error: unconstrained generic constant
  --> $DIR/issue-67739.rs:11:15
   |
LL |         [0u8; mem::size_of::<Self::Associated>()];
   |               ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   |
help: try adding a `where` bound
   |
LL |     fn associated_size(&self) -> usize where [(); mem::size_of::<Self::Associated>()]: {
   |                                        +++++++++++++++++++++++++++++++++++++++++++++++

error: aborting due to 1 previous error