Patterns come in two forms: refutable and irrefutable. Patterns that will match for any possible value passed are irrefutable. An example would be x in the statement let x = 5; because x matches ...

The manual_flatten lint doesn't take into account nested refutable patterns. warning: variant `Unwanted` is never constructed --> src/main.rs:3:5 | 1 | enum Data { | ---- variant in this enum 2 | ...