Introduce support for read-only, write-only, and read-write binary files
in Rust debugfs. This adds:

- BinaryWriter and BinaryReader traits for writing to and reading from
  user slices in binary form.
- New Dir methods: read_binary_file(), write_binary_file(),
  `read_write_binary_file`.
- Corresponding FileOps implementations: BinaryReadFile,
  BinaryWriteFile, BinaryReadWriteFile.

This allows kernel modules to expose arbitrary binary data through
debugfs, with proper support for offsets and partial reads/writes.

Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Matthew Maurer <mmaurer@google.com>
Signed-off-by: Danilo Krummrich <dakr@kernel.org>
---
 rust/kernel/debugfs.rs          |  66 ++++++++++++++-
 rust/kernel/debugfs/file_ops.rs | 146 +++++++++++++++++++++++++++++++-
 rust/kernel/debugfs/traits.rs   |  68 ++++++++++++++-
 3 files changed, 273 insertions(+), 7 deletions(-)

diff --git a/rust/kernel/debugfs.rs b/rust/kernel/debugfs.rs
index 381c23b3dd83..95cd3376ecbe 100644
--- a/rust/kernel/debugfs.rs
+++ b/rust/kernel/debugfs.rs
@@ -21,12 +21,15 @@
 use core::ops::Deref;
 
 mod traits;
-pub use traits::{Reader, Writer};
+pub use traits::{BinaryReader, BinaryWriter, Reader, Writer};
 
 mod callback_adapters;
 use callback_adapters::{FormatAdapter, NoWriter, WritableAdapter};
 mod file_ops;
-use file_ops::{FileOps, ReadFile, ReadWriteFile, WriteFile};
+use file_ops::{
+    BinaryReadFile, BinaryReadWriteFile, BinaryWriteFile, FileOps, ReadFile, ReadWriteFile,
+    WriteFile,
+};
 #[cfg(CONFIG_DEBUG_FS)]
 mod entry;
 #[cfg(CONFIG_DEBUG_FS)]
@@ -150,6 +153,32 @@ pub fn read_only_file<'a, T, E: 'a>(
         self.create_file(name, data, file_ops)
     }
 
+    /// Creates a read-only binary file in this directory.
+    ///
+    /// The file's contents are produced by invoking [`BinaryWriter::write_to_slice`] on the value
+    /// initialized by `data`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// # use kernel::c_str;
+    /// # use kernel::debugfs::Dir;
+    /// # use kernel::prelude::*;
+    /// # let dir = Dir::new(c_str!("my_debugfs_dir"));
+    /// let file = KBox::pin_init(dir.read_binary_file(c_str!("foo"), [0x1, 0x2]), GFP_KERNEL)?;
+    /// # Ok::<(), Error>(())
+    /// ```
+    pub fn read_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryWriter + Send + Sync + 'static,
+    {
+        self.create_file(name, data, &T::FILE_OPS)
+    }
+
     /// Creates a read-only file in this directory, with contents from a callback.
     ///
     /// `f` must be a function item or a non-capturing closure.
@@ -206,6 +235,22 @@ pub fn read_write_file<'a, T, E: 'a>(
         self.create_file(name, data, file_ops)
     }
 
+    /// Creates a read-write binary file in this directory.
+    ///
+    /// Reading the file uses the [`BinaryWriter`] implementation.
+    /// Writing to the file uses the [`BinaryReader`] implementation.
+    pub fn read_write_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryWriter + BinaryReader + Send + Sync + 'static,
+    {
+        let file_ops = &<T as BinaryReadWriteFile<_>>::FILE_OPS;
+        self.create_file(name, data, file_ops)
+    }
+
     /// Creates a read-write file in this directory, with logic from callbacks.
     ///
     /// Reading from the file is handled by `f`. Writing to the file is handled by `w`.
@@ -248,6 +293,23 @@ pub fn write_only_file<'a, T, E: 'a>(
         self.create_file(name, data, &T::FILE_OPS)
     }
 
+    /// Creates a write-only binary file in this directory.
+    ///
+    /// The file owns its backing data. Writing to the file uses the [`BinaryReader`]
+    /// implementation.
+    ///
+    /// The file is removed when the returned [`File`] is dropped.
+    pub fn write_binary_file<'a, T, E: 'a>(
+        &'a self,
+        name: &'a CStr,
+        data: impl PinInit<T, E> + 'a,
+    ) -> impl PinInit<File<T>, E> + 'a
+    where
+        T: BinaryReader + Send + Sync + 'static,
+    {
+        self.create_file(name, data, &T::FILE_OPS)
+    }
+
     /// Creates a write-only file in this directory, with write logic from a callback.
     ///
     /// `w` must be a function item or a non-capturing closure.
diff --git a/rust/kernel/debugfs/file_ops.rs b/rust/kernel/debugfs/file_ops.rs
index 50fead17b6f3..ebdd2427d2ce 100644
--- a/rust/kernel/debugfs/file_ops.rs
+++ b/rust/kernel/debugfs/file_ops.rs
@@ -1,13 +1,14 @@
 // SPDX-License-Identifier: GPL-2.0
 // Copyright (C) 2025 Google LLC.
 
-use super::{Reader, Writer};
+use super::{BinaryReader, BinaryWriter, Reader, Writer};
 use crate::debugfs::callback_adapters::Adapter;
+use crate::fs::file;
 use crate::prelude::*;
 use crate::seq_file::SeqFile;
 use crate::seq_print;
 use crate::uaccess::UserSlice;
-use core::fmt::{Display, Formatter, Result};
+use core::fmt;
 use core::marker::PhantomData;
 
 #[cfg(CONFIG_DEBUG_FS)]
@@ -65,8 +66,8 @@ fn deref(&self) -> &Self::Target {
 
 struct WriterAdapter<T>(T);
 
-impl<'a, T: Writer> Display for WriterAdapter<&'a T> {
-    fn fmt(&self, f: &mut Formatter<'_>) -> Result {
+impl<'a, T: Writer> fmt::Display for WriterAdapter<&'a T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         self.0.write(f)
     }
 }
@@ -245,3 +246,140 @@ impl<T: Reader + Sync> WriteFile<T> for T {
         unsafe { FileOps::new(operations, 0o200) }
     };
 }
+
+extern "C" fn blob_read<T: BinaryWriter>(
+    file: *mut bindings::file,
+    buf: *mut c_char,
+    count: usize,
+    ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY:
+    // - `file` is a valid pointer to a `struct file`.
+    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
+    let this = unsafe { &*((*file).private_data.cast::<T>()) };
+
+    // SAFETY:
+    // `ppos` is a valid `file::Offset` pointer.
+    // We have exclusive access to `ppos`.
+    let pos = unsafe { file::Offset::from_raw(ppos) };
+
+    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
+
+    let ret = || -> Result<isize> {
+        let written = this.write_to_slice(&mut writer, pos)?;
+
+        Ok(written.try_into()?)
+    }();
+
+    match ret {
+        Ok(n) => n,
+        Err(e) => e.to_errno() as isize,
+    }
+}
+
+/// Representation of [`FileOps`] for read only binary files.
+pub(crate) trait BinaryReadFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryWriter + Sync> BinaryReadFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            read: Some(blob_read::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_read()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o400) }
+    };
+}
+
+extern "C" fn blob_write<T: BinaryReader>(
+    file: *mut bindings::file,
+    buf: *const c_char,
+    count: usize,
+    ppos: *mut bindings::loff_t,
+) -> isize {
+    // SAFETY:
+    // - `file` is a valid pointer to a `struct file`.
+    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
+    let this = unsafe { &*((*file).private_data.cast::<T>()) };
+
+    // SAFETY:
+    // `ppos` is a valid `file::Offset` pointer.
+    // We have exclusive access to `ppos`.
+    let pos = unsafe { file::Offset::from_raw(ppos) };
+
+    let mut reader = UserSlice::new(UserPtr::from_ptr(buf.cast_mut().cast()), count).reader();
+
+    let ret = || -> Result<isize> {
+        let read = this.read_from_slice(&mut reader, pos)?;
+
+        Ok(read.try_into()?)
+    }();
+
+    match ret {
+        Ok(n) => n,
+        Err(e) => e.to_errno() as isize,
+    }
+}
+
+/// Representation of [`FileOps`] for write only binary files.
+pub(crate) trait BinaryWriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryReader + Sync> BinaryWriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            write: Some(blob_write::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_write()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o200) }
+    };
+}
+
+/// Representation of [`FileOps`] for read/write binary files.
+pub(crate) trait BinaryReadWriteFile<T> {
+    const FILE_OPS: FileOps<T>;
+}
+
+impl<T: BinaryWriter + BinaryReader + Sync> BinaryReadWriteFile<T> for T {
+    const FILE_OPS: FileOps<T> = {
+        let operations = bindings::file_operations {
+            read: Some(blob_read::<T>),
+            write: Some(blob_write::<T>),
+            llseek: Some(bindings::default_llseek),
+            open: Some(bindings::simple_open),
+            // SAFETY: `file_operations` supports zeroes in all fields.
+            ..unsafe { core::mem::zeroed() }
+        };
+
+        // SAFETY:
+        // - The private data of `struct inode` does always contain a pointer to a valid `T`.
+        // - `simple_open()` stores the `struct inode`'s private data in the private data of the
+        //   corresponding `struct file`.
+        // - `blob_read()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `blob_write()` re-creates a reference to `T` from the `struct file`'s private data.
+        // - `default_llseek()` does not access the `struct file`'s private data.
+        unsafe { FileOps::new(operations, 0o600) }
+    };
+}
diff --git a/rust/kernel/debugfs/traits.rs b/rust/kernel/debugfs/traits.rs
index ab009eb254b3..bd38eb988d51 100644
--- a/rust/kernel/debugfs/traits.rs
+++ b/rust/kernel/debugfs/traits.rs
@@ -3,9 +3,11 @@
 
 //! Traits for rendering or updating values exported to DebugFS.
 
+use crate::fs::file;
 use crate::prelude::*;
 use crate::sync::Mutex;
-use crate::uaccess::UserSliceReader;
+use crate::transmute::{AsBytes, FromBytes};
+use crate::uaccess::{UserSliceReader, UserSliceWriter};
 use core::fmt::{self, Debug, Formatter};
 use core::str::FromStr;
 use core::sync::atomic::{
@@ -39,6 +41,44 @@ fn write(&self, f: &mut Formatter<'_>) -> fmt::Result {
     }
 }
 
+/// Trait for types that can be written out as binary.
+pub trait BinaryWriter {
+    /// Writes the binary form of `self` into `writer`.
+    ///
+    /// `offset` is the requested offset into the binary representation of `self`.
+    ///
+    /// On success, returns the number of bytes written in to `writer`.
+    fn write_to_slice(
+        &self,
+        writer: &mut UserSliceWriter,
+        offset: &mut file::Offset,
+    ) -> Result<usize>;
+}
+
+// Base implementation for any `T: AsBytes`.
+impl<T: AsBytes> BinaryWriter for T {
+    fn write_to_slice(
+        &self,
+        writer: &mut UserSliceWriter,
+        offset: &mut file::Offset,
+    ) -> Result<usize> {
+        writer.write_slice_file(self.as_bytes(), offset)
+    }
+}
+
+// Delegate for `Mutex<T>`: Support a `T` with an outer mutex.
+impl<T: BinaryWriter> BinaryWriter for Mutex<T> {
+    fn write_to_slice(
+        &self,
+        writer: &mut UserSliceWriter,
+        offset: &mut file::Offset,
+    ) -> Result<usize> {
+        let guard = self.lock();
+
+        guard.write_to_slice(writer, offset)
+    }
+}
+
 /// A trait for types that can be updated from a user slice.
 ///
 /// This works similarly to `FromStr`, but operates on a `UserSliceReader` rather than a &str.
@@ -66,6 +106,32 @@ fn read_from_slice(&self, reader: &mut UserSliceReader) -> Result {
     }
 }
 
+/// Trait for types that can be constructed from a binary representation.
+pub trait BinaryReader {
+    /// Reads the binary form of `self` from `reader`.
+    ///
+    /// `offset` is the requested offset into the binary representation of `self`.
+    ///
+    /// On success, returns the number of bytes read from `reader`.
+    fn read_from_slice(
+        &self,
+        reader: &mut UserSliceReader,
+        offset: &mut file::Offset,
+    ) -> Result<usize>;
+}
+
+impl<T: AsBytes + FromBytes> BinaryReader for Mutex<T> {
+    fn read_from_slice(
+        &self,
+        reader: &mut UserSliceReader,
+        offset: &mut file::Offset,
+    ) -> Result<usize> {
+        let mut this = self.lock();
+
+        reader.read_slice_file(this.as_bytes_mut(), offset)
+    }
+}
+
 macro_rules! impl_reader_for_atomic {
     ($(($atomic_type:ty, $int_type:ty)),*) => {
         $(
-- 
2.51.0

On Wed, Oct 22, 2025 at 04:30:40PM +0200, Danilo Krummrich wrote:
> Introduce support for read-only, write-only, and read-write binary files
> in Rust debugfs. This adds:
> 
> - BinaryWriter and BinaryReader traits for writing to and reading from
>   user slices in binary form.
> - New Dir methods: read_binary_file(), write_binary_file(),
>   `read_write_binary_file`.
> - Corresponding FileOps implementations: BinaryReadFile,
>   BinaryWriteFile, BinaryReadWriteFile.
> 
> This allows kernel modules to expose arbitrary binary data through
> debugfs, with proper support for offsets and partial reads/writes.
> 
> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> Signed-off-by: Danilo Krummrich <dakr@kernel.org>

> +extern "C" fn blob_read<T: BinaryWriter>(
> +    file: *mut bindings::file,
> +    buf: *mut c_char,
> +    count: usize,
> +    ppos: *mut bindings::loff_t,
> +) -> isize {
> +    // SAFETY:
> +    // - `file` is a valid pointer to a `struct file`.
> +    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
> +    let this = unsafe { &*((*file).private_data.cast::<T>()) };
> +
> +    // SAFETY:
> +    // `ppos` is a valid `file::Offset` pointer.
> +    // We have exclusive access to `ppos`.
> +    let pos = unsafe { file::Offset::from_raw(ppos) };
> +
> +    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
> +
> +    let ret = || -> Result<isize> {
> +        let written = this.write_to_slice(&mut writer, pos)?;
> +
> +        Ok(written.try_into()?)

Hmm ... a conversion? Sounds like write_to_slice() has the wrong return
type.

Alice

On Thu Oct 23, 2025 at 10:26 AM CEST, Alice Ryhl wrote:
> On Wed, Oct 22, 2025 at 04:30:40PM +0200, Danilo Krummrich wrote:
>> Introduce support for read-only, write-only, and read-write binary files
>> in Rust debugfs. This adds:
>> 
>> - BinaryWriter and BinaryReader traits for writing to and reading from
>>   user slices in binary form.
>> - New Dir methods: read_binary_file(), write_binary_file(),
>>   `read_write_binary_file`.
>> - Corresponding FileOps implementations: BinaryReadFile,
>>   BinaryWriteFile, BinaryReadWriteFile.
>> 
>> This allows kernel modules to expose arbitrary binary data through
>> debugfs, with proper support for offsets and partial reads/writes.
>> 
>> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
>> Reviewed-by: Matthew Maurer <mmaurer@google.com>
>> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
>
>> +extern "C" fn blob_read<T: BinaryWriter>(
>> +    file: *mut bindings::file,
>> +    buf: *mut c_char,
>> +    count: usize,
>> +    ppos: *mut bindings::loff_t,
>> +) -> isize {
>> +    // SAFETY:
>> +    // - `file` is a valid pointer to a `struct file`.
>> +    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
>> +    let this = unsafe { &*((*file).private_data.cast::<T>()) };
>> +
>> +    // SAFETY:
>> +    // `ppos` is a valid `file::Offset` pointer.
>> +    // We have exclusive access to `ppos`.
>> +    let pos = unsafe { file::Offset::from_raw(ppos) };
>> +
>> +    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
>> +
>> +    let ret = || -> Result<isize> {
>> +        let written = this.write_to_slice(&mut writer, pos)?;
>> +
>> +        Ok(written.try_into()?)
>
> Hmm ... a conversion? Sounds like write_to_slice() has the wrong return
> type.

write_to_slice() returns the number of bytes written as usize, which seems
correct, no?

On Thu, Oct 23, 2025 at 12:09 PM Danilo Krummrich <dakr@kernel.org> wrote:
>
> On Thu Oct 23, 2025 at 10:26 AM CEST, Alice Ryhl wrote:
> > On Wed, Oct 22, 2025 at 04:30:40PM +0200, Danilo Krummrich wrote:
> >> Introduce support for read-only, write-only, and read-write binary files
> >> in Rust debugfs. This adds:
> >>
> >> - BinaryWriter and BinaryReader traits for writing to and reading from
> >>   user slices in binary form.
> >> - New Dir methods: read_binary_file(), write_binary_file(),
> >>   `read_write_binary_file`.
> >> - Corresponding FileOps implementations: BinaryReadFile,
> >>   BinaryWriteFile, BinaryReadWriteFile.
> >>
> >> This allows kernel modules to expose arbitrary binary data through
> >> debugfs, with proper support for offsets and partial reads/writes.
> >>
> >> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> >> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> >> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> >
> >> +extern "C" fn blob_read<T: BinaryWriter>(
> >> +    file: *mut bindings::file,
> >> +    buf: *mut c_char,
> >> +    count: usize,
> >> +    ppos: *mut bindings::loff_t,
> >> +) -> isize {
> >> +    // SAFETY:
> >> +    // - `file` is a valid pointer to a `struct file`.
> >> +    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
> >> +    let this = unsafe { &*((*file).private_data.cast::<T>()) };
> >> +
> >> +    // SAFETY:
> >> +    // `ppos` is a valid `file::Offset` pointer.
> >> +    // We have exclusive access to `ppos`.
> >> +    let pos = unsafe { file::Offset::from_raw(ppos) };
> >> +
> >> +    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
> >> +
> >> +    let ret = || -> Result<isize> {
> >> +        let written = this.write_to_slice(&mut writer, pos)?;
> >> +
> >> +        Ok(written.try_into()?)
> >
> > Hmm ... a conversion? Sounds like write_to_slice() has the wrong return
> > type.
>
> write_to_slice() returns the number of bytes written as usize, which seems
> correct, no?

Yes, you're right, I think usize is the right value. The cast is
unfortunate, but it can't really be avoided. In practice it should
never fail because slice lengths always fit in an isize, but isize
isn't the right type.

Alice

On Thu, Oct 23, 2025 at 12:21:24PM +0200, Alice Ryhl wrote:
> On Thu, Oct 23, 2025 at 12:09 PM Danilo Krummrich <dakr@kernel.org> wrote:
> >
> > On Thu Oct 23, 2025 at 10:26 AM CEST, Alice Ryhl wrote:
> > > On Wed, Oct 22, 2025 at 04:30:40PM +0200, Danilo Krummrich wrote:
> > >> Introduce support for read-only, write-only, and read-write binary files
> > >> in Rust debugfs. This adds:
> > >>
> > >> - BinaryWriter and BinaryReader traits for writing to and reading from
> > >>   user slices in binary form.
> > >> - New Dir methods: read_binary_file(), write_binary_file(),
> > >>   `read_write_binary_file`.
> > >> - Corresponding FileOps implementations: BinaryReadFile,
> > >>   BinaryWriteFile, BinaryReadWriteFile.
> > >>
> > >> This allows kernel modules to expose arbitrary binary data through
> > >> debugfs, with proper support for offsets and partial reads/writes.
> > >>
> > >> Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > >> Reviewed-by: Matthew Maurer <mmaurer@google.com>
> > >> Signed-off-by: Danilo Krummrich <dakr@kernel.org>
> > >
> > >> +extern "C" fn blob_read<T: BinaryWriter>(
> > >> +    file: *mut bindings::file,
> > >> +    buf: *mut c_char,
> > >> +    count: usize,
> > >> +    ppos: *mut bindings::loff_t,
> > >> +) -> isize {
> > >> +    // SAFETY:
> > >> +    // - `file` is a valid pointer to a `struct file`.
> > >> +    // - The type invariant of `FileOps` guarantees that `private_data` points to a valid `T`.
> > >> +    let this = unsafe { &*((*file).private_data.cast::<T>()) };
> > >> +
> > >> +    // SAFETY:
> > >> +    // `ppos` is a valid `file::Offset` pointer.
> > >> +    // We have exclusive access to `ppos`.
> > >> +    let pos = unsafe { file::Offset::from_raw(ppos) };
> > >> +
> > >> +    let mut writer = UserSlice::new(UserPtr::from_ptr(buf.cast()), count).writer();
> > >> +
> > >> +    let ret = || -> Result<isize> {
> > >> +        let written = this.write_to_slice(&mut writer, pos)?;
> > >> +
> > >> +        Ok(written.try_into()?)
> > >
> > > Hmm ... a conversion? Sounds like write_to_slice() has the wrong return
> > > type.
> >
> > write_to_slice() returns the number of bytes written as usize, which seems
> > correct, no?
> 
> Yes, you're right, I think usize is the right value. The cast is
> unfortunate, but it can't really be avoided. In practice it should
> never fail because slice lengths always fit in an isize, but isize
> isn't the right type.
> 
> Alice

Reviewed-by: Alice Ryhl <aliceryhl@google.com>