Error code E0604

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

Erroneous code example:

#![allow(unused)]
fn main() {
0u32 as char; // error: only `u8` can be cast as `char`, not `u32`
}

char is a Unicode Scalar Value, an integer value from 0 to 0xD7FF and 0xE000 to 0x10FFFF. (The gap is for surrogate pairs.) Only u8 always fits in those ranges so only u8 may be cast to char.

To allow larger values, use char::from_u32, which checks the value is valid.

#![allow(unused)]
fn main() {
assert_eq!(86u8 as char, 'V'); // ok!
assert_eq!(char::from_u32(0x3B1), Some('α')); // ok!
assert_eq!(char::from_u32(0xD800), None); // not a USV.
}

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