Use Vec::drain_filter when available

[martinthomson]

This code has bothered me for a while, because it seems inefficient (two inverse filters means redundancy). It's better than dumb iteration, but it isn't perfect.

neqo/neqo-qpack/src/decoder.rs

Lines 68 to 75 in 44dfdbf

	let r = self
	.blocked_streams
	.iter()
	.filter(|(_, req)| *req <= base)
	.map(|(id, _)| *id)
	.collect::<Vec<_>>();
	self.blocked_streams.retain(|(_, req)| *req > base);
	Ok(r)

Turns out that Vec::drain_filter is coming. When that becomes stable, we should move to using that. drain_filter(|i| {/* logic here */}).collect() will be much nicer.

[MightyPork]

ftr, I solve a similar problem with the combination of drain.filter_map.collect, moving the removed elements to a separate vec (or consuming in other ways) in the filter_map callback

rust-lang/rust-clippy#4433 (comment)

[larseggert]

@martinthomson this is now

neqo/neqo-qpack/src/decoder.rs

Lines 93 to 98 in 32ef2c3

	let r = self
	.blocked_streams
	.iter()
	.filter_map(|(id, req)| if *req <= base_new { Some(*id) } else { None })
	.collect::<Vec<_>>();
	self.blocked_streams.retain(|(_, req)| *req > base_new);

Anything left to do?

[martinthomson]

This would be better with drain_filter(), but what we have is fine. Given that drain_filter is taking SOOOO long to stablize, we might just give up on it.