[PATCH 5/5] rust: alloc: add Vec::retain

[Alice Ryhl <aliceryhl@google.com>]

This adds a common Vec method called `retain` that removes all elements
that don't match a certain condition. Rust Binder uses it to find all
processes that match a given pid.

The stdlib retain method takes &T rather than &mut T and has a separate
retain_mut for the &mut T case. However, this is considered an API
mistake that can't be fixed now due to backwards compatibility. There's
no reason for us to repeat that mistake.

To verify the correctness of this implementation, you may run the
following program in userspace:

    fn retain<T>(vec: &mut Vec<T>, f: impl Fn(&T) -> bool) {
        let mut num_kept = 0;
        let mut next_to_check = 0;
        while let Some(to_check) = vec.get_mut(next_to_check) {
            if f(to_check) {
                vec.swap(num_kept, next_to_check);
                num_kept += 1;
            }
            next_to_check += 1;
        }
        vec.truncate(num_kept);
    }

    fn verify(c: &[bool]) {
        let mut vec1: Vec<usize> = (0..c.len()).collect();
        let mut vec2: Vec<usize> = (0..c.len()).collect();

        vec1.retain(|i| c[*i]);
        retain(&mut vec2, |i| c[*i]);

        assert_eq!(vec1, vec2);
    }

    // Used to loop through all 2^n bit vectors.
    fn add(value: &mut [bool]) -> bool {
        let mut carry = true;
        for v in value {
            let new_v = carry != *v;
            carry = carry && *v;
            *v = new_v;
        }
        carry
    }

    fn main() {
        for len in 0..10 {
            let mut retain = vec![false; len];
            while !add(&mut retain) {
                verify(&retain);
            }
        }
        println!("ok!");
    }

Signed-off-by: Alice Ryhl <aliceryhl@google.com>
---
 rust/kernel/alloc/kvec.rs | 14 ++++++++++++++
 1 file changed, 14 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index 303198509885f5e24b74da5a92382b518de3e1c0..00dabea8ea6c8a742a7fc95954d8de58be124493 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -588,6 +588,20 @@ pub fn drain_all(&mut self) -> DrainAll<'_, T> {
             elements: self.spare_capacity_mut()[..len].iter_mut(),
         }
     }
+
+    /// Removes all elements that don't match the provided closure.
+    pub fn retain(&mut self, mut f: impl FnMut(&mut T) -> bool) {
+        let mut num_kept = 0;
+        let mut next_to_check = 0;
+        while let Some(to_check) = self.get_mut(next_to_check) {
+            if f(to_check) {
+                self.swap(num_kept, next_to_check);
+                num_kept += 1;
+            }
+            next_to_check += 1;
+        }
+        self.truncate(num_kept);
+    }
 }
 
 impl<T: Clone, A: Allocator> Vec<T, A> {

-- 
2.49.0.rc1.451.g8f38331e32-goog