Suggested replacement code for E0139 results in memory unsafety

[wthrowe]

The message for E0139 suggests replacing let y: SomeType = unsafe { mem::transmute(v) }; with let y: SomeType = unsafe { ptr::read(&v as *const _ as *const SomeType) };. Actually performing such a replacement will result in double drops, since the transmute version consumes v but the read version does not. (It presumably needs a mem::forget(v) or something.)