error[E0053]: method `len` has an incompatible type for trait
  --> $DIR/unsafe-trait-impl.rs:18:5
   |
LL |     unsafe fn len(&self) -> u32;
   |     ---------------------------- type in trait
...
LL |     fn len(&self) -> u32 { *self }
   |     ^^^^^^^^^^^^^^^^^^^^ expected unsafe fn, found normal fn
   |
   = note: expected type `unsafe fn(&u32) -> u32`
              found type `fn(&u32) -> u32`

error: aborting due to previous error

For more information about this error, try `rustc --explain E0053`.