linux-kernel - [PATCH v3 3/4] rust: alloc: refactor `Vec::truncate` using `dec

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Message-Id: <20250407-vec-set-len-v3-3-c5da0d03216e@gmail.com>
Date: Mon, 07 Apr 2025 10:52:33 -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 v3 3/4] rust: alloc: refactor `Vec::truncate` using
 `dec_len`

Use `checked_sub` to satisfy the safety requirements of `dec_len` and
replace nearly the whole body of `truncate` with a call to `dec_len`.

Signed-off-by: Tamir Duberstein <tamird@...il.com>
---
 rust/kernel/alloc/kvec.rs | 24 ++++++++----------------
 1 file changed, 8 insertions(+), 16 deletions(-)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 844154d310f0..d0b46aa36169 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -493,23 +493,15 @@ pub fn reserve(&mut self, additional: usize, flags: Flags) -> Result<(), AllocEr
     /// # Ok::<(), Error>(())
     /// ```
     pub fn truncate(&mut self, len: usize) {
-        if len >= self.len() {
-            return;
+        if let Some(count) = self.len().checked_sub(len) {
+            // SAFETY: `count` is `self.len() - len` so it is guaranteed to be less than or
+            // equal to `self.len()`.
+            let ptr: *mut [T] = unsafe { self.dec_len(count) };
+
+            // SAFETY: the contract of `dec_len` guarantees that the elements in `ptr` are
+            // valid elements whose ownership has been transferred to the caller.
+            unsafe { ptr::drop_in_place(ptr) };
         }
-
-        let drop_range = len..self.len();
-
-        // SAFETY: `drop_range` is a subrange of `[0, len)` by the bounds check above.
-        let ptr: *mut [T] = unsafe { self.get_unchecked_mut(drop_range) };
-
-        // SAFETY: By the above bounds check, it is guaranteed that `len < self.capacity()`.
-        unsafe { self.set_len(len) };
-
-        // SAFETY:
-        // - the dropped values are valid `T`s by the type invariant
-        // - we are allowed to invalidate [`new_len`, `old_len`) because we just changed the
-        //   len, therefore we have exclusive access to [`new_len`, `old_len`)
-        unsafe { ptr::drop_in_place(ptr) };
     }
 }
 

-- 
2.49.0