linux-kernel - Re: [PATCH v9 3/5] rust: debugfs: Support `PinInit` backing for `File`s.

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Message-ID: <2a0f2965-af37-4ffe-8806-3bb469361b1d@de.bosch.com>
Date: Tue, 19 Aug 2025 07:51:35 +0200
From: Dirk Behme <dirk.behme@...bosch.com>
To: Matthew Maurer <mmaurer@...gle.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>, "Alice
 Ryhl" <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>, "Danilo
 Krummrich" <dakr@...nel.org>, 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>
CC: <linux-kernel@...r.kernel.org>, <rust-for-linux@...r.kernel.org>
Subject: Re: [PATCH v9 3/5] rust: debugfs: Support `PinInit` backing for
 `File`s.

Hi Matthew,

On 09/07/2025 21:09, Matthew Maurer wrote:
> This allows `File`s to own their data, allowing DebugFS files to be
> managed in sync with the data that backs them. Because DebugFS files are
> intended to actually own data and provide access, `File`s still maintain
> the same lifecycle for provided data when `CONFIG_DEBUG_FS` is disabled.
> 
> Signed-off-by: Matthew Maurer <mmaurer@...gle.com>
> ---
>  rust/kernel/debugfs.rs | 149 ++++++++++++++++++++++++++++++++++++++-----------
>  1 file changed, 117 insertions(+), 32 deletions(-)
> 
> diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
> index e5b6497d1deb67671d22ffd90cd5baa855bb9257..a1a84dd309216f455ae8fe3d3c0fd00f957f82a9 100644
> --- a/rust/kernel/debugfs.rs
> +++ b/rust/kernel/debugfs.rs
> @@ -5,12 +5,13 @@
>  //!
>  //! C header: [`include/linux/debugfs.h`](srctree/include/linux/debugfs.h)
>  
> -#[cfg(CONFIG_DEBUG_FS)]
> -use crate::prelude::GFP_KERNEL;
> +use crate::prelude::*;
>  use crate::str::CStr;
>  #[cfg(CONFIG_DEBUG_FS)]
>  use crate::sync::Arc;
>  use core::fmt::Display;
> +use core::marker::PhantomPinned;
> +use core::ops::Deref;
>  
>  #[cfg(CONFIG_DEBUG_FS)]
>  mod display_file;
> @@ -63,40 +64,78 @@ fn create(_name: &CStr, _parent: Option<&Dir>) -> Self {
>      }
>  
>      #[cfg(CONFIG_DEBUG_FS)]
> -    fn create_file<T: Display + Sized + Sync>(&self, name: &CStr, data: &'static T) -> File {
> -        let Some(parent) = &self.0 else {
> -            return File {
> +    /// Creates a DebugFS file which will own the data produced by the initializer provided in
> +    /// `data`.
> +    ///
> +    /// # Safety
> +    ///
> +    /// The provided vtable must be appropriate for implementing a seq_file if provided
> +    /// with a private data pointer which provides shared access to a `T`.
> +    unsafe fn create_file<'a, T: Sync, E, TI: PinInit<T, E>>(
> +        &self,
> +        name: &'a CStr,
> +        data: TI,
> +        vtable: &'static bindings::file_operations,
> +    ) -> impl PinInit<File<T>, E> + use<'_, 'a, T, E, TI> {

Rebasing my test code from an older version of this series to this v9
(this is the most recent one?) here in rust/kernel/debugfs.rs and in
samples/rust/rust_debugfs.rs I get errors for each place where
'use<...>' is used:

error[E0658]: precise captures on `impl Trait` are experimental
  --> rust/kernel/debugfs.rs:81:37
   |
81 |     ) -> impl PinInit<File<T>, E> + use<'_, 'a, T, E, TI> {
   |                                     ^^^
   |
   = note: see issue #123432
<https://github.com/rust-lang/rust/issues/123432> for more information
   = help: add `#![feature(precise_capturing)]` to the crate attributes
to enable
   = note: this compiler was built on 2024-09-04; consider upgrading it
if it is out of date

rustc is

rustc 1.81.0 (eeb90cda1 2024-09-04)

I tried to work around this by adding

--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -36,6 +36,9 @@
 //
 // To be determined.
 #![feature(used_with_arg)]
+
+#![feature(precise_capturing)]
+

This seems to help for rust/kernel/debugfs.rs but not for
samples/rust/rust_debugfs.rs.

Any hint?

Best regards

Dirk