Feature request: add support for `open_memstream` (and eventually `open_wmemstream`) on Linux

[dodomorandi]

I was trying to implement an abstraction over open_memstream, and I realized it is less trivial than I initially thought. In fact, the stream opened by the OS is able to change the values of the pointed arguments ptr and sizeloc when fflush and fclose are called.

I think that the implementation of open_memstream (and eventually open_wmemstream) inside Miri would help a lot to check the invariances and the soundness of a safe abstraction, which requires carefully written pinned structures (because it's UB to move the structure containing the ptr and size passed to the function as pointers) and some attention to avoid breaking aliasing rules (it should be UB to call fflush or fflush if you have any reference to ptr or size).

[RalfJung]

Thanks for the report!

Since this is not an API exposed by the Rust standard library, it will have fairly low priority I am afraid. It also does not show up in #2057, so it does not seem very widely used. We could consider contributions as long as they don't complicate the overall codebase. However, the API is a bit odd -- apparently it must return a FILE*, and is explicitly documented not to return a file descriptor. We don't have any operations on FILE* right now, we only support direct use of file descriptors, so a bunch of work would be needed on top of these functions to even support basic read/write/close of FILE*.