linux-kernel - Re: [PATCH v5 5/6] rust: rbtree: add `RBTreeCursor`

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Date: Tue, 18 Jun 2024 12:11:15 -0700
From: Boqun Feng <boqun.feng@...il.com>
To: Matt Gilbride <mattgilbride@...gle.com>
Cc: Miguel Ojeda <ojeda@...nel.org>, Alex Gaynor <alex.gaynor@...il.com>,
	Wedson Almeida Filho <wedsonaf@...il.com>,
	Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...sung.com>,
	Alice Ryhl <aliceryhl@...gle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Arve Hjønnevåg <arve@...roid.com>,
	Todd Kjos <tkjos@...roid.com>, Martijn Coenen <maco@...roid.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Carlos Llamas <cmllamas@...gle.com>,
	Suren Baghdasaryan <surenb@...gle.com>,
	Christian Brauner <brauner@...nel.org>,
	Rob Landley <rob@...dley.net>, Davidlohr Bueso <dave@...olabs.net>,
	Michel Lespinasse <michel@...pinasse.org>,
	rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 5/6] rust: rbtree: add `RBTreeCursor`

On Thu, Jun 06, 2024 at 02:50:08PM +0000, Matt Gilbride wrote:
[...]
> +
> +    /// Returns a cursor over the tree nodes based on the given key.
> +    ///
> +    /// If the given key exists, the cursor starts there.
> +    /// Otherwise it starts with the first larger key in sort order.
> +    /// If there is no larger key, it returns [`None`].
> +    pub fn cursor_lower_bound(&mut self, key: &K) -> Option<RBTreeCursor<'_, K, V>>
> +    where
> +        K: Ord,
> +    {
> +        let mut node = self.root.rb_node;
> +        let mut best_match: Option<NonNull<Node<K, V>>> = None;
> +        while !node.is_null() {
> +            // SAFETY: By the type invariant of `Self`, all non-null `rb_node` pointers stored in `self`
> +            // point to the links field of `Node<K, V>` objects.
> +            let this = unsafe { container_of!(node, Node<K, V>, links) }.cast_mut();
> +            // SAFETY: `this` is a non-null node so it is valid by the type invariants.
> +            let this_key = unsafe { &(*this).key };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let left_child = unsafe { (*node).rb_left };
> +            // SAFETY: `node` is a non-null node so it is valid by the type invariants.
> +            let right_child = unsafe { (*node).rb_right };
> +            if key == this_key {
> +                // INVARIANT:
> +                // - `node` is a valid node in the [`RBTree`] pointed to by `self`.
> +                // - Due to the type signature of this function, the returned [`RBTreeCursor`]
> +                //   borrows mutably from `self`.
> +                return Some(RBTreeCursor {
> +                    tree: self,
> +                    current: node,
> +                });
> +            } else {
> +                node = if key > this_key {
> +                    right_child
> +                } else {
> +                    let is_better_match = match best_match {
> +                        None => true,
> +                        Some(best) => {
> +                            // SAFETY: `best` is a non-null node so it is valid by the type invariants.
> +                            let best_key = unsafe { &(*best.as_ptr()).key };
> +                            best_key > this_key
> +                        }
> +                    };
> +                    if is_better_match {
> +                        best_match = NonNull::new(this);
> +                    }
> +                    left_child
> +                }
> +            };

These two lines above should really be:

+                };
+            }

, right? Any lint we can use to catch this?

Regards,
Boqun

> +        }
[...]