Fail to format match arm when other arm has long line

Rustfmt is not formatting the `let a=0;` part in this reduced testcase:
```rust
fn f() {
    let mut a = Vec::new();
    a.push(match 0 {
        0 => {let a=0;""}
        _ => r#"
--------------------loong line----------------------------------------------------------------------.
"#,
    });
}
```

I am using rustfmt version:
rustfmt 0.99.6-nightly (750b252 2018-10-18)