go/ast,go/format: newline missing between modified comment and function

[ldemailly]

Go version

go version go1.21.8 darwin/arm64

but same with 1.22 and Go dev branch on playground

What did you do?

Mutating node.Comments.List.Text produces incorrect formatting if adding newlines to /* */ style comments:

https://go.dev/play/p/IoJy719eLRt

As part of breaking too long lines, I stepped on the issue that if I cut a multi line comment that is before a function, once output back a new line between the comment and the function is missing eg

/*
 * multi
 * line comment.
 */func main() {
}

is produced

What did you see happen?

/*
 * multi
 * line comment.
 */func main() {

What did you expect to see?

/*
 * multi
 * line comment.
*/
func main() {

[ldemailly]

Same with go 1.22 and same with dev version (per playground)

Wondering if it's related to #20744 (but would love a work around in any case)

I think it happens with string literals as well so maybe a bug in my understanding of how to insert newlines

actual complete code ldemailly/lll-fixer#1

[dr2chase]

@griesemer @mvdan

[ldemailly]

Debugging (blindly, not knowing the code at all) a bit when adding a new line the buffer in printer:

"package main\f\f\xff/*\xff\f\xff * multi\xff\f\xff * line comment.\xff\f\xff */\xfffunc main() {\f}\n"

vs with X instead of \n in the update

"package main\f\f\xff/*\xff\f\xff * multiX * line comment.\xff\f\xff */\xff\nfunc main() {\f}\n"

in printer.go print()

			n := nlimit(next.Line - p.pos.Line)

next.Line - p.pos.Line is 1 when inserting an X and 0 when inserting a new line which then cause if n > 0 { to not trigger so \n is not added

Not sure how to 'dirty' the pos to ensure it gets recalculated?