Due to the the lack of native strings in WebAssembly different Wasm compilers have different memory layouts and string encodings. For example assemblyscript uses ucs2 for the sake fo compatibility with JavaScript. This obliges to carefully work with memory bounds, string length estimation due to difference in host native and guest string encodings.
For the specific goal of working with Strings in rust and assemblyscript I've created this project: https://github.com/onsails/wasmer-as.