Finds local variables that cannot be automatically moved due to constness.

Under certain conditions, local values are automatically moved out when returning from a function. A common mistake is to declare local lvalue variables const, which prevents the move.

Example [1]:

StatusOr<std::vector<int>> Cool() {
  std::vector<int> obj = ...;
  return obj;  // calls StatusOr::StatusOr(std::vector<int>&&)

StatusOr<std::vector<int>> NotCool() {
  const std::vector<int> obj = ...;
  return obj;  // calls `StatusOr::StatusOr(const std::vector<int>&)`

The former version (Cool) should be preferred over the latter (Uncool) as it will avoid allocations and potentially large memory copies.


In the example above, StatusOr::StatusOr(T&&) have the same semantics as long as the copy and move constructors for T have the same semantics. Note that there is no guarantee that S::S(T&&) and S::S(const T&) have the same semantics for any single S, so we’re not providing automated fixes for this check, and judgement should be exerted when making the suggested changes.


Another case where the move cannot happen is the following:

StatusOr<std::vector<int>> Uncool() {
  std::vector<int>&& obj = ...;
  return obj;  // calls `StatusOr::StatusOr(const std::vector<int>&)`

In that case the fix is more consensual: just return std::move(obj). This is handled by the -Wreturn-std-move warning.