Unsafe really needs to be addressed for Rust to be a language where it's possible to run untrusted third party code safely. badge 
+ Which Crates Will Work Off-the-Shelf with WebAssembly? It is easiest to list the things that do not currently work with WebAssembly; crates which avoid these things tend to be portable to WebAssembly and usually Just Work. A good rule of thumb is that if a crate supports embedded and #![no_std] usage, it probably also supports WebAssembly.
i.e. Crates that do not rely on the standard library tend to work well with WebAssembly. page 