cmd/gofmt: odd comment alignment

[cznic]

What does 'go version' print?
go version 1.3.2 // According to fmt.Println(runtime.Version()) in playground.

What steps reproduce the problem?
If possible, include a link to a program on play.golang.org.

1.
Open http://play.golang.org/p/E22kYjkJpS

2.
Click button "Format"

What happened?
Comment at line 4 jumps to the right.

What should have happened instead?
Comment at line 4 should have probably stay where it was.

Please provide any additional information below.
My hypothesis is that gofmt wrongly computes the length of line 5 using also len() of
the string literal as if it was a quoted string; in an attempt to align the comments at
L4 and L7 as if they are adjacent. But the raw string literal has newlines embedded so
they are not.

[ianlancetaylor]

Comment 1:

Labels changed: added repo-main, release-none.

[griesemer]

Comment 2:

Owner changed to @griesemer.

Status changed to Accepted.

[griesemer]

@cznic I believe your hypothesis is correct. The following example https://play.golang.org/p/-4qfL0qblf7 exhibits the same problem and the alignment of the 3rd foo is based on the string length of the raw string on the next line (the "abc_0123456789_" has the same length). This is really done in the tabwriter package, but gofmt could perhaps do something about it.

[griesemer]

The fix is probably to recognize newlines in escaped text sequences in the tabwriter and then flush the buffer contents up to the newline but keep the escape mode on: No matter the previous content, when the escaped text sequence containing a newline is written out, the resulting line break is going to break the existing formatting/column alignment.

This should probably only be done if enabled by a new flag in order to not break existing code that might rely on the current behavior.

go/printer then would simply have to set that flag.

Probably not too hard to do but requires thorough understanding of tabwriter internals (which is somewhat subtle).

Marking for 1.11 in case I find the muse to do it. Not urgent.