[PATCH] rust/alloc: add Vec::into_boxed_slice()

[David Rheinsberg <david@readahead.eu>]

Add `Vec::into_boxed_slice()` similar to
`std::vec::Vec::into_boxed_slice()` [1].

There is currently no way to easily consume the allocation of a vector.
However, it is very convenient to use `Vec` to initialize a dynamically
sized array and then "seal" it, so it can be passed along as a Box:

    fn create_from(src: &[T]) -> Result<KBox<[U]>, AllocError> {
        let v = Vec::with_capacity(n, GFP_KERNEL)?;

        for i in src {
            v.push(foo(i)?, GFP_KERNEL)?;
        }

        Ok(v.into_boxed_slice())
    }

A valid alternative is to use `Box::new_uninit()` rather than
`Vec::with_capacity()`, and eventually convert the box via
`Box::assume_init()`. This works but needlessly requires unsafe code,
awkward drop handling, etc. Using `Vec` is the much simpler solution.

[1] https://doc.rust-lang.org/std/vec/struct.Vec.html#method.into_boxed_slice

Signed-off-by: David Rheinsberg <david@readahead.eu>
---
 rust/kernel/alloc/kvec.rs | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs
index ac8d6f763ae8..537818bb66d0 100644
--- a/rust/kernel/alloc/kvec.rs
+++ b/rust/kernel/alloc/kvec.rs
@@ -826,6 +826,23 @@ pub fn resize(&mut self, new_len: usize, value: T, flags: Flags) -> Result<(), A
             }
         }
     }
+
+    /// Converts the vector into [`Box<[T], A>`].
+    ///
+    /// Excess capacity is retained in the allocation, but lost until the box
+    /// is dropped.
+    pub fn into_boxed_slice(self) -> Box<[T], A> {
+        let (buf, len, _cap) = self.into_raw_parts();
+        let slice = ptr::slice_from_raw_parts_mut(buf, len);
+
+        // SAFETY:
+        // - `slice` has been allocated with `A`
+        // - `slice` is suitably aligned
+        // - `slice` has at least a length of `len`
+        // - all elements within `slice` are initialized values of `T`
+        // - `len` does not exceed `isize::MAX`
+        unsafe { Box::from_raw(slice) }
+    }
 }
 
 impl<T, A> Drop for Vec<T, A>
-- 
2.53.0