`normalize_doc_attributes` sometimes comments out code

The simplest test case I've been able to come up with so far is this:

mod a {
    mod b {
        #[doc = "test"]fn foo() -> () { }
    }
}

with config

normalize_doc_attributes=true
max_width=10

This example gets reformatted to the following, although it does report an error because the line exceeds the maximum width.

mod a {
    mod b {
        ///testfn foo() -> (){
        }
    }
}

I'm still not sure exactly what triggers this bug, but have found a lot of variants for this example which don't trigger it. Here are a few that might be significant:

#[doc = "test"]fn foo() -> () { }

mod a {
    mod b {
        #[doc = "test"]fn foo() { }
    }
}

mod a {
    mod b {
        #[doc = "test"]
        fn foo() -> () { }
    }
}

mod a {
    mod b {
        /// test
        fn foo() -> () { }
    }
}

In these examples, removing max_width=10 fixes the issue, but I encountered this originally in a situation where rustfmt was inexplicably being run on autogenerated files in the target directory that were part of the wayland-protocol crate. I haven't figured out what was causing rustfmt to process those files in the first place, but in that case I did have max_width=80 instead.

I have no clue what's happening or how to fix it at this point, but I'm pretty sure it has to do with formatting lines that are longer than max_width combined with some unknown other requirement. I'll try to look into this more soon and see if I can figure out how to fix it.

Reproduce the nested-module example with normalize_doc_attributes=true and max_width=10, then inspect rustfmt's documentation-attribute normalization and line-width handling. Add a regression test for the reported input and verify that formatting does not merge the documentation text with the following function or comment out code.