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Pretty-printer inserts an extra newline after multiline trailing comments #37201

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@petrochenkov

Description

@petrochenkov

Example:

fn main() {
    let a = 10; // Comment 1
                // Comment 2
}

=>

fn main() {
    let a =
        10; // Comment 1
            // Comment 2

}

Affected tests:
[pretty] pretty\issue-20427.rs
[pretty] pretty\issue-22992.rs
[pretty] pretty\issue-23338-ensure-param-drop-order.rs
[pretty] pretty\issue-27401-dropflag-reinit.rs

The issue disappears if comments are not packed into ibox (see try!(self.ibox(0)) and self.end() in pprust.rs/fn print_comment/comments::Trailing), but formatting become worse and other tests fail.
I spent whole day investigating how these boxes work and reading Derek C. Oppen's "Pretty Printing" but still have no idea why this extra newline appears.

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    A-prettyArea: Pretty printing (including `-Z unpretty`)E-easyCall for participation: Easy difficulty. Experience needed to fix: Not much. Good first issue.E-needs-testCall for participation: An issue has been fixed and does not reproduce, but no test has been added.

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