The coerced type does not outlive the value being coerced to.
Example of erroneous code:
#![feature(coerce_unsized)] #![feature(unsize)] use std::marker::Unsize; use std::ops::CoerceUnsized; // error: lifetime of the source pointer does not outlive lifetime bound of the // object type impl<'a, 'b, T, S> CoerceUnsized<&'a T> for &'b S where S: Unsize<T> {}
During a coercion, the “source pointer” (the coerced type) did not outlive the “object type” (value being coerced to). In the above example, 'b
is not a subtype of 'a
. This error can currently only be encountered with the unstable CoerceUnsized
trait which allows custom coercions of unsized types behind a smart pointer to be implemented.