E0604 - Error codes index
Description: Book listing all Rust error codes

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# 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.