linux-kernel - Re: [PATCH] rust: rbtree: simplify finding `current` in `remove

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Message-ID: <aGeEcOEYXiLju-Lj@google.com>
Date: Fri, 4 Jul 2025 07:36:16 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: "Onur Özkan" <work@...rozkan.dev>
Cc: rust-for-linux@...r.kernel.org, linux-kernel@...r.kernel.org, 
	ojeda@...nel.org, alex.gaynor@...il.com, boqun.feng@...il.com, 
	gary@...yguo.net, bjorn3_gh@...tonmail.com, lossin@...nel.org, 
	a.hindborg@...nel.org, tmgross@...ch.edu, dakr@...nel.org, 
	mattgilbride@...gle.com, wedsonaf@...il.com, daniel@...lak.dev, 
	tamird@...il.com
Subject: Re: [PATCH] rust: rbtree: simplify finding `current` in `remove_current`

On Fri, Jul 04, 2025 at 08:45:39AM +0300, Onur Özkan wrote:
> The previous version used a verbose `match` to get
> `current`, which may be slightly confusing at first
> glance.
> 
> This change makes it shorter and more clearly expresses
> the intent: prefer `next` if available, otherwise fall
> back to `prev`.
> 
> Signed-off-by: Onur Özkan <work@...rozkan.dev>
> ---
>  rust/kernel/rbtree.rs | 14 +++-----------
>  1 file changed, 3 insertions(+), 11 deletions(-)
> 
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 8d978c896747..8f1052552132 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -769,18 +769,10 @@ pub fn remove_current(self) -> (Option<Self>, RBTreeNode<K, V>) {
>          // the tree cannot change. By the tree invariant, all nodes are valid.
>          unsafe { bindings::rb_erase(&mut (*this).links, addr_of_mut!(self.tree.root)) };
> 
> -        let current = match (prev, next) {
> -            (_, Some(next)) => next,
> -            (Some(prev), None) => prev,
> -            (None, None) => {
> -                return (None, node);
> -            }
> -        };
> -
>          (
> -            // INVARIANT:
> -            // - `current` is a valid node in the [`RBTree`] pointed to by `self.tree`.
> -            Some(Self {
> +            next.or(prev).map(|current| Self {
> +                // INVARIANT:
> +                // - `current` is a valid node in the [`RBTree`] pointed to by `self.tree`.
>                  current,
>                  tree: self.tree,
>              }),

I'm okay with this change, but the INVARIANT: comment usually goes
before the `StructName {` declaration rather than on the field. For
example, what about this?

	// INVARIANT:
	// - `current` is a valid node in the [`RBTree`] pointed to by `self.tree`.
	let cursor = next.or(prev).map(|current| Self {
	    current,
	    tree: self.tree,
	});
	
	(cursor, node)

Alice