Using := assignment on a struct field gives a helpful message from gc, but less helpful
from gofmt.

From gc:  prog.go:12: non-name t.a on left side of :=
From gofmt:  prog.go:12:2: expected identifier

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

--------------------
package main

import "fmt"

type T struct {
    a int
    b int
}

func main() {
    var t T
    t.a := 7 // here is the error
    fmt.Println(t)
}