[rtfeldman]

The platform header spelled out main!'s argument type as the tag union behind OsStr, while main_for_host! passes a List(OsStr) and every example annotates main! with List(OsStr). That only type-checks because the compiler currently lets a written-out tag union stand in for a named type with the same tags, and a planned compiler change makes a written-out type mean exactly what it says. This names OsStr in the header instead, and writes main_for_host!'s annotation the same way.

Nothing changes for apps on current compilers: apps that annotate main! with List(OsStr), leave it unannotated, or call OsStr.from_raw on each argument all keep compiling. Once the compiler change lands, the arguments are simply OsStr values, so an app that called OsStr.from_raw on them passes them to OsStr functions directly.