Message-ID: <20260102125357.402c0e15.gary@garyguo.net>
Date: Fri, 2 Jan 2026 12:53:57 +0000
From: Gary Guo <gary@...yguo.net>
To: Kari Argillander <kari.argillander@...il.com>
Cc: Daniel Almeida <daniel.almeida@...labora.com>, Filipe Xavier
 <felipeaggger@...il.com>, Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor
 <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>,
 Björn Roy Baron <bjorn3_gh@...tonmail.com>, Benno Lossin
 <benno.lossin@...ton.me>, Andreas Hindborg <a.hindborg@...nel.org>, Alice
 Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, Danilo
 Krummrich <dakr@...nel.org>, rust-for-linux@...r.kernel.org,
 felipe_life@...e.com, linux-kernel@...r.kernel.org, Lyude Paul
 <lyude@...hat.com>
Subject: Re: [PATCH v7] rust: add new macro for common bitmap operations

On Thu, 1 Jan 2026 23:39:26 +0200
Kari Argillander <kari.argillander@...il.com> wrote:

> to 1.1.2026 klo 22.23 Daniel Almeida (daniel.almeida@...labora.com) kirjoitti:
> > > On 1 Jan 2026, at 16:14, Kari Argillander <kari.argillander@...il.com> wrote:
> > >
> > > On Thu, 1 Jan 2026 at 20:21, Filipe Xavier <felipeaggger@...il.com> wrote:
> > >  
> > >> +/// // Combine multiple permissions using operation OR (`|`).
> > >> +/// let read_write: Permissions = Permission::Read | Permission::Write;
> > >> +///
> > >> +/// assert!(read_write.contains(Permission::Read));
> > >> +/// assert!(read_write.contains(Permission::Write));
> > >> +/// assert!(!read_write.contains(Permission::Execute));
> > >> +/// assert!(read_write.contains_any(Permission::Read | Permission::Execute));
> > >> +/// assert!(read_write.contains_all(Permission::Read | Permission::Write));
> > >> +///
> > >> +/// // Removing a permission with operation AND (`&`).
> > >> +/// let read_only: Permissions = read_write & Permission::Read;
> > >> +/// assert!(read_only.contains(Permission::Read));
> > >> +/// assert!(!read_only.contains(Permission::Write));
> > >> +///
> > >> +/// // Toggling permissions with XOR (`^`).
> > >> +/// let toggled: Permissions = read_only ^ Permission::Read;
> > >> +/// assert!(!toggled.contains(Permission::Read));
> > >> +///
> > >> +/// // Inverting permissions with negation (`!`).
> > >> +/// let negated = !read_only;
> > >> +/// assert!(negated.contains(Permission::Write));
> > >> +/// assert!(!negated.contains(Permission::Read));  
> > >
> > > I really like the OR (`|`) operator. Personally, I don’t like anything
> > > else. When
> > > a function or expression gets longer, something like let negated = !read_only;
> > > already feels strange to read.
> > >
> > > My suggestion is that everything else should be functions, so we would write:
> > >
> > > ```rust
> > >    let xxx = Permission::Read | Permission::Write;
> > >    xxx.remove(Permission::Write | Permission::Read);
> > >    xxx.toggle(Permission::Execute);
> > >    xxx.invert();
> > > ```
> > >
> > > This way we avoid expressions like:
> > >
> > > ```rust
> > >    let a = !(!xxx | yyy ^ zzz);
> > > ```
> > >  
> > “  
> > > At that point, no one understands what is really happening anymore. People start
> > > wondering whether operator precedence or evaluation order matters, and may even
> > > have to go read the source code to figure it out. 
> >
> > I disagree. How would that look like given your suggestion?
> >  
> > >
> > > By only supporting OR (|) as an operator and expressing everything else as
> > > explicit function calls, the code becomes much easier to read, review, and
> > > maintain.
> > >
> > >    Argillander  
> >
> > How is A | B readable but not A & B or A ^ B?  
> 
> Because that is just a way to express multiple / group of Flags and nothing
> more. And thinking of this it could be even & operator. So we only allow
> creating / enabling / disabling / toggling for a group of flags. Everything
> still needs to be explicit. So we would end up with something like
> 
> ```rust
>     Flags::new(A | B);
>     toggle(A | B);
>     disable(A | B); // Or remove() (I do not have opinion)
>     enable(A | B);
> 
>     // I would even consider & operator here as then you read it quite nicely
>     // toggle (A and B)
>     toggle(A & B);
> ```
> 
> Most annoying thing with flags is that if you need to enable 8 flags then some
> operator becomes a must. Still using those in everywhere is error prone and
> confusing. So I totally agreed that we need some way to express group of flags
> without always doing something like "t.enable(Flag::A).enable(Flag::B)"
> 
> If this way is used it is really nice that every function get's documentation.
> If we only allow | or & there is very little chance that someone mix what those
> means as it is always documented function call. Using operators too freely feels
> very C++ in my opinion. Rust is very explicit language so let's not change that
> but let's still make it usable for our cases.

I don't see why using operators is C++ specific. You do use these
operators in C, too.

I would rather write `flag & mask` rather than
`flag.remove(mask.invert())`.

Using operators make the bitflags nature explicit to the users and I think
that's a pro not a con.

Best,
Gary

> 
> If this way is used it is really nice that every function get's documentation.
> If we only allow | or & there is very little chance that someone mix what those
> means as it is always documented function call. Using operators too freely feels
> very C++ in my opinion. Rust is very explicit language so let's not change that
> but let's still make it usable for our cases.
> 
>     Argillander

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