error[E0283]: type annotations needed
  --> $DIR/issue-34979.rs:6:13
   |
LL | trait Foo {}
   | --------- required by this bound in `Foo`
...
LL |     &'a (): Foo,
   |             ^^^ cannot infer type for reference `&'a ()`
   |
   = note: cannot satisfy `&'a (): Foo`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0283`.