A long string in a match arm breaks formatting for the whole match expression

Describe the bug

If an arm of a match expression contains a string with >= 87 characters, formatting fails on the whole match

To Reproduce

match 0 {
    0 => {"hello"} 1 => { ""}
      _ => {"veeeeeeeeeeery loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong string"}
};

Expected behavior

 match 0 {
    0 => "hello",
    1 => "",
    _ => {
        "veeeeeeeeeeery loooooooooooooooooooooooooooooooooooooooooooooooooooooooooooooong string"
    }
};

If the third string is less than 86 characters, formatting works as expected.

Meta

• rustfmt version: rustfmt 1.4.36-stable (7de6968 2021-02-07)
• From where did you install rustfmt?: rustup
• How do you run rustfmt: rustfmt, cargo-fmt, and rust-analyzer formatting in VSCode.

Start by running rustfmt on the match-expression reproduction in the issue and inspect the match-arm formatting path that handles long strings. Done means the provided example formats successfully, including the long-string arm, without breaking formatting for the whole match expression; add a regression test if the existing test layout identifies a suitable location.