go: small refactoring for maps (#2619)

entc/gen/func.go

@@ -246,10 +246,10 @@ type graphScope struct {
 //	{{ end}}
 //
 func extend(v interface{}, kv ...interface{}) (interface{}, error) {
-	scope := make(map[interface{}]interface{})
 	if len(kv)%2 != 0 {
 		return nil, fmt.Errorf("invalid number of parameters: %d", len(kv))
 	}
+	scope := make(map[interface{}]interface{}, len(kv)/2)
 	for i := 0; i < len(kv); i += 2 {
 		scope[kv[i]] = kv[i+1]
 	}

schema/field/annotation.go

@@ -63,10 +63,10 @@ func (a Annotation) Merge(other schema.Annotation) schema.Annotation {
 	default:
 		return a
 	}
+	if a.StructTag == nil && len(ant.StructTag) > 0 {
+		a.StructTag = make(map[string]string, len(ant.StructTag))
+	}
 	for k, v := range ant.StructTag {
-		if a.StructTag == nil {
-			a.StructTag = make(map[string]string)
-		}
 		a.StructTag[k] = v
 	}
 	if len(ant.ID) > 0 {