Use interpolation:

``` ruby
warn "ActiveIf#options is deprecated and is never used at: #{caller[1]}"
```

This may raise `TypeError` when `caller.size == 1`

``` ruby
ary = ["a", "b"]
warn "foo " + ary[1] #=> "foo b"
warn "foo #{ary[1]}" #=> "foo b"

ary = ["a"]
warn "bar " + ary[1] #=> no implicit conversion of nil into String (TypeError)
warn "bar #{ary[1]}" #=> "bar "
```