Cargo links all the rust code together in a static binary, but the C/C++ libraries can be linked dynamically, and in the case of something like Qt, most certain

Cargo links all the rust code together in a static binary, but the C/C++ libraries can be linked dynamically, and in the case of something like Qt, most certainly are. Since most important part of Qt are LGPL, you can use this crate and other Qt binding crates to develop proprietary applications. (Only if you wish to use one of the few parts that are "only" GPL, then you need to release the final product and all its parts under the GPL, or acquire a Qt license)