The macro relied on an expression of the form Enum::Variant always being a
variant of the enum. However, it may also be an associated integer constant, in
which case there's no guarantee that the value of said constant consists only of
bits valid for this bitflag type.

Thus, code like this could create an invalid BitFlags<Test>, which would cause
iterating over it to trigger undefined behavior. As the debug formatter
internally iterates over the value, it is also affected.

use enumflags2::{bitflags, make_bitflags};

#[bitflags]
#[repr(u8)]
#[derive(Copy, Clone, Debug)]
enum Test {
    A = 1,
    B = 2,
}

impl Test {
    const C: u8 = 69;
}

fn main() {
    let x = make_bitflags!(Test::{C});
    // printing or iterating over x is UB
}

References

• https://github.com/meithecatte/enumflags2/releases/tag/v0.7.7
• https://rustsec.org/advisories/RUSTSEC-2023-0035.html