Tracking Issue i64 to u64 (as)

[AveNRU]

let x: i64 = -10;
let y: u64 = x as u64;
println!("{}", y); // 18446744073709551606

Please help with error when do not worry transform i64 to u64

[zachs18]

The as cast operator does bitcasts for integer types, and the 64-bit 2's complement representation of -10 is 0b1111111111111111111111111111111111111111111111111111111111110110, so as casting it to u64 gives the u64 with that same bit pattern (18446744073709551606).

If you want to keep the same number value, and get an error if it is not representable, you could use TryFrom/TryInto instead of as-casting, e.g.

let x: i64 = -10;
let y: Result<u64, _> = x.try_into(); // or u64::try_from(x)
println!("{:?}", y); // Err(TryFromIntError(()))
let x: i64 = 10;
let y: u64 = x.try_into().unwrap();
println!("{:?}", y); // 10

(Tracking issues are generally for tracking the implementation of things like new features, not for bug reports (which should use one of the other issue templates) or debugging questions (which should generally go on the user forum))

@rustbot label -C-tracking-issue -S-tracking-unimplemented +C-discussion

[AveNRU]

Sorry for tags

[ChrisDenton]

I would suggest asking rust questions on one of the community pages https://www.rust-lang.org/community or other sites like stackoverflow.

[AveNRU]

https://www.collabora.com/news-and-blog/blog/2025/05/06/matt-godbolt-sold-me-on-rust-by-showing-me-c-plus-plus/
This question was discussed by Matt Godbolt above when comparing c++ and rust. Is it possible to fully protect via as

[ShE3py]

Clippy has lints that detect as conversions:

• clippy::cast_possible_wrap unsigned -> signed
• clippy::cast_possible_truncation big int -> smaller int
• clippy::as_conversions all of them

You can enable them by promoting them to warnings/errors:
https://doc.rust-lang.org/rustc/lints/levels.html