cmd/gofmt: formatting of struct literal depends inconsistently on lengths of field names

[cloneable]

http://play.golang.org/p/UlzyF8U_nZ

func main() {
    z := MyStruct{
        Foo:      "foo",
        Bar:      "bar",
        Name:     "name",
        LongName: "longname",
        Baz:      "baz",
    }
    y := MyStruct{
        Foo:                   "foo",
        Bar:                   "bar",
        NameXX:                "name",
        LongNameXXXXXXXXXXXXX: "longname",
        Baz: "baz",
    }
    z := MyStruct{
        Foo:  "foo",
        Bar:  "bar",
        Name: "name",
        LongNameXXXXXXXXXXXXX: "longname",
        Baz: "baz",
    }
}

[robpike]

Comment 1:

Labels changed: added repo-tools.

Owner changed to @griesemer.

[robpike]

Comment 2:

Status changed to Accepted.

[cloneable]

Comment 3:

What's the expected output for (2) and (3)?
(2) looks like a bug, but I could see (3) as an optimization where alignment is broken
up into two groups due to a long field name in-between. However, I couldn't find test
input for this in testdata/. So this is a bug as well?

[griesemer]

Comment 4:

This is working as intended.
There is explicit code to check for large size discrepancies of keys in composite
literals, see: http://golang.org/src/pkg/go/printer/nodes.go#L195
In short, if the difference in size between two adjacent keys is more than a factor of
4, the alignment is broken. This avoids values aligned all the way to the far right in
very large composite literals simply because one key somewhere is very long. It's a
heuristic alright, and the factor (4) can be debated, but it's been working reasonably
well so far.
In this specific case, NameXX has length 6, LongNameXXXXXXXXXXXXX has length 21, and 4*6
> 21, so they is no break of alignment. Name has length and thus the difference between
LongNameXXXXXXXXXXXXX and Name is more than a factor of 4, and then the alignment is
broken. Same holds for LongNameXXXXXXXXXXXXX and Baz.

Status changed to WorkingAsIntended.

[griesemer]

Comment 5:

Let me rephrase the last paragraph correctly:
In this specific case, NameXX has length 6, LongNameXXXXXXXXXXXXX has length 21, and 4*6
> 21, so there is no break of alignment. Name has length 4 and thus the difference
between LongNameXXXXXXXXXXXXX and Name is more than a factor of 4, and then the
alignment is broken. Same holds for LongNameXXXXXXXXXXXXX and Baz.