error[E0261]: use of undeclared lifetime name `'a`
  --> $DIR/unresolved_lifetimes_error.rs:5:13
   |
LL |     let a: &'a ();
   |             ^^ undeclared lifetime
   |
help: consider introducing lifetime `'a` here
   |
LL | fn foo<'a>() -> [(); {
   |       ++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0261`.