error[E0005]: refutable pattern in local binding
  --> $DIR/exhaustive_patterns.rs:20:9
   |
LL |     let Either::A(()) = foo();
   |         ^^^^^^^^^^^^^ pattern `Either::B(_)` not covered
   |
   = note: `let` bindings require an "irrefutable pattern", like a `struct` or an `enum` with only one variant
   = note: for more information, visit https://doc.rust-lang.org/book/ch18-02-refutability.html
note: `Either<(), !>` defined here
  --> $DIR/exhaustive_patterns.rs:10:6
   |
LL | enum Either<A, B> {
   |      ^^^^^^
LL |     A(A),
LL |     B(inner::Wrapper<B>),
   |     - not covered
   = note: pattern `Either::B(_)` is currently uninhabited, but this variant contains private fields which may become inhabited in the future
   = note: the matched value is of type `Either<(), !>`
help: you might want to use `if let` to ignore the variant that isn't matched
   |
LL |     if let Either::A(()) = foo() { todo!() };
   |     ++                           +++++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0005`.