It will not add one if there isn't one already. (I should probably rename this function, work in progress..). But the type is an array of the correct length.
Ah, okay. In that case I feel like the only sane way to approach this is to completely abolish null-terminated strings, and reimplement everything (including stuff like printf's format and arguments) in terms of strv. Otherwise if there has to be a 2cstr function, it should be an allocating one.
Reimplementing everything is not an option in C. But one should be able to pass string views directly to printf.
The allocating version is what string_dup does. strv2cstr is certainly more dangerous. But as since the size is encoded in the return type, compilers can add bounds checking (and partially already do so).