[WIP] feat: use json-iterator replace encoding/json for performance

generate.go

@@ -52,7 +52,7 @@ func genUnmarshalJSON(mtyp *marshalerType) Function {
 		input    = Name(m.scope.newIdent("input"))
 		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		dec      = Name(m.scope.newIdent("dec"))
-		json     = Name(m.scope.parent.packageName("encoding/json"))
+		json     = Name("jsonIter")
 	)
 	fn := Function{
 		Receiver:    recv,
@@ -80,7 +80,7 @@ func genMarshalJSON(mtyp *marshalerType) Function {
 		recv     = m.receiver()
 		intertyp = m.intermediateType(m.scope.newIdent(m.mtyp.orig.Obj().Name()))
 		enc      = Name(m.scope.newIdent("enc"))
-		json     = Name(m.scope.parent.packageName("encoding/json"))
+		json = Name("jsonIter")
 	)
 	fn := Function{
 		Receiver:    recv,

main.go

@@ -179,6 +179,7 @@ import (
 	"io"
 	"os"
 	"reflect"
+	"slices"
 	"strings"
 
 	"github.com/garslo/gogen"
@@ -298,6 +299,12 @@ func generate(mtyp *marshalerType, cfg *Config) ([]byte, error) {
 	fmt.Fprintln(w)
 	mtyp.scope.writeImportDecl(w)
 	fmt.Fprintln(w)
+
+	if slices.Contains(cfg.Formats, "json") {
+		fmt.Fprintln(w, "var jsonIter = jsoniter.ConfigCompatibleWithStandardLibrary")
+		fmt.Fprintln(w)
+	}
+
 	if mtyp.override != nil {
 		writeUseOfOverride(w, mtyp.override, mtyp.scope.qualify)
 	}
@@ -360,7 +367,7 @@ func newMarshalerType(fs *token.FileSet, imp types.Importer, typ *types.Named) *
 	mtyp.scope.addReferences(styp)
 
 	// Add packages which are always needed.
-	mtyp.scope.addImport("encoding/json")
+	mtyp.scope.addImport("github.com/json-iterator/go")
 	mtyp.scope.addImport("errors")
 
 	for i := 0; i < styp.NumFields(); i++ {

types_util.go

@@ -9,8 +9,11 @@ import (
 	"fmt"
 	"go/types"
 	"io"
+	"os"
 	"sort"
 	"strconv"
+
+	"golang.org/x/tools/go/packages"
 )
 
 // walkNamedTypes runs the callback for all named types contained in the given type.
@@ -215,6 +218,20 @@ func (s *fileScope) writeImportDecl(w io.Writer) {
 // addImport loads a package and adds it to the import set.
 func (s *fileScope) addImport(path string) {
 	pkg, err := s.imp.Import(path)
+	if err != nil {
+		// Fallback to module-aware importer via go/packages
+		cfg := &packages.Config{
+			Mode: packages.NeedTypes | packages.NeedImports | packages.NeedDeps,
+			// Preserve build environment so that go/packages can resolve modules.
+			Env:   os.Environ(),
+			Tests: false,
+		}
+		pkgs, perr := packages.Load(cfg, path)
+		if perr == nil && len(pkgs) > 0 && pkgs[0].Types != nil {
+			pkg = pkgs[0].Types
+			err = nil
+		}
+	}
 	if err != nil {
 		panic(fmt.Errorf("can't import %q: %v", path, err))
 	}