unused: flag unused variables with initializers

[dominikh]

The rewrite of unused no longer flags the following code:

package pkg
var x = [3]int{1,2,3}

This is caused by the SSA that go/ssa produces:

func init():
0:                                                                entry P:0 S:2
	t0 = *init$guard                                                   bool
	if t0 goto 2 else 1
1:                                                           init.start P:1 S:1
	*init$guard = true:bool
	t1 = &x[0:int]                                                     *int
	t2 = &x[1:int]                                                     *int
	t3 = &x[2:int]                                                     *int
	*t1 = 1:int
	*t2 = 2:int
	*t3 = 3:int
	jump 2
2:                                                            init.done P:2 S:0
	return

Because the init function is taking addresses of values in x, x is clearly used.

This is not a problem for slices, because they generate different code (an array gets constructed, sliced, and assigned to the variable. Assignment alone isn't considered a use.)

This is, however, also a problem for structs, as

var x = t{1}

generates

t1 = &x.x [#0]                                                     *int
*t1 = 1:int

Another annoying side effect of this is that function calls used as part of the initialization are marked as used by the init function, not the variable.