warning: unreachable pattern
  --> $DIR/packed_pattern.rs:16:9
   |
LL |         Foo { field: (5, 6, 7, 8) } => {},
   |         --------------------------- matches all the values already
LL |         FOO => unreachable!(),
   |         ^^^ unreachable pattern
   |
   = note: `#[warn(unreachable_patterns)]` on by default

warning: 1 warning emitted