Specifically, there are two 'weak lang items' that need to be defined. Lang items are not yet stable. So while you can build libraries with only core, the final binary still needs to be compiled with nightly.
For now.
Furthermore, you can use a no_std library with an application that uses std just fine. The use case not quite covered is "I want an application with no standard library at all, period."
For now.
Furthermore, you can use a no_std library with an application that uses std just fine. The use case not quite covered is "I want an application with no standard library at all, period."