`posix_fallocate` not supported

[nazar-pc]

error: unsupported operation: can't call foreign function `posix_fallocate` on OS `linux`
   --> /home/nazar-pc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/fs2-0.4.3/src/unix.rs:103:24
    |
103 |     let ret = unsafe { libc::posix_fallocate(file.as_raw_fd(), 0, len as libc::off_t) };
    |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ unsupported operation occurred here
    |
    = help: this means the program tried to do something Miri does not support; it does not indicate a bug in the program
    = note: BACKTRACE on thread `tests::other_op`:
    = note: inside `fs2::unix::allocate` at /home/nazar-pc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/fs2-0.4.3/src/unix.rs:103:24: 103:86
    = note: inside `<std::fs::File as fs2::FileExt>::allocate` at /home/nazar-pc/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/fs2-0.4.3/src/lib.rs:104:9: 104:33

It is meant to pre-allocate disk space to ensure writes do not fail within provided length with lack of disk space afterward. I'd say it is mostly the same as File::set_len() except it avoids sparse files on disk.

[RalfJung]

I think the best we can do here is implement it like set_len. Would that be useful?

We're generally avoiding non-portable implementations unless they are needed for something super common, so the part about the disk space is not something we're likely going to get.

[nazar-pc]

I think set_len()-like implementation is reasonable and helpful already