fmt: clarify when %#q does not use backquotes

[gopherbot]

by proebsting:

Why does fmt.Printf("%#q", "a\nb") fail to use backquotes rather
than falling back on double quotes?  There's nothing special about newlines in
backquoted strings, is there?

Thanks.

[bradfitz]

Comment 1:

I think that would be surprising and would probably break the Go 1 API promise, in a
way. %q is often used in tests to show failures, showing empty strings or strings with
accidental binary data.  I wouldn't like to see `` vs "" flipping back and forth,
depending on other binary data or chars needs to be escaped, or \r, etc.  Having one way
seems best, even if not ideal.

Status changed to WorkingAsIntended.

[minux]

Comment 2:

fmt just asks strconv.CanBackquote wether a given string can be safely
represented if it's enclosed in backquotes.
I don't know why "\n" is not allowed, perhaps CanBackquote just want
to make sure the string contain only one line if printed out?

[rsc]

Comment 3:

Yes, the goal is to avoid multiline output from %#q.

[gopherbot]

Comment 4 by proebsting:

I would argue the documentation is misleading then.  It says the following:
#   alternate format: add leading 0 for octal (%#o), 0x for hex (%#x);
    0X for hex (%#X); suppress 0x for %p (%#p);
    print a raw (backquoted) string if possible for %q (%#q);
Clearly, it is possible to print the string with backquotes.

[rsc]

Comment 5:

Labels changed: added priority-later, go1.1, removed priority-triage, go1.1maybe.

Status changed to Accepted.

[bradfitz]

Comment 6:

Wow, I stand corrected. I had never seen that. (# on %q)

[minux]

Comment 7:

https://golang.org/cl/7387044

Status changed to Started.

[minux]

Comment 8:

This issue was closed by revision 4692711.

Status changed to Fixed.