Long casts get's moved to a new line

[drager]

When running rustfmt on long lines that contains a cast, the formatting gets a bit weird. I copied the code I formatted to a playground that can be found here: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4bc27f6604fa0ca8cce228b99e81223c (this wont compile). This is my formatted version, using rustfmt 1.0.3-stable (d6829d6 2019-02-14). I also tried rustfmt 1.2.1-nightly (b860fea 2019-04-18) which seems to be the one at the playground which also results in the same formatting. I think the weird formatting is that as Box<dyn Backend> gets moved to a new line with weird indention.

[ytmimi]

Backport PR #5296 seems to solve this