Remove `Option<W>` in `BufWriter`

[Lucretiel]

rust/src/libstd/io/buffered.rs

Line 451 in eeaf497

inner: Option<W>,

BufWriter doesn't really need to hold its inner writer as an Option; it seems like the only reason it does so is to make it easier to call into_inner, here:

rust/src/libstd/io/buffered.rs

Lines 648 to 653 in eeaf497

	pub fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>> {
	match self.flush_buf() {
	Err(e) => Err(IntoInnerError(self, e)),
	Ok(()) => Ok(self.inner.take().unwrap()),
	}
	}

While it's unfortunate to add unsafe, there's already some in buffered.rs, and it seems like the better choice here would be to use ptr::read and mem::forget, especially because this method is probably infrequently used, and making this change would remove the numerous unwraps that litter the BufWriter implementation.

[Lucretiel]

Just tried to implement this; it turns out to be nontrivial, because you need to mem::forget(self) in a way that doesn't leak self.buf.

[Amanieu]

You can use ptr::drop_in_place on self.buf before forget(self).