From: Hang Shu <hangshu847@gmail.com>

The returned keys and values of cursor methods should be bound by
the lifetime of the rbtree itself ('a), not the lifetime of the cursor.

Without this adjustment, examples like the following fail to compile:

fn test_rbtree_cursor(rbtree: &mut RBTree<i32, i32>) -> &i32 {
    rbtree.try_create_and_insert(1, 1, GFP_KERNEL).unwrap();
    let mut cursor = rbtree.cursor_front().unwrap();
    // compile error
    // cannot return value referencing local variable `cursor`
    cursor.peek_next().unwrap().1
}

This modification ensures that references to tree elements remain valid
independently of the cursor's scope,
aligning with the actual lifetime dependencies in the data structure.

The changes will be applied to multiple similar methods
throughout the Cursor implementation to maintain consistency.

Fixes: 98c14e40e07a ("rust: rbtree: add cursor")
Signed-off-by: Hang Shu <hangshu847@gmail.com>
---
 rust/kernel/rbtree.rs | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
index 9e178dacddf1..702a1b6ef7a9 100644
--- a/rust/kernel/rbtree.rs
+++ b/rust/kernel/rbtree.rs
@@ -742,7 +742,7 @@ unsafe impl<'a, K: Sync, V: Sync> Sync for Cursor<'a, K, V> {}
 
 impl<'a, K, V> Cursor<'a, K, V> {
     /// The current node
-    pub fn current(&self) -> (&K, &V) {
+    pub fn current(&self) -> (&'a K, &'a V) {
         // SAFETY:
         // - `self.current` is a valid node by the type invariants.
         // - We have an immutable reference by the function signature.
@@ -750,7 +750,7 @@ pub fn current(&self) -> (&K, &V) {
     }
 
     /// The current node, with a mutable value
-    pub fn current_mut(&mut self) -> (&K, &mut V) {
+    pub fn current_mut(&mut self) -> (&'a K, &'a mut V) {
         // SAFETY:
         // - `self.current` is a valid node by the type invariants.
         // - We have an mutable reference by the function signature.
@@ -831,16 +831,16 @@ fn mv(self, direction: Direction) -> Option<Self> {
     }
 
     /// Access the previous node without moving the cursor.
-    pub fn peek_prev(&self) -> Option<(&K, &V)> {
+    pub fn peek_prev(&self) -> Option<(&'a K, &'a V)> {
         self.peek(Direction::Prev)
     }
 
     /// Access the next node without moving the cursor.
-    pub fn peek_next(&self) -> Option<(&K, &V)> {
+    pub fn peek_next(&self) -> Option<(&'a K, &'a V)> {
         self.peek(Direction::Next)
     }
 
-    fn peek(&self, direction: Direction) -> Option<(&K, &V)> {
+    fn peek(&self, direction: Direction) -> Option<(&'a K, &'a V)> {
         self.get_neighbor_raw(direction).map(|neighbor| {
             // SAFETY:
             // - `neighbor` is a valid tree node.
@@ -850,16 +850,16 @@ fn peek(&self, direction: Direction) -> Option<(&K, &V)> {
     }
 
     /// Access the previous node mutably without moving the cursor.
-    pub fn peek_prev_mut(&mut self) -> Option<(&K, &mut V)> {
+    pub fn peek_prev_mut(&mut self) -> Option<(&'a K, &'a mut V)> {
         self.peek_mut(Direction::Prev)
     }
 
     /// Access the next node mutably without moving the cursor.
-    pub fn peek_next_mut(&mut self) -> Option<(&K, &mut V)> {
+    pub fn peek_next_mut(&mut self) -> Option<(&'a K, &'a mut V)> {
         self.peek_mut(Direction::Next)
     }
 
-    fn peek_mut(&mut self, direction: Direction) -> Option<(&K, &mut V)> {
+    fn peek_mut(&mut self, direction: Direction) -> Option<(&'a K, &'a mut V)> {
         self.get_neighbor_raw(direction).map(|neighbor| {
             // SAFETY:
             // - `neighbor` is a valid tree node.
-- 
2.43.0

On Fri, Nov 7, 2025 at 6:07 AM Hang Shu <m18080292938@163.com> wrote:
>
> From: Hang Shu <hangshu847@gmail.com>
>
> The returned keys and values of cursor methods should be bound by
> the lifetime of the rbtree itself ('a), not the lifetime of the cursor.
>
> Without this adjustment, examples like the following fail to compile:
>
> fn test_rbtree_cursor(rbtree: &mut RBTree<i32, i32>) -> &i32 {
>     rbtree.try_create_and_insert(1, 1, GFP_KERNEL).unwrap();
>     let mut cursor = rbtree.cursor_front().unwrap();
>     // compile error
>     // cannot return value referencing local variable `cursor`
>     cursor.peek_next().unwrap().1
> }
>
> This modification ensures that references to tree elements remain valid
> independently of the cursor's scope,
> aligning with the actual lifetime dependencies in the data structure.
>
> The changes will be applied to multiple similar methods
> throughout the Cursor implementation to maintain consistency.
>
> Fixes: 98c14e40e07a ("rust: rbtree: add cursor")
> Signed-off-by: Hang Shu <hangshu847@gmail.com>
> ---
>  rust/kernel/rbtree.rs | 16 ++++++++--------
>  1 file changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/rust/kernel/rbtree.rs b/rust/kernel/rbtree.rs
> index 9e178dacddf1..702a1b6ef7a9 100644
> --- a/rust/kernel/rbtree.rs
> +++ b/rust/kernel/rbtree.rs
> @@ -742,7 +742,7 @@ unsafe impl<'a, K: Sync, V: Sync> Sync for Cursor<'a, K, V> {}
>
>  impl<'a, K, V> Cursor<'a, K, V> {
>      /// The current node
> -    pub fn current(&self) -> (&K, &V) {
> +    pub fn current(&self) -> (&'a K, &'a V) {
>          // SAFETY:
>          // - `self.current` is a valid node by the type invariants.
>          // - We have an immutable reference by the function signature.
> @@ -750,7 +750,7 @@ pub fn current(&self) -> (&K, &V) {
>      }
>
>      /// The current node, with a mutable value
> -    pub fn current_mut(&mut self) -> (&K, &mut V) {
> +    pub fn current_mut(&mut self) -> (&'a K, &'a mut V) {

This would allow me to call current_mut() twice on the same cursor to
get two mutable references to the same value. That is not okay.

If you want to have methods that return a reference with the tree's
lifetime instead of the cursor's, then you need to add new methods
(probably called into_*) rather than modify the existing ones.

Alice

On Fri,  7 Nov 2025 05:06:56 +0000
Hang Shu <m18080292938@163.com> wrote:

> From: Hang Shu <hangshu847@gmail.com>
> 

Jfyi, there is no need for adding this.

> The returned keys and values of cursor methods should be bound by
> the lifetime of the rbtree itself ('a), not the lifetime of the
> cursor.
>