Add Vec::drain_filter
This implements the API proposed in #43244.
So I spent like half a day figuring out how to implement this in some awesome super-optimized unsafe way, which had me very confident this was worth putting into the stdlib.
Then I looked at the impl for `retain`, and was like "oh dang". I compared the two and they basically ended up being the same speed. And the `retain` impl probably translates to DoubleEndedIter a lot more cleanly if we ever want that.
So now I'm not totally confident this needs to go in the stdlib, but I've got two implementations and an amazingly robust test suite, so I figured I might as well toss it over the fence for discussion.
add a note to Vec's Extend<&T> impl about its slice specialization
From the regular documentation view, it's not at all apparent that [this specialization](5669c9988f/src/liballoc/vec.rs (L1879-L1891)) exists for `slice::Iter`. This adds a documentation blurb to the Extend impl itself to note that this optimization exists.
When trying to use dedup_by to merge some auxiliary information from
removed elements into kept elements, I was surprised to observe that
vec.dedup_by(same_bucket) calls same_bucket(a, b) where b appears
before a in the vector, and discards a when true is returned. This
argument order is probably a bug, but since it has already been
stabilized, I guess we should document it as a feature and move on.
(Vec::dedup also uses == with this unexpected argument order, but I
figure that’s not important since == is expected to be symmetric with
no side effects.)
Signed-off-by: Anders Kaseorg <andersk@mit.edu>
Replaced by adding extra imports, adding hidden code (`# ...`), modifying
examples to be runnable (sorry Homura), specifying non-Rust code, and
converting to should_panic, no_run, or compile_fail.
Remaining "```ignore"s received an explanation why they are being ignored.
