Add let_cxx_vector to construct CxxVector on stack

[compiler-errors]

Implement let_cxx_vector macro, very similarly to let_cxx_string, to enable constructing a CxxVector<T> on the stack.

Helps with calling methods that take std::vector as an out variable.

[compiler-errors]

@hanusek's #961 reminded me that this PR was open.

@dtolnay: is this change something you consider reasonable, and if so, can I get a review on it sometime? If it's not worthwhile or is fundamentally broken in some way, I'll just close it.

[NlightNFotis]

Hi @dtolnay, pinging you here, as evaluating this one for going in would be an incredible help to a lot of workflows (at least for me).

PR looks clean, has tests, could do with a rebase on top of recent master but no merge conflicts.

What would it take for this to be taken forward?

[NlightNFotis]

Hi @dtolnay, many thanks for a very prompt response.

In light of this decision, what would you suggest is the best way to construct a CxxVector<CxxString> and pass it to the C++ side?

Would I be better off just creating the Rust equivalents and translate them into the C++ APIs in the .cpp file?

Thanks again,

[dtolnay]

You can do that or you can obtain an empty UniquePtr<CxxVector<CxxString>> from C++ to put elements into. Or, via a callback, obtain a Pin<&mut CxxVector<CxxString>> for the Rust code to fill.