Cope with types that have deleted move-constructors

[adetaylor]

This is a problem encountered on an internal codebase, so I can't give details here, but recording that this bug exists for my own records. Once it's minimized I will post a test case here.

[adetaylor]

Minimization command for the record

~/autocxx/target/release/autocxx-reduce -I . -h column_list.h -d 'generate!("spanner::ReadOptions")' -o min.h -p "move-insertable" --compile -d 'block!("proto2:Message")' -c=-std=c++17 `cat customopts-min.txt` --gen-cmd $(pwd)/custom_autocxx_gen -k --clang $(pwd)/custom_clang -- --n 64

[adetaylor]

Problems here include:
gen0.cc:1383:15: error: call to deleted constructor of '::xyz' ::new (out) ::xyz(::std::move(v->back())); called from this cxx-generated code:

void cxxbridge1$std$vector$xyz$pop_back(::std::vector<::xyz> *v, ::xyz *out) noexcept {
  ::new (out) ::xyz(::std::move(v->back()));
  v->pop_back();
}

and
error: static_assert failed due to requirement '__is_cpp17_move_insertable<std::allocator<xyz>, void>::value' "The specified type does not meet the requirements of Cpp17MoveInsertable"

from:

void cxxbridge1$std$vector$xyz$push_back(::std::vector<::xyz> *v, ::xyz *value) noexcept {
  v->push_back(::std::move(*value));
  ::rust::destroy(value);
}

The problem here stems from types which have:

• Deleted move constructors (e.g. using DISALLOW_COPY_AND_ASSIGN)
• Deleted copy constructors (which may be because a field has a deleted copy constructor)

This seems to be OK when we impl UniquePtr<T> but causes problems when we impl CxxVector<T> in the cxx::bridge.

I think for now we should cease to generate impl CxxVector<T> unless there's a generate_vec!(..) directive.

In the future we might be able to use the wisdom that we'll need to learn in areas around #458 and #258 to determine if there are move constructors, and if so, reinstate auto vector generation.

[martinboehme]

You may be able to query whether the move or copy constructor are deleted using the code introduced as part of #426 -- but this may not work if they are implicitly deleted (I don't recall), so it may be best to just avoid generating impl CxxVector<T>.