gofmt -w x.go

with x.go:

package p

// aligned
var _ = T1{
    a:                    x,
    b:                    y,
    cccccccccccccccccccc: z,
}

// not aligned
var _ = T2{
    a: x,
    b: y,
    ccccccccccccccccccccc: z,
}

// aligned
var a = T3{
    aaaaaaaaaaaaaaaaaaaa: x,
    b:                    y,
    c:                    z,
}

// still aligned
var a = T4{
    aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa: x,
    b:                                       y,
    c:                                       z,
}

gofmt aligns "similarly sized" composite literal elements; it breaks the
grouping when an internal threshold is reached. This works fine for T1, T2, and T3. But
the algorithm is not working for T4.