A cast to char was attempted on a type other than u8.

Erroneous code example:

0u32 as char; // error: only `u8` can be cast as `char`, not `u32`

As the error message indicates, only u8 can be cast into char. Example:

let c = 86u8 as char; // ok!
assert_eq!(c, 'V');

For more information about casts, take a look at the Type cast section in The Reference Book.