bjoernager/rust
bjoernager/rust
1
Fork 0
Code Activity

Rollup merge of #60130 - khuey:efficient_last, r=sfackler

Browse source 
Add implementations of last in terms of next_back on a bunch of DoubleEndedIterators

Provided a `DoubleEndedIterator` has finite length, `Iterator::last` is equivalent to `DoubleEndedIterator::next_back`. But searching forwards through the iterator when it's unnecessary is obviously not good for performance. I ran into this on one of the collection iterators.

I tried adding appropriate overloads for a bunch of the iterator adapters like filter, map, etc, but I ran into a lot of type inference failures after doing so.

The other interesting case is what to do with `Repeat`. Do we consider it part of the contract that `Iterator::last` will loop forever on it? The docs do say that the iterator will be evaluated until it returns None. This is also relevant for the adapters, it's trivially easy to observe whether a `Map` adapter invoked its closure a zillion times or just once for the last element.
This commit is contained in:
Mazdak Farrokhzad 2019-05-14 22:00:09 +02:00 • committed by GitHub
commit bab03cecfe
No known key found for this signature in database
GPG key ID: 4AEE18F83AFDEB23
14 changed files with 159 additions and 0 deletions
Download patch file Download diff file Expand all files Collapse all files

6
src/libstd/env.rs
View file

@ -746,6 +746,10 @@ impl Iterator for Args {
self.inner.next().map(|s| s.into_string().unwrap())
}
fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
#[inline]
fn last(mut self) -> Option<String> {
self.next_back()
}
}
#[stable(feature = "env", since = "1.0.0")]
@ -781,6 +785,8 @@ impl Iterator for ArgsOs {
type Item = OsString;
fn next(&mut self) -> Option<OsString> { self.inner.next() }
fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
#[inline]
fn last(mut self) -> Option<OsString> { self.next_back() }
}
#[stable(feature = "env", since = "1.0.0")]