Rust errors / Types and traits
Rust compiler error · checked with rustc 1.99.0
Rust error E0604: only `u8` can be cast as `char`, not `u32`
What it meansA cast to char was attempted on a type other than u8.
What rustc prints
error[E0604]: only `u8` can be cast as `char`, not `u32` --> main.rs:3:1 | 3 | 0u32 as char; // error: only `u8` can be cast as `char`, not `u32` | ^^^^^^^^^^^^ invalid cast | help: consider using `char::from_u32` instead | 3 - 0u32 as char; // error: only `u8` can be cast as `char`, not `u32` 3 + char::from_u32(0u32); // error: only `u8` can be cast as `char`, not `u32` |
Real output of rustc 1.99.0 compiling the official example below (2021 edition). Your names and line numbers will differ; the first line is the part to match.
The compiler team's explanation
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`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.
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.
From rustc --explain E0604 (1.99.0), part of the Rust project, MIT or Apache-2.0. Official page.
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