fix: do not report types implementing (Un)Marshaler

builtins.go

@@ -3,34 +3,144 @@ package musttag
 // builtins is a set of functions supported out of the box.
 var builtins = []Func{
 	// https://pkg.go.dev/encoding/json
-	{Name: "encoding/json.Marshal", Tag: "json", ArgPos: 0},
-	{Name: "encoding/json.MarshalIndent", Tag: "json", ArgPos: 0},
-	{Name: "encoding/json.Unmarshal", Tag: "json", ArgPos: 1},
-	{Name: "(*encoding/json.Encoder).Encode", Tag: "json", ArgPos: 0},
-	{Name: "(*encoding/json.Decoder).Decode", Tag: "json", ArgPos: 0},
+	{
+		Name:           "encoding/json.Marshal",
+		Tag:            "json",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/json.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "encoding/json.MarshalIndent",
+		Tag:            "json",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/json.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "encoding/json.Unmarshal",
+		Tag:            "json",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"encoding/json.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "(*encoding/json.Encoder).Encode",
+		Tag:            "json",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/json.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "(*encoding/json.Decoder).Decode",
+		Tag:            "json",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/json.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
 
 	// https://pkg.go.dev/encoding/xml
-	{Name: "encoding/xml.Marshal", Tag: "xml", ArgPos: 0},
-	{Name: "encoding/xml.MarshalIndent", Tag: "xml", ArgPos: 0},
-	{Name: "encoding/xml.Unmarshal", Tag: "xml", ArgPos: 1},
-	{Name: "(*encoding/xml.Encoder).Encode", Tag: "xml", ArgPos: 0},
-	{Name: "(*encoding/xml.Decoder).Decode", Tag: "xml", ArgPos: 0},
-	{Name: "(*encoding/xml.Encoder).EncodeElement", Tag: "xml", ArgPos: 0},
-	{Name: "(*encoding/xml.Decoder).DecodeElement", Tag: "xml", ArgPos: 0},
+	{
+		Name:           "encoding/xml.Marshal",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "encoding/xml.MarshalIndent",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "encoding/xml.Unmarshal",
+		Tag:            "xml",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"encoding/xml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "(*encoding/xml.Encoder).Encode",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "(*encoding/xml.Decoder).Decode",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "(*encoding/xml.Encoder).EncodeElement",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Marshaler", "encoding.TextMarshaler"},
+	},
+	{
+		Name:           "(*encoding/xml.Decoder).DecodeElement",
+		Tag:            "xml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding/xml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
 
 	// https://github.com/go-yaml/yaml
-	{Name: "gopkg.in/yaml.v3.Marshal", Tag: "yaml", ArgPos: 0},
-	{Name: "gopkg.in/yaml.v3.Unmarshal", Tag: "yaml", ArgPos: 1},
-	{Name: "(*gopkg.in/yaml.v3.Encoder).Encode", Tag: "yaml", ArgPos: 0},
-	{Name: "(*gopkg.in/yaml.v3.Decoder).Decode", Tag: "yaml", ArgPos: 0},
+	{
+		Name:           "gopkg.in/yaml.v3.Marshal",
+		Tag:            "yaml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"gopkg.in/yaml.v3.Marshaler"},
+	},
+	{
+		Name:           "gopkg.in/yaml.v3.Unmarshal",
+		Tag:            "yaml",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"gopkg.in/yaml.v3.Unmarshaler"},
+	},
+	{
+		Name:           "(*gopkg.in/yaml.v3.Encoder).Encode",
+		Tag:            "yaml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"gopkg.in/yaml.v3.Marshaler"},
+	},
+	{
+		Name:           "(*gopkg.in/yaml.v3.Decoder).Decode",
+		Tag:            "yaml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"gopkg.in/yaml.v3.Unmarshaler"},
+	},
 
 	// https://github.com/BurntSushi/toml
-	{Name: "github.com/BurntSushi/toml.Unmarshal", Tag: "toml", ArgPos: 1},
-	{Name: "github.com/BurntSushi/toml.Decode", Tag: "toml", ArgPos: 1},
-	{Name: "github.com/BurntSushi/toml.DecodeFS", Tag: "toml", ArgPos: 2},
-	{Name: "github.com/BurntSushi/toml.DecodeFile", Tag: "toml", ArgPos: 1},
-	{Name: "(*github.com/BurntSushi/toml.Encoder).Encode", Tag: "toml", ArgPos: 0},
-	{Name: "(*github.com/BurntSushi/toml.Decoder).Decode", Tag: "toml", ArgPos: 0},
+	{
+		Name:           "github.com/BurntSushi/toml.Unmarshal",
+		Tag:            "toml",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"github.com/BurntSushi/toml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "github.com/BurntSushi/toml.Decode",
+		Tag:            "toml",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"github.com/BurntSushi/toml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "github.com/BurntSushi/toml.DecodeFS",
+		Tag:            "toml",
+		ArgPos:         2,
+		ifaceWhitelist: []string{"github.com/BurntSushi/toml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "github.com/BurntSushi/toml.DecodeFile",
+		Tag:            "toml",
+		ArgPos:         1,
+		ifaceWhitelist: []string{"github.com/BurntSushi/toml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
+	{
+		Name:           "(*github.com/BurntSushi/toml.Encoder).Encode",
+		Tag:            "toml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"encoding.TextMarshaler"},
+	},
+	{
+		Name:           "(*github.com/BurntSushi/toml.Decoder).Decode",
+		Tag:            "toml",
+		ArgPos:         0,
+		ifaceWhitelist: []string{"github.com/BurntSushi/toml.Unmarshaler", "encoding.TextUnmarshaler"},
+	},
 
 	// https://github.com/mitchellh/mapstructure
 	{Name: "github.com/mitchellh/mapstructure.Decode", Tag: "mapstructure", ArgPos: 1},

musttag.go

@@ -24,6 +24,10 @@
 	Name   string // Name is the full name of the function, including the package.
 	Tag    string // Tag is the struct tag whose presence should be ensured.
 	ArgPos int    // ArgPos is the position of the argument to check.
+
+	// a list of interface names (including the package);
+	// if at least one is implemented by the argument, no check is performed.
+	ifaceWhitelist []string
 }
 
 func (fn Func) shortName() string {
@@ -93,7 +97,7 @@
 	pass.Reportf(st.Pos, format, st.Name, fn.Tag, fn.shortName(), fnPos)
 }
 
-var cleanFullName = regexp.MustCompile(`([^*/(]+/vendor/)`)
+var trimVendor = regexp.MustCompile(`([^*/(]+/vendor/)`)
 
 // run starts the analysis.
 func run(pass *analysis.Pass, mainModule string, funcs map[string]Func) (any, error) {
@@ -117,7 +121,7 @@
 			return // not a static call.
 		}
 
-		name := cleanFullName.ReplaceAllString(callee.FullName(), "")
+		name := trimVendor.ReplaceAllString(callee.FullName(), "")
 		fn, ok := funcs[name]
 		if !ok {
 			return // the function is not supported.
@@ -144,13 +148,21 @@
 			initialPos = arg.Pos()
 		}
 
+		typ := pass.TypesInfo.TypeOf(arg)
+		if typ == nil {
+			return // no type info found.
+		}
+
+		if implementsInterface(typ, fn.ifaceWhitelist, pass.Pkg.Imports()) {
+			return // the type implements a Marshaler interface, nothing to check; see issue #64.
+		}
+
 		checker := checker{
 			mainModule: mainModule,
 			seenTypes:  make(map[string]struct{}),
 		}
 
-		t := pass.TypesInfo.TypeOf(arg)
-		st, ok := checker.parseStructType(t, initialPos)
+		st, ok := checker.parseStructType(typ, initialPos)
 		if !ok {
 			return // not a struct argument.
 		}
@@ -257,3 +269,50 @@
 
 	return nil, true
 }
+
+func implementsInterface(typ types.Type, ifaces []string, imports []*types.Package) bool {
+	findScope := func(pkgName string) (*types.Scope, bool) {
+		// fast path: check direct imports (e.g. looking for "encoding/json.Marshaler").
+		for _, direct := range imports {
+			if pkgName == trimVendor.ReplaceAllString(direct.Path(), "") {
+				return direct.Scope(), true
+			}
+		}
+		// slow path: check indirect imports (e.g. looking for "encoding.TextMarshaler").
+		for _, direct := range imports {
+			for _, indirect := range direct.Imports() {
+				if pkgName == trimVendor.ReplaceAllString(indirect.Path(), "") {
+					return indirect.Scope(), true
+				}
+			}
+		}
+		return nil, false
+	}
+
+	for _, ifacePath := range ifaces {
+		// "encoding/json.Marshaler" -> "encoding/json" + "Marshaler"
+		idx := strings.LastIndex(ifacePath, ".")
+		if idx == -1 {
+			continue
+		}
+		pkgName, ifaceName := ifacePath[:idx], ifacePath[idx+1:]
+
+		scope, ok := findScope(pkgName)
+		if !ok {
+			continue
+		}
+		obj := scope.Lookup(ifaceName)
+		if obj == nil {
+			continue
+		}
+		iface, ok := obj.Type().Underlying().(*types.Interface)
+		if !ok {
+			continue
+		}
+		if types.Implements(typ, iface) {
+			return true
+		}
+	}
+
+	return false
+}

testdata/src/builtins/builtins.go

@@ -76,13 +76,44 @@ type User struct { /* want
 	Email string `json:"email" xml:"email" yaml:"email" toml:"email" mapstructure:"email" db:"email" custom:"email"`
 }
 
+// TODO: Unmarshaler should be implemented using pointer semantics.
+
+type TextMarshaler struct{ NoTag string }
+
+func (TextMarshaler) MarshalText() ([]byte, error) { return nil, nil }
+func (TextMarshaler) UnmarshalText([]byte) error   { return nil }
+
+type Marshaler struct{ NoTag string }
+
+func (Marshaler) MarshalJSON() ([]byte, error)                              { return nil, nil }
+func (Marshaler) UnmarshalJSON([]byte) error                                { return nil }
+func (Marshaler) MarshalXML(e *xml.Encoder, start xml.StartElement) error   { return nil }
+func (Marshaler) UnmarshalXML(d *xml.Decoder, start xml.StartElement) error { return nil }
+func (Marshaler) MarshalYAML() (any, error)                                 { return nil, nil }
+func (Marshaler) UnmarshalYAML(*yaml.Node) error                            { return nil }
+func (Marshaler) UnmarshalTOML(any) error                                   { return nil }
+
 func testJSON() {
 	var user User
 	json.Marshal(user)
 	json.MarshalIndent(user, "", "")
 	json.Unmarshal(nil, &user)
 	json.NewEncoder(nil).Encode(user)
 	json.NewDecoder(nil).Decode(&user)
+
+	var m Marshaler
+	json.Marshal(m)
+	json.MarshalIndent(m, "", "")
+	json.Unmarshal(nil, &m)
+	json.NewEncoder(nil).Encode(m)
+	json.NewDecoder(nil).Decode(&m)
+
+	var tm TextMarshaler
+	json.Marshal(tm)
+	json.MarshalIndent(tm, "", "")
+	json.Unmarshal(nil, &tm)
+	json.NewEncoder(nil).Encode(tm)
+	json.NewDecoder(nil).Decode(&tm)
 }
 
 func testXML() {
@@ -94,6 +125,24 @@ func testXML() {
 	xml.NewDecoder(nil).Decode(&user)
 	xml.NewEncoder(nil).EncodeElement(user, xml.StartElement{})
 	xml.NewDecoder(nil).DecodeElement(&user, &xml.StartElement{})
+
+	var m Marshaler
+	xml.Marshal(m)
+	xml.MarshalIndent(m, "", "")
+	xml.Unmarshal(nil, &m)
+	xml.NewEncoder(nil).Encode(m)
+	xml.NewDecoder(nil).Decode(&m)
+	xml.NewEncoder(nil).EncodeElement(m, xml.StartElement{})
+	xml.NewDecoder(nil).DecodeElement(&m, &xml.StartElement{})
+
+	var tm TextMarshaler
+	xml.Marshal(tm)
+	xml.MarshalIndent(tm, "", "")
+	xml.Unmarshal(nil, &tm)
+	xml.NewEncoder(nil).Encode(tm)
+	xml.NewDecoder(nil).Decode(&tm)
+	xml.NewEncoder(nil).EncodeElement(tm, xml.StartElement{})
+	xml.NewDecoder(nil).DecodeElement(&tm, &xml.StartElement{})
 }
 
 func testYAML() {
@@ -102,6 +151,12 @@ func testYAML() {
 	yaml.Unmarshal(nil, &user)
 	yaml.NewEncoder(nil).Encode(user)
 	yaml.NewDecoder(nil).Decode(&user)
+
+	var m Marshaler
+	yaml.Marshal(m)
+	yaml.Unmarshal(nil, &m)
+	yaml.NewEncoder(nil).Encode(m)
+	yaml.NewDecoder(nil).Decode(&m)
 }
 
 func testTOML() {
@@ -112,6 +167,21 @@ func testTOML() {
 	toml.DecodeFile("", &user)
 	toml.NewEncoder(nil).Encode(user)
 	toml.NewDecoder(nil).Decode(&user)
+
+	var m Marshaler
+	toml.Unmarshal(nil, &m)
+	toml.Decode("", &m)
+	toml.DecodeFS(nil, "", &m)
+	toml.DecodeFile("", &m)
+	toml.NewDecoder(nil).Decode(&m)
+
+	var tm TextMarshaler
+	toml.Unmarshal(nil, &tm)
+	toml.Decode("", &tm)
+	toml.DecodeFS(nil, "", &tm)
+	toml.DecodeFile("", &tm)
+	toml.NewEncoder(nil).Encode(tm)
+	toml.NewDecoder(nil).Decode(&tm)
 }
 
 func testMapstructure() {