One of their (more minor admittedly) goals this year is to improve/stabilize no_std as well as make std more portable. Looking forward to another great year in rust!
(This link will change in one or two releases; I landed a new "Unstable Book" a few days ago.)
The TL;DR is Rust expects certain stuff to be implemented in Rust code to work, and that's provided by the standard library. When you don't use the standard library, libcore implements all but a few of them. Implementing them is not yet stable.