Incorrectly modifies associated type in macro body

https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=4d7f756328bf278741b41d51f006cf8a

Formatting:
```rust
macro_rules! m {
    () => {
        type Type;
    };
}
```
Incorrectly produces:
```rust
macro_rules! m {
    () => {
        type Type = impl ;
    };
}
```

`m!` is intended to be used like so:
```rust
pub trait Trait {
    m! {}
}
```