feat: add support for inline struct tag option

Author: Quentin-M

encode_test.go

@@ -1703,6 +1703,64 @@ func TestAnonymousFields(t *testing.T) {
 			return S{}
 		},
 		want: `{}`,
+	}, {
+		// Structs with an inline tag option should be embedded.
+		label: "InlinedStruct",
+		makeInput: func() interface{} {
+			type (
+				S2 struct{ X int }
+				S  struct {
+					S2 `json:",inline"`
+				}
+			)
+			return S{S2: S2{X: 1}}
+		},
+		want: `{"X":1}`,
+	}, {
+		// Pointers to structs with an inline tag option should be embedded.
+		label: "InlinedStructPointer",
+		makeInput: func() interface{} {
+			type (
+				S2 struct{ X int }
+				S  struct {
+					*S2 `json:",inline"`
+				}
+			)
+			return S{S2: &S2{X: 1}}
+		},
+		want: `{"X":1}`,
+	}, {
+		// Non-anymous generic struct with an inline tag option should
+		// be embedded.
+		label: "InlinedStructGeneric",
+		makeInput: func() interface{} {
+			type (
+				S1 struct {
+					X int
+				}
+				S2 struct {
+					Y int
+				}
+				S[T any] struct {
+					Embedded T `json:",inline"`
+					S2
+				}
+			)
+			return S[S1]{Embedded: S1{X: 1}, S2: S2{Y: 2}}
+		},
+		want: `{"X":1,"Y":2}`,
+	}, {
+		// Non-struct with an inline tag option should be serialized
+		label: "InlinedStructPointer",
+		makeInput: func() interface{} {
+			type (
+				S struct {
+					X int `json:",inline"`
+				}
+			)
+			return S{X: 1}
+		},
+		want: `{"X":1}`,
 	}}
 
 	for i, tt := range tests {

internal/encoder/code.go

@@ -1015,6 +1015,9 @@ func isEmbeddedStruct(field *StructFieldCode) bool {
 	if !field.isAnonymous {
 		return false
 	}
+	if field.tag.IsInline {
+		return true
+	}
 	t := field.typ
 	if t.Kind() == reflect.Ptr {
 		t = t.Elem()

internal/encoder/compiler.go

@@ -637,7 +637,7 @@ func (c *Compiler) structFieldCode(structCode *StructCode, tag *runtime.StructTa
 		key:           tag.Key,
 		tag:           tag,
 		offset:        field.Offset,
-		isAnonymous:   field.Anonymous && !tag.IsTaggedKey && toElemType(fieldType).Kind() == reflect.Struct,
+		isAnonymous:   (field.Anonymous || tag.IsInline) && !tag.IsTaggedKey && toElemType(fieldType).Kind() == reflect.Struct,
 		isTaggedKey:   tag.IsTaggedKey,
 		isNilableType: c.isNilableType(fieldType),
 		isNilCheck:    true,

internal/runtime/struct_field.go

@@ -31,6 +31,7 @@ func IsIgnoredStructField(field reflect.StructField) bool {
 
 type StructTag struct {
 	Key         string
+	IsInline    bool
 	IsTaggedKey bool
 	IsOmitEmpty bool
 	IsString    bool
@@ -84,6 +85,8 @@ func StructTagFromField(field reflect.StructField) *StructTag {
 				st.IsOmitEmpty = true
 			case "string":
 				st.IsString = true
+			case "inline":
+				st.IsInline = true
 			}
 		}
 	}