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

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Message-ID: <D8IMCFZJ2G7C.28DHUZGDB596P@proton.me>
Date: Mon, 17 Mar 2025 14:42:23 +0000
From: Benno Lossin <benno.lossin@...ton.me>
To: Alice Ryhl <aliceryhl@...gle.com>, Tamir Duberstein <tamird@...il.com>
Cc: 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 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?

---
Cheers,
Benno