Summary

Using size_of::<$T>() * 8 is a fully generic way in macros to get the size of some type $T in bits. This allows the macro to be used for any kind of type. If this macro is then used on a numeric type clippy should not tell you to use BITS because it's possible, perhaps even likely, that the macro is also used for non-numeric types.

Lint Name

clippy::manual_bits

Reproducer

macro_rules! some_macro {
    ($T: ty) => {
        let bits = size_of::<$T>() * 8;
        dbg!(bits);
    }
}

fn main() {
    some_macro!(u32);
    some_macro!(String);
}

Version

rustc 1.80.1 (3f5fd8dd4 2024-08-06)
binary: rustc
commit-hash: 3f5fd8dd41153bc5fdca9427e9e05be2c767ba23
commit-date: 2024-08-06
host: aarch64-apple-darwin
release: 1.80.1
LLVM version: 18.1.7

Additional Labels

No response