error[E0282]: type annotations needed for `[_; 3]`
  --> $DIR/slice-patterns-irrefutable.rs:43:9
   |
LL |     let b;
   |         ^
LL |
LL |     [a, b] = Default::default();
   |      - type must be known at this point
   |
help: consider giving `b` an explicit type, where the placeholders `_` are specified
   |
LL |     let b: [_; 3];
   |          ++++++++

error: aborting due to 1 previous error

For more information about this error, try `rustc --explain E0282`.