How to truncate if `const _ = !0;` was used #443

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opened 2025-04-24 12:22:47 -05:00 by rusty-snake · 1 comment

rusty-snake commented

2025-04-24 12:22:47 -05:00

(Migrated from github.com)

I'm in the situation where a bitflags type from a third-party crate uses const _ = !0; (i.e. I can not modify the bitflags! definition and add/remove flags). I need the complement of all known/defined flags. However I do not see a way to truncate/... theses flags in this case or do a complement_defined or access FooFlags::_ or whatever. I can not even tell if there are extra bits set.

I'm in the situation where a bitflags type from a third-party crate uses `const _ = !0;` (i.e. I can not modify the `bitflags!` definition and add/remove flags). I need the `complement` of all known/defined flags. However I do not see a way to `truncate`/... theses flags in this case or do a `complement_defined` or access `FooFlags::_` or whatever. I can not even tell if there are extra bits set.

KodrAus commented

2025-04-28 17:07:41 -05:00

(Migrated from github.com)

If the library is using an unnamed _ = !0 flag then there aren't really any unknown flags or bits so to speak. The underlying definition could introduce new flags before the library adds them, so any set bits should be considered valid.

Something you can do though is get a flags value that only includes named flags by iterating over them, like in this example:

let defined = <MyFlags as bitflags::Flags>::FLAGS
    .iter()
    .filter(|f| !f.name().is_empty())
    .fold(MyFlags::empty(), |acc, f| acc | *f.value());

We could maybe define a Flags::all_named constructor that produced this value, and optimize it for the bitflags! macro.

If the library is using an unnamed `_ = !0` flag then there aren't really any unknown flags or bits so to speak. The underlying definition could introduce new flags before the library adds them, so any set bits should be considered valid. Something you can do though is get a flags value that only includes named flags by iterating over them, like [in this example](https://play.rust-lang.org/?version=stable&mode=debug&edition=2024&gist=e8155b2a5a9d36a1ade3d5b1468e1573): ```rust let defined = <MyFlags as bitflags::Flags>::FLAGS .iter() .filter(|f| !f.name().is_empty()) .fold(MyFlags::empty(), |acc, f| acc | *f.value()); ``` We could maybe define a `Flags::all_named` constructor that produced this value, and optimize it for the `bitflags!` macro.

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How to truncate if const _ = !0; was used #443

How to truncate if `const _ = !0;` was used #443