Auto merge of #24533 - servo:rustup, r=nox
Auto merge of #24533 - servo:rustup, r=nox

Update smallvec

The old version fails to build with the `union` unstable feature on today’s Rust Nightly:

https://tools.taskcluster.net/groups/Mq9abu-VT4OA3-Vq0Pe3dQ/tasks/Ri2K_wPMQRak5Y108_wDpA/runs/0/logs/public%2Flogs%2Flive.log#L1324

splav/servo,KiChjang/servo,saneyuki/servo,saneyuki/servo,splav/servo,KiChjang/servo,saneyuki/servo,splav/servo,DominoTree/servo,saneyuki/servo,KiChjang/servo,DominoTree/servo,saneyuki/servo,DominoTree/servo,DominoTree/servo,DominoTree/servo,splav/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,KiChjang/servo,splav/servo,KiChjang/servo,splav/servo,DominoTree/servo,DominoTree/servo,splav/servo,DominoTree/servo,KiChjang/servo,splav/servo,splav/servo,saneyuki/servo,saneyuki/servo,KiChjang/servo,DominoTree/servo,saneyuki/servo,DominoTree/servo,saneyuki/servo,saneyuki/servo,splav/servo