error: lifetime may not live long enough
  --> $DIR/regions-infer-invariance-due-to-mutability-4.rs:10:5
   |
LL | fn to_longer_lifetime<'r>(b_isize: Invariant<'r>) -> Invariant<'static> {
   |                       -- lifetime `'r` defined here
LL |     b_isize
   |     ^^^^^^^ returning this value requires that `'r` must outlive `'static`
   |
   = note: requirement occurs because of the type `Invariant<'_>`, which makes the generic argument `'_` invariant
   = note: the struct `Invariant<'a>` is invariant over the parameter `'a`
   = help: see <https://doc.rust-lang.org/nomicon/subtyping.html> for more information about variance

error: aborting due to 1 previous error