fix: Some minor fixes for RawVector (#11693)

Summary:
The patch addresses a couple of issues with `RawVector` class:
1. Const version of `operator[]` returns `T` by value, which seems to be an unfortunate implementation mistake
2. Added an additional requirement for `T` to be `std::trivially_copyable_v<T>`. It makes little sense to use `raw_vector` for non-trivially copyable T:s, so explicitly forbid it.

Pull Request resolved: #11693

Reviewed By: yuandagits

Differential Revision: D66683059

Pulled By: xiaoxmeng

fbshipit-source-id: e5016a3bce54c4580c52946e8a0f1c8a3e7c828f

velox/common/base/RawVector.h

@@ -20,17 +20,22 @@
 #include "velox/common/base/BitUtil.h"
 #include "velox/common/base/SimdUtil.h"
 
+#include <type_traits>
+
 namespace facebook::velox {
 
 /// Class template similar to std::vector with no default construction and a
 /// SIMD load worth of padding below and above the data. The idea is that one
 /// can access the data at full SIMD width at both ends.
+///
+/// `T` should name a trivially copyable and trivially destructible type.
 template <typename T>
 class raw_vector {
  public:
-  raw_vector() {
-    static_assert(std::is_trivially_destructible<T>::value);
-  }
+  static_assert(
+      std::is_trivially_destructible_v<T> && std::is_trivially_copyable_v<T>);
+
+  raw_vector() = default;
 
   explicit raw_vector(int32_t size) {
     resize(size);
@@ -92,7 +97,7 @@ class raw_vector {
     return data_[index];
   }
 
-  T operator[](int32_t index) const {
+  const T& operator[](int32_t index) const {
     return data_[index];
   }