fix: do not report types implementing (Un)Marshaler

builtins.go

@@ -3,7 +3,7 @@ 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.Marshal", Tag: "json", ArgPos: 0, ifaceWhitelist: []string{"Marshaler"}},
 	{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},

musttag.go

@@ -24,13 +24,25 @@ type Func struct {
 	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 interfaces from the same package;
+	// if at least one is implemented by the argument, no check is performed.
+	ifaceWhitelist []string
 }
 
 func (fn Func) shortName() string {
 	name := strings.NewReplacer("*", "", "(", "", ")", "").Replace(fn.Name)
 	return path.Base(name)
 }
 
+func (fn Func) pkgPath() string {
+	name := strings.NewReplacer("*", "", "(", "", ")", "").Replace(fn.Name)
+	if idx := strings.LastIndex(name, "."); idx != -1 {
+		return name[:idx]
+	}
+	return ""
+}
+
 // New creates a new musttag analyzer.
 // To report a custom function provide its description via Func,
 // it will be added to the builtin ones.
@@ -144,13 +156,38 @@ func run(pass *analysis.Pass, mainModule string, funcs map[string]Func) (any, er
 			initialPos = arg.Pos()
 		}
 
+		argType := pass.TypesInfo.TypeOf(arg)
+		if argType == nil {
+			return // no type info found.
+		}
+
+		for _, pkg := range pass.Pkg.Imports() {
+			if pkg.Path() != fn.pkgPath() {
+				continue
+			}
+			for _, ifaceName := range fn.ifaceWhitelist {
+				obj := pkg.Scope().Lookup(ifaceName)
+				if obj == nil {
+					continue
+				}
+				iface, ok := obj.Type().Underlying().(*types.Interface)
+				if !ok {
+					continue
+				}
+				if types.Implements(argType, iface) {
+					pass.Reportf(initialPos, "implements %s", iface)
+					return // the argument implements an (Un)Marshaler interface, no need to check; see issue #64.
+				}
+			}
+			break
+		}
+
 		checker := checker{
 			mainModule: mainModule,
 			seenTypes:  make(map[string]struct{}),
 		}
 
-		t := pass.TypesInfo.TypeOf(arg)
-		st, ok := checker.parseStructType(t, initialPos)
+		st, ok := checker.parseStructType(argType, initialPos)
 		if !ok {
 			return // not a struct argument.
 		}

testdata/src/tests/tests.go

@@ -94,3 +94,13 @@ func nothingToReport() {
 	json.NewEncoder(nil).Encode(Foo{})
 	json.NewDecoder(nil).Decode(&Foo{})
 }
+
+type marshaler struct{}
+
+func (marshaler) MarshalJSON() ([]byte, error) { return nil, nil }
+
+func implementsInterface() {
+	var m marshaler
+	json.Marshal(m)
+	json.Marshal(marshaler{})
+}