Rustfmt misaligns doc comment between rustfmt::skip modules

[dtolnay]

mod modules {
    #[rustfmt::skip]
    #[path = "/dev/null"]
    mod first;

    /// ...
    /// ...
    #[rustfmt::skip]
    #[path = "/dev/null"]
    mod second;
}

Running rustfmt on this file turns it into:

mod modules {
    #[rustfmt::skip]
    #[path = "/dev/null"]
    mod first; /// ...
               /// ...
    #[rustfmt::skip]
    #[path = "/dev/null"]
    mod second;
}

<     mod first;
<
<     /// ...
<     /// ...
---
>     mod first; /// ...
>                /// ...

Observations:

• The #[path = "/dev/null"] attribute is not necessary for the repro. It is just to make this reproducible with a single input file.
• Both #[rustfmt::skip] are necessary for the repro. Without either one, the problem does not occur.
• Both ; are necessary for the repro. If either module uses {​} instead of ;, the problem does not occur.
• The outer module is necessary.

[dtolnay]

This currently affects syn as of https://github.com/dtolnay/syn/tree/2.0.66. Running cargo fmt in that repo produces the incorrect diff seen in dtolnay/syn#1672.

The oldest version of rustfmt that exhibits this issue is the one that shipped with Rust 1.38; rustfmt in older Rust releases leaves the original code unmodified.

[dtolnay]

According to bisect, this regressed between nightly-2019-07-25 and nightly-2019-07-31 (the dates in between are missing the rustfmt component). The only PR touching rust-lang/rust's src/tools/rustfmt in that range is rust-lang/rust#62805, so the relevant rustfmt commit range is 66c27c9...9e960e7.

#3691 looks suspiciously relevant based on these changes it causes to some of the tests.