error[E0016]: blocks in constant functions are limited to items and tail expressions
  --> $DIR/const-fn-error.rs:16:19
   |
LL |     let mut sum = 0;
   |                   ^

error[E0015]: calls in constant functions are limited to constant functions, tuple structs and tuple variants
  --> $DIR/const-fn-error.rs:18:14
   |
LL |     for i in 0..x {
   |              ^^^^

error[E0019]: constant function contains unimplemented expression type
  --> $DIR/const-fn-error.rs:18:14
   |
LL |     for i in 0..x {
   |              ^^^^

error[E0080]: constant evaluation error
  --> $DIR/const-fn-error.rs:18:14
   |
LL |     for i in 0..x {
   |              ^^^^ calling non-const fn `<I as std::iter::IntoIterator><std::ops::Range<usize>>::into_iter`
...
LL |     let a : [i32; f(X)];
   |                   ---- inside call to `f`
   |
note: for constant expression here
  --> $DIR/const-fn-error.rs:29:13
   |
LL |     let a : [i32; f(X)];
   |             ^^^^^^^^^^^

error: aborting due to 4 previous errors

Some errors occurred: E0015, E0016, E0019, E0080.
For more information about an error, try `rustc --explain E0015`.