Fix: the typ of emptyInterface does not match ptr(#519, #510)

[12Clock]

fix: The typ of emptyInterface does not match ptr

Issue:
When struct A contains a single field that is a pointer to struct B (e.g., A { B *B }),encoding an instance of A{} into our custom emptyInterface causes a value-pointer mismatch:

• emptyInterface.typ points to the type descriptor of A
• emptyInterface.ptr points directly to the embedded B field (type *B)

Solution:
Encode &A{} (pointer to struct) instead of A{} (struct value). This ensures:

1. EmptyInterface.typ correctly points to *A
2. EmptyInterface.ptr holds the address of the A instance, which contains a valid *B field
3. Type assertions and reflection operations on the interface now work as expected

Fixes #519, #510

[12Clock]

Benchmark

Before fixing:

goos: darwin
goarch: arm64
pkg: benchmark
cpu: Apple M3 Pro
Benchmark_Encode_LargeStruct_GoJson-12    	  101646	     10220 ns/op	   14143 B/op	     319 allocs/op
Benchmark_Encode_MediumStruct_GoJson-12    	 2392053	       486.4 ns/op	     608 B/op	      15 allocs/op
Benchmark_Encode_SmallStruct_GoJson-12    	 8177296	       130.6 ns/op	     256 B/op	       2 allocs/op
PASS

After fixing:

goos: darwin
goarch: arm64
pkg: benchmark
cpu: Apple M3 Pro
Benchmark_Encode_LargeStruct_GoJson-12    	  107654	     10314 ns/op	   14147 B/op	     319 allocs/op
Benchmark_Encode_MediumStruct_GoJson-12    	 2375899	       495.5 ns/op	     608 B/op	      15 allocs/op
Benchmark_Encode_SmallStruct_GoJson-12    	 8273752	       138.5 ns/op	     256 B/op	       2 allocs/op
PASS

Testing

Passed all encode tests except Issue147

TestIssue147
    encode_test.go:1795: json: error calling MarshalJSON for type *json_test.testNullStr: unexpected character 'b'

Although Issue147 was wrong when testing, I think it was as expected. 'json_test.testNullStr' should return "b" instead of b

type testNullStr string

func (v *testNullStr) MarshalJSON() ([]byte, error) {
	if *v == "" {
		return []byte("null"), nil
	}

	return []byte(*v), nil
}

func TestIssue147(t *testing.T) {
	type T struct {
		Field1 string      `json:"field1"`
		Field2 testNullStr `json:"field2,omitempty"`
	}
	got, err := json.Marshal(T{
		Field1: "a",
		Field2: "b",
	})
	if err != nil {
		t.Fatal(err)
	}
	expect, _ := stdjson.Marshal(T{
		Field1: "a",
		Field2: "b",
	})
	if !bytes.Equal(expect, got) {
		t.Fatalf("expect %q but got %q", string(expect), string(got))
	}
}

[12Clock]

@goccy Could you take a look at this PR when you have a moment?

[goccy]

With this approach, allocation occurs every time in the case of a struct, which is not ideal in terms of performance.

[12Clock]

@goccy Is it feasible to change 'rv.Kind() == reflect.Struct' to 'rv.Kind() == reflect.Struct && rv.NumField() == 1'?

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Codecov Report

❌ Patch coverage is 91.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.71%. Comparing base (9872089) to head (93c6385).
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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #551      +/-   ##
==========================================
- Coverage   76.73%   74.71%   -2.03%     
==========================================
  Files          55       55              
  Lines       18926    18935       +9     
==========================================
- Hits        14523    14147     -376     
- Misses       3771     4108     +337     
- Partials      632      680      +48     

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