Hello,

I have tried to submit a PR ( rust-lang/rust#47069 ) on the Rust repo after running rustfmt on the libarena crate but it seems like there are some formatting issues:

• on lines 136-137 the construct intrinsics -- as *mut T has been split on two lines
• but on line 367 the construct intrinsics -- as *mut u8 has been put on the same line and there is a comma added after as *mut u8

It seems as if it is not an expected result, so I've decided to submit this issue (my very first issue).

I've used rustfmt-nightly 0.3.4 without options to get this.