Add some unsafe helper to track errors

internal/errors/unsafe.go

@@ -0,0 +1,37 @@
+package errors
+
+import (
+	"fmt"
+	"reflect"
+	"unsafe"
+)
+
+const maxInt = uintptr(int(^uint(0) >> 1))
+
+
+func UnsafeSubsliceOffset(data []byte, subslice []byte) int {
+	datap := (*reflect.SliceHeader)(unsafe.Pointer(&data))
+	hlp := (*reflect.SliceHeader)(unsafe.Pointer(&subslice))
+
+
+	if hlp.Data < datap.Data {
+		panic(fmt.Errorf("subslice address (%d) is before data address (%d)", hlp.Data, datap.Data))
+	}
+	offset := hlp.Data - datap.Data
+
+	if offset > maxInt {
+		panic(fmt.Errorf("slice offset larger than int (%d)", offset))
+	}
+
+	intoffset := int(offset)
+
+	if intoffset >= datap.Len {
+		panic(fmt.Errorf("slice offset (%d) is farther than data length (%d)", intoffset, datap.Len))
+	}
+
+	if intoffset + hlp.Len > datap.Len {
+		panic(fmt.Errorf("slice ends (%d+%d) is farther than data length (%d)", intoffset, hlp.Len, datap.Len))
+	}
+
+	return intoffset
+}

internal/errors/unsafe_test.go

@@ -0,0 +1,79 @@
+package errors_test
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/pelletier/go-toml/v2/internal/errors"
+)
+
+func TestUnsafeSubsliceOffsetValid(t *testing.T) {
+	examples := []struct{
+		desc string
+		test func() ([]byte, []byte)
+		offset int
+	}{
+		{
+			desc: "simple",
+			test: func() ([]byte, []byte) {
+				data := []byte("hello")
+				return data, data[1:]
+			},
+			offset: 1,
+		},
+	}
+
+	for _, e := range examples {
+		t.Run(e.desc, func(t *testing.T) {
+			d, s := e.test()
+			offset := errors.UnsafeSubsliceOffset(d, s)
+			assert.Equal(t, e.offset, offset)
+		})
+	}
+}
+
+func TestUnsafeSubsliceOffsetInvalid(t *testing.T) {
+	examples := []struct{
+		desc string
+		test func() ([]byte, []byte)
+	}{
+		{
+			desc: "unrelated arrays",
+			test: func() ([]byte, []byte) {
+				return []byte("one"), []byte("two")
+			},
+		},
+		{
+			desc: "slice starts before data",
+			test: func() ([]byte, []byte) {
+				full := []byte("hello world")
+				return full[5:], full[1:]
+			},
+		},
+		{
+			desc: "slice starts after data",
+			test: func() ([]byte, []byte) {
+				full := []byte("hello world")
+				return full[:3], full[5:]
+			},
+		},
+		{
+			desc: "slice ends after data",
+			test: func() ([]byte, []byte) {
+				full := []byte("hello world")
+				return full[:5], full[3:8]
+			},
+		},
+	}
+
+	for _, e := range examples {
+		t.Run(e.desc, func(t *testing.T) {
+			d, s := e.test()
+			require.Panics(t, func() {
+				errors.UnsafeSubsliceOffset(d, s)
+			})
+		})
+	}
+}