struct {
+		Known string `toml:"known"`
+	}
+	data := []byte("known = \"x\"\nzeta = 1\nalpha = 2\nmu = 3\n")
+	for range 20 {
+		var c C
+		err := NewDecoder().DisallowUnknownFields().Decode(data, &c)
+		if err == nil {
+			t.Fatal("expected error for unknown fields")
+		}
+		if !strings.Contains(err.Error(), `unknown field "alpha"`) {
+			t.Fatalf("err = %v, want the smallest unknown key alpha", err)
+		}
+	}
+}
+
 func TestSkippedFieldTag(t *testing.T) {
 	type C struct {
 		Keep string `toml:"keep"`