cmd/gofmt: -r 'bar(x) -> bar(x)' mistakenly replaces a comment with a newline in closures inside function

[qfel]

What version of Go are you using (go version)?

go version go1.9.2 linux/amd64

What did you do?

$ gofmt -r 'bar(x) -> bar(x)' /tmp/test.go

/tmp/test.go is:

package main

func main() {
    // Call foo().
    foo()
    bar(func() {
        // Call foo().
        foo()
    })
}

What did you expect to see?

Same contents

What did you see instead?

package main

func main() {
        // Call foo().
        foo()
        bar(func() {

                foo()
        })

}

Notice the comment in the closure passed to bar was replaced with an empty line, and an empty line was added after the call to bar.

The identity rewrite rule is for simplicity, this happens with more useful rewrites that act on bar too.

[odeke-em]

/cc @griesemer

[m4ns0ur]

The "recreate comments list" is filtering the comment inside the closure

[griesemer]

@m4ns0ur Thanks for investigating. In this example, while bar(x) is replaced with the same looking expression, it's nevertheless a different, new expression, and as a result (I suspect) the comment that was associated with the old bar(x) is gone. If my suspicion is true, this is working as intended (even though not as expected).

I don't know why there's a new empty line before the closing }, though.

gofmt -r is somewhat experimental, unfortunately.

[dominikh]

Related: #7417