linux-kernel - [PATCH v2 2/4] rust: alloc: add `Vec::dec

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Message-Id: <20250318-vec-set-len-v2-2-293d55f82d18@gmail.com>
Date: Tue, 18 Mar 2025 16:13:54 -0400
From: Tamir Duberstein <tamird@...il.com>
To: Danilo Krummrich <dakr@...nel.org>, 
 Andrew Ballance <andrewjballance@...il.com>, 
 Alice Ryhl <aliceryhl@...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>, 
 Benno Lossin <benno.lossin@...ton.me>, 
 Andreas Hindborg <a.hindborg@...nel.org>, Trevor Gross <tmgross@...ch.edu>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
 Tamir Duberstein <tamird@...il.com>
Subject: [PATCH v2 2/4] rust: alloc: add `Vec::dec_len`

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`.

Signed-off-by: Tamir Duberstein <tamird@...il.com>
---
 rust/kernel/alloc/kvec.rs | 19 +++++++++++++++++++
 1 file changed, 19 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ce58ee66c99b..97cc5ab11e2a 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -198,6 +198,25 @@ pub unsafe fn set_len(&mut self, new_len: usize) {
         self.len = new_len;
     }
 
+    /// Decreases `self.len` by `count`.
+    ///
+    /// Returns a mutable slice to the elements forgotten by the vector. It is the caller's
+    /// responsibility to drop these elements if necessary.
+    ///
+    /// # Safety
+    ///
+    /// - `count` must be less than or equal to `self.len`.
+    unsafe fn dec_len(&mut self, count: usize) -> &mut [T] {
+        debug_assert!(count <= self.len());
+        // INVARIANT: We relinquish ownership of the elements within the range `[self.len - count,
+        // self.len)`, hence the updated value of `set.len` represents the exact number of elements
+        // stored within `self`.
+        self.len -= count;
+        // SAFETY: The memory after `self.len()` is guaranteed to contain `count` initialized
+        // elements of type `T`.
+        unsafe { slice::from_raw_parts_mut(self.as_mut_ptr().add(self.len), count) }
+    }
+
     /// Returns a slice of the entire vector.
     #[inline]
     pub fn as_slice(&self) -> &[T] {

-- 
2.48.1