Rollup of 5 pull requests#59653

Closed

Centril wants to merge 14 commits into

rust-lang:masterfrom

Centril:rollup-0kr2w0l

Commits on Mar 11, 2019

Add initial implementation of 'sort_at_index' for slices -- analog to C++'s std::nth_element (a.k.a. quickselect)
Mokosha
committed

Commits on Mar 29, 2019

improve worst-case performance of BTreeSet difference and intersection
ssomers
committed

Commits on Apr 1, 2019

Commits on Apr 2, 2019