error[E0308]: mismatched types
  --> $DIR/fully-qualified-type-name4.rs:6:12
   |
LL | fn bar(x: usize) -> Option<usize> {
   |                     ------------- expected `Option<usize>` because of return type
LL |     return x;
   |            ^ expected `Option<usize>`, found `usize`
   |
   = note: expected enum `Option<usize>`
              found type `usize`
help: try wrapping the expression in `Some`
   |
LL |     return Some(x);
   |            +++++ +

error: aborting due to previous error

For more information about this error, try `rustc --explain E0308`.