I think an important thing to make sense of this is that Rust doesn't guarantee that what Manishearth did will work! Just that if it doesn't work, the code will

I think an important thing to make sense of this is that Rust doesn't guarantee that what Manishearth did will work! Just that if it doesn't work, the code will not compile. It could have been the case that this vector was mutated or deleted at some point during the lifetime of his new struct; in that case, his code would not have compiled.