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

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Message-ID: <Z9k9I6mp11Z358vz@google.com>
Date: Tue, 18 Mar 2025 09:30:11 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: Tamir Duberstein <tamird@...il.com>
Cc: Benno Lossin <benno.lossin@...ton.me>, Danilo Krummrich <dakr@...nel.org>, 
	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 09:53:04AM -0400, Tamir Duberstein wrote:
> On Mon, Mar 17, 2025 at 8:59 AM Alice Ryhl <aliceryhl@...gle.com> 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`.
> >
> > Alice
> 
> Isn't it fine to keep this function unsafe given that it can break
> invariants in its current form? As expressed earlier, I would prefer
> not to make it safe by using saturating_sub.

I guess that's okay. But I would think that with the exception of
`Vec::pop`, the interface you want for Vec methods that decrease the
length is set_len, not dec_len. That is the case for clear, truncate,
and drain.

Even for the Vec methods that actually use

	set_len(original_len - removed_cnt)

they make this call after having previously called set_len(0).

Alice