Fix false positives where errors are returned

Fixes: #5

errchkjson.go

@@ -43,6 +43,12 @@ func (e *errchkjson) run(pass *analysis.Pass) (interface{}, error) {
 				return true
 			}
 
+			// if the error is returned, it is the caller's responsibility to check
+			// the return value.
+			if _, ok := n.(*ast.ReturnStmt); ok {
+				return false
+			}
+
 			ce, ok := n.(*ast.CallExpr)
 			if ok {
 				fn, _ := typeutil.Callee(pass.TypesInfo, ce).(*types.Func)

testdata/src/nosafe/a.go

@@ -612,3 +612,12 @@ func NotJSONMarshal() {
 	f := func() bool { return false }
 	_ = f()
 }
+
+// Issue 5
+type T struct {
+	s string
+}
+
+func (t T) MarshalJSON() ([]byte, error) {
+	return json.Marshal(t.s) // not an error because it is the caller's responsibility to check the error
+}

testdata/src/standard/a.go

@@ -612,3 +612,12 @@ func NotJSONMarshal() {
 	f := func() bool { return false }
 	_ = f()
 }
+
+// Issue 5
+type T struct {
+	f64 float64
+}
+
+func (t T) MarshalJSON() ([]byte, error) {
+	return json.Marshal(t.f64) // not an error because it is the caller's responsibility to check the error
+}