linux-kernel - Re: [PATCH v8 4/6] rust: debugfs: Support arbitrary owned backing for File

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Message-ID: <CAGSQo038u_so+_pMRYj0K546zNfO5-eqoXFivXsEF6ACK=Y5cw@mail.gmail.com>
Date: Mon, 30 Jun 2025 10:34:19 -0700
From: Matthew Maurer <mmaurer@...gle.com>
To: Danilo Krummrich <dakr@...nel.org>
Cc: 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>, Alice Ryhl <aliceryhl@...gle.com>, 
	Trevor Gross <tmgross@...ch.edu>, Greg Kroah-Hartman <gregkh@...uxfoundation.org>, 
	"Rafael J. Wysocki" <rafael@...nel.org>, Sami Tolvanen <samitolvanen@...gle.com>, 
	Timur Tabi <ttabi@...dia.com>, Benno Lossin <lossin@...nel.org>, linux-kernel@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, Dirk Behme <dirk.behme@...bosch.com>
Subject: Re: [PATCH v8 4/6] rust: debugfs: Support arbitrary owned backing for File

On Mon, Jun 30, 2025 at 10:30 AM Danilo Krummrich <dakr@...nel.org> wrote:
>
> On 6/28/25 1:18 AM, Matthew Maurer wrote:
> > +    fn create_file<D: ForeignOwnable>(&self, _name: &CStr, data: D) -> File
> > +    where
> > +        for<'a> D::Borrowed<'a>: Display,
> > +    {
> > +        File {
> > +            _foreign: ForeignHolder::new(data),
> > +        }
> >       }
>
> What's the motivation for the ForeignHolder abstraction? Why not just make it
> File<D> and store data directly?

1. A `File<D>` can't be held in collection data structures as easily
unless all your files contain the *same* backing type.
2. None of the APIs or potential APIs for `File` care about which type
it's wrapping, nor are they supposed to. If nothing you can do with a
`File` is different depending on the backing type, making it
polymorphic is just needlessly confusing.