error[E0515]: cannot return value referencing local data `*b`
  --> $DIR/regions-trait-variance.rs:38:5
   |
LL |     let bb: &B = &*b;
   |                  --- `*b` is borrowed here
LL |     make_a(bb)
   |     ^^^^^^^^^^ returns a value referencing data owned by the current function

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0515`.