Mark a type as allowing any value

[Morganamilo]

Would it be possible to mark a bitflag as allowing any value?

This is useful to me when wrapping C apis that use bitflags where we as the user of the API may not be aware of all the possible flags. I would like to take the cint and turn it into a bitflags type, perserving the original value, while only defining the flag bits that I am aware of/are public/care about.

[Morganamilo]

Thinking about it I could just bypass the unsafe unchecked function by wrapping it in a safe one.

However the saftey sections says:

The caller of from_bits_unchecked() has to ensure that all bits correspond to a defined flag or that extra bits are valid for this bitflags type.

Is there actually any reason this invariant has to be held? If I violate it is there any UB issues?

[KodrAus]

Hey @Morganamilo 👋

This method has been a source of confusion since it was introduced and isn't actually possible to make any safety guarantees on. We're going to be deprecating it in favor of a from_bits_preserve function.

In the meantime you can add an inherent method to your flags type:

bitflags! {
    /// baz
    pub struct Flags: u32 {
        const A = 0b00000001;
        #[doc = "bar"]
        const B = 0b00000010;
        const C = 0b00000100;
        #[doc = "foo"]
        const ABC = Flags::A.bits() | Flags::B.bits() | Flags::C.bits();
    }
}

impl Flags {
    pub fn from_bits_preserve(bits: u32) -> Self {
        Flags { bits }
    }
}

[Morganamilo]

Glad it's being worked on. Thanks.