Running cargo fmt seems to break some macro expansion

[trinity-1686a]

Running cargo fmt on Plume-org/Plume breaks it. It seems to remove curly braces around the macro invocation, which make macro expansion fail with unexpected end of macro invocation

Step to reproduce :

$ git clone https://github.com/Plume-org/Plume
$ cd Plume
$ cargo check #compile fine
$ cargo fmt --all
$ cargo check #fail

Reproducible with rustfmt 0.8.2-nightly (5e59925 2018-07-02) (specified by rust-toolchain) and rustfmt 0.99.2-nightly (5c9a2b6 2018-08-07) via +nightly

[topecongiro]

A minimal example:

fn main() {
    foo!(|_| { () });
}

becomes

fn main() {
    foo!(|_| ());
}

[eddyb]

Another case is stringify!((foo+)), where rustfmt removes the +.
Why does it even change that? Because it looks like a trait object type?

[topecongiro]

Because it looks like a trait object type?

I think so. I am surprised to find out that the trailing plus is allowed on a trait object type.

[eddyb]

@topecongiro Pretty much on any separated list, the separator can be trailing, in Rust.