Span for "reinterpret_cast called on types with different size" error is incorrect

[jdm]

I'll find some time to make a testcase, but I'm currently seeing this:

/Users/jdm/src/servo/src/servo/servo.rc:1:0: 1:0 error: reinterpret_cast called on types with different size: uint (8) to {rc: uint,td: *core::sys::type_desc,next: *(),prev: *(),payload: js::rust::compartment} (40)
/Users/jdm/src/servo/src/servo/servo.rc:1 #[link(name = "servo",
                                          ^

[nikomatsakis]

This is almost certainly a dup of #1972.