error[E0381]: used binding `x` is possibly-uninitialized
  --> $DIR/borrowck-while.rs:4:12
   |
LL |     let mut x: isize;
   |         ----- binding declared here but left uninitialized
LL |     while 1 == 1 { x = 10; }
   |           ------ if this condition isn't met and the `while` loop runs 0 times, `x` is not initialized
LL |     return x;
   |            ^ `x` used here but it is possibly-uninitialized

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0381`.