error[E0133]: mutation of layout constrained field is unsafe and requires unsafe function or block
  --> $DIR/unsafe-assign.rs:9:5
   |
LL |     foo.0.0 = 0;
   |     ^^^^^^^^^^^ mutation of layout constrained field
   |
   = note: mutating layout constrained fields cannot statically be checked for valid values

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0133`.