error: Undefined Behavior: constructing invalid value: encountered invalid reference metadata: total size is bigger than largest supported object
  --> $DIR/too-big-unsized.rs:LL:CC
   |
LL |         let _x: &MySlice = mem::transmute((ptr, isize::MAX as usize));
   |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ constructing invalid value: encountered invalid reference metadata: total size is bigger than largest supported object
   |
   = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
   = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
   = note: backtrace:
   = note: inside `main` at $DIR/too-big-unsized.rs:LL:CC

note: some details are omitted, run with `MIRIFLAGS=-Zmiri-backtrace=full` for a verbose backtrace

error: aborting due to previous error