linux-kernel - Re: [PATCH 2/2] rust: alloc: add `Vec::dec

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Message-ID: <CAJ-ks9kXySdc2OdZ3uxwvRGYKSyBUNtg=bxovxGrOmLyZ=Rt8Q@mail.gmail.com>
Date: Mon, 17 Mar 2025 12:21:13 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: Benno Lossin <benno.lossin@...ton.me>, Alice Ryhl <aliceryhl@...gle.com>, 
	Andrew Ballance <andrewjballance@...il.com>, Miguel Ojeda <ojeda@...nel.org>, 
	Alex Gaynor <alex.gaynor@...il.com>, Boqun Feng <boqun.feng@...il.com>, 
	Gary Guo <gary@...yguo.net>, Björn Roy Baron <bjorn3_gh@...tonmail.com>, 
	Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>, 
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/2] rust: alloc: add `Vec::dec_len`

On Mon, Mar 17, 2025 at 12:17 PM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On Mon, Mar 17, 2025 at 10:44:25AM -0400, Tamir Duberstein wrote:
> > On Mon, Mar 17, 2025 at 10:42 AM Benno Lossin <benno.lossin@...ton.me> wrote:
> > >
> > > On Mon Mar 17, 2025 at 1:59 PM CET, Alice Ryhl wrote:
> > > > On Mon, Mar 17, 2025 at 11:47:50AM +0000, Alice Ryhl wrote:
> > > >> On Mon, Mar 17, 2025 at 07:34:44AM -0400, Tamir Duberstein wrote:
> > > >> > On Mon, Mar 17, 2025 at 6:04 AM Benno Lossin <benno.lossin@...ton.me> wrote:
> > > >> > >
> > > >> > > On Sun Mar 16, 2025 at 11:32 PM CET, Tamir Duberstein wrote:
> > > >> > > > Add `Vec::dec_len` that reduces the length of the receiver. This method
> > > >> > > > is intended to be used from methods that remove elements from `Vec` such
> > > >> > > > as `truncate`, `pop`, `remove`, and others. This method is intentionally
> > > >> > > > not `pub`.
> > > >> > >
> > > >> > > I think it should be `pub`. Otherwise we're loosing functionality
> > > >> > > compared to now. If one decides to give the raw pointer to some C API
> > > >> > > that takes ownership of the pointer, then I want them to be able to call
> > > >> > > `dec_len` manually.
> > > >> >
> > > >> > This is premature. It is trivial to make this function pub when the need arises.
> > > >>
> > > >> Normally I'd agree with Benno, but in this case I think having it
> > > >> private is preferable. The function is safe, so it's too easy for
> > > >> end-users to confuse it with truncate.
> > > >
> > > > Thinking more about this ... I think we should have `set_len` and
> > > > `inc_len` instead. That way, both methods are unsafe so people will not
> > > > accidentally use `set_len` when they meant to use `truncate`.
> > >
> > > I agree for this on the public API. The way I usually saw `set_len`
> > > being used for decrementing was truncation without dropping the old
> > > values. And that is going to be `vec.dec_len(vec.len())` with the
> > > current design. `vec.set_len(0);` is much clearer in that respect.
> > >
> > > But for the internals, I'd say that `dec_len` is nicer, so for `pop` one
> > > would then use `self.dec_len(1)`.
> > >
> > > How about we keep `set_len` and make `dec_len` a private, safe helper?
> >
> > This discussion is _way_ too speculative for my taste. If you'd like
> > to do this kind of thing, I'm happy to drop this patch or the series.
> > I'm not comfortable adding API whose usage I haven't seen and don't
> > understand.
>
> Seems like setting the length of a vector is a hard thing to do. :)
>
> I advocate for a middle ground.
>
> (1) Let's keep dec_len() a private and safe helper, it clearly improves the
> internals.

I don't agree with making it safe by using saturating_sub. I prefer
that the caller must justify that they aren't calling it with count >
self.len().

>
> (2) Introduce set_len() as a public API and defer the question on how to support
> dec_len() in a public API once the need arises.

Do we have line of sight on a public caller?