error[E0072]: recursive type `Foo` has infinite size
  --> $DIR/issue-17431-6.rs:5:1
   |
LL | enum Foo { X(Mutex<Option<Foo>>) }
   | ^^^^^^^^                  --- recursive without indirection
   |
help: insert some indirection (e.g., a `Box`, `Rc`, or `&`) to break the cycle
   |
LL | enum Foo { X(Mutex<Option<Box<Foo>>>) }
   |                           ++++   +

error[E0391]: cycle detected when computing when `Foo` needs drop
  --> $DIR/issue-17431-6.rs:5:1
   |
LL | enum Foo { X(Mutex<Option<Foo>>) }
   | ^^^^^^^^
   |
   = note: ...which immediately requires computing when `Foo` needs drop again
   = note: cycle used when computing whether `Foo` needs drop
   = note: see https://rustc-dev-guide.rust-lang.org/overview.html#queries and https://rustc-dev-guide.rust-lang.org/query.html for more information

error: aborting due to 2 previous errors

Some errors have detailed explanations: E0072, E0391.
For more information about an error, try `rustc --explain E0072`.