Can you be more specific? I don't even know what you mean by "powerful library". If you mean a library that's been developed and debugged for a long time, then sure, C++ currently has the advantage there, but that's a transient state and nonspecific to the language itself. If you mean a library that does wild, earth-moving things then I would call that a liability, not an advantage. The language features that allow such things are sources of bugs that Rust designers have elected not to introduce in the first place.

The question of, "In April 2019, which language and ecosystem are more reliable?" is a perfectly valid one. Stronger language semantics vs decades of library refinement. It's not at all obvious. But the answer to "Is Rust or C++ a better language in the long run?" seems clear to me.