rustfmt can't parse negative const generic parameters

[bgourlie]

cargo +nightly fmt fails to parse the following code:

#![feature(const_generics)]

fn main() {
    let i = test::<-1>();
    println!("Hello, {}!", i);
}

fn test<const T: i8>() -> i8 {
    {T}
}

output:

error: expected identifier, found `<-`
 --> /Users/brian/Repos/untitled3/src/main.rs:4:19
  |
4 |     let i = test::<-1>();
  |                   ^^ expected identifier

[topecongiro]

Cannot-be-parsed part is fixed on the master, though with suboptimal formatting:

 fn test<const T: i8>() -> i8 {
-    {T}
+    {
+        T
+    }
 }