go/parser: better error message for struct.field := value

[jscrockett01]

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)
}

[rsc]

Comment 1:

I agree that go/parser should do a better job here. It's especially important since so
many tools parse the Go code with go/parser before passing it to gc.

Labels changed: added priority-later, removed priority-triage.

Owner changed to @griesemer.

Status changed to Accepted.

[griesemer]

Comment 2:

This issue was closed by revision 82accf4.

Status changed to Fixed.