Re: [PATCH 2/2] rust: alloc: add `Vec::dec_len`

[Tamir Duberstein]

On Tue, Mar 18, 2025 at 5:30 AM Alice Ryhl <aliceryhl@xxxxxxxxxx> wrote:
>
> 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@xxxxxxxxxx> 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@xxxxxxxxx> 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).

The methods you're describing are all on Vec, right? In other words,
their usage calls for a private `dec_len` or `set_len`. As I've said
repeatedly in the course of this discussion: I would prefer not to
introduce `dec_len` at all here. It (or `set_len`) can be introduced
in the series that adds truncate or your patch that adds clear, where
its signature can be properly scrutinized in the context of an actual
caller.

Tamir