reflect: add SliceAt

DO NOT SUBMIT

Fixes #61308

Change-Id: I707e788591aa4545ceef340aab424a9ef24dcc07

Author: qiulaidongfeng

src/reflect/all_test.go

@@ -8512,3 +8512,19 @@ func TestClear(t *testing.T) {
 		})
 	}
 }
+
+func TestSliceAt(t *testing.T) {
+	s := make([]string, 10)
+	et := TypeOf(s).Elem()
+	len := 5
+	v := SliceAt(et, unsafe.Pointer(unsafe.SliceData(s)), len)
+	if v.Len() != len {
+		t.Fatalf("Value.Len() got %d, want %d", v.Len(), len)
+	}
+	if v.Cap() != len {
+		t.Fatalf("Value.Cap() got %d, want %d", v.Cap(), len)
+	}
+	if v.UnsafePointer() != unsafe.Pointer(unsafe.SliceData(s)) {
+		t.Fatalf("v.UnsafePointer(%p) != unsafe.SliceData(%p)", v.UnsafePointer(), unsafe.SliceData(s))
+	}
+}

src/reflect/value.go

@@ -3297,6 +3297,18 @@ func New(typ Type) Value {
 	return Value{pt, ptr, fl}
 }
 
+// SliceAt returns a Value representing
+// a slice whose underlying data starts at p,
+// with length and capacity equal to len.
+// This is like unsafe.Slice.
+func SliceAt(typ Type, p unsafe.Pointer, len int) Value {
+	fl := flag(Slice)
+	st := SliceOf(typ)
+	s := &unsafeheader.Slice{Data: p, Len: len, Cap: len}
+	t := &st.(*rtype).t
+	return Value{t, unsafe.Pointer(s), fl}
+}
+
 // NewAt returns a Value representing a pointer to a value of the
 // specified type, using p as that pointer.
 func NewAt(typ Type, p unsafe.Pointer) Value {