Inner attributes in block expressions get deleted

[matthew-mcallister]

If you pass in input like this...

let x = {
    #![my_attr]
    do_something()
};
let x = #[my_attr] {
    do_something()
};

... what you get is this...

let x = { do_something() };
let x = #[my_attr]
{ do_something() };

The second case is just bad formatting due to assuming that the attribute appears on its own line. More concerning is the first case, in which actually meaningful code gets deleted.

[topecongiro]

We probably should disallow rewriting a block in a single line if there are any attributes.

[topecongiro]

Closed via #2075.