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There are a couple of > issues with this trait in the kernel, namely: > > - Rust's specialization feature is unstable. This prevents us to > optimze for the special case where `I::IntoIter` equals `Vec`'s > `IntoIter` type. > - We also can't use `I::IntoIter`'s type ID either to work around this, > since `FromIterator` doesn't require this type to be `'static`. > - `FromIterator::from_iter` does return `Self` instead of > `Result`, hence we can't properly handle allocation > failures. > - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle > additional allocation flags. > > Instead, provide `IntoIter::collect`, such that we can at least convert > `IntoIter` into a `Vec` again. > > Reviewed-by: Alice Ryhl > Signed-off-by: Danilo Krummrich A question is how useful is this? The way this can be used seems fairly limited: you `into_iter`, consume a few elements, and the `collect()`? It feels whatever user this serves, it would make more sense for them to use a `VecDeque` and just `pop_front()`. Also, inline comments below. > --- > rust/kernel/alloc/kvec.rs | 86 +++++++++++++++++++++++++++++++++++++++ > 1 file changed, 86 insertions(+) > > diff --git a/rust/kernel/alloc/kvec.rs b/rust/kernel/alloc/kvec.rs > index e91761c5c52d..686e969463f8 100644 > --- a/rust/kernel/alloc/kvec.rs > +++ b/rust/kernel/alloc/kvec.rs > @@ -690,6 +690,92 @@ impl IntoIter > fn as_raw_mut_slice(&mut self) -> *mut [T] { > ptr::slice_from_raw_parts_mut(self.ptr, self.len) > } > + > + fn into_raw_parts(self) -> (*mut T, NonNull, usize, usize) { > + let me = ManuallyDrop::new(self); > + let ptr = me.ptr; > + let buf = me.buf; > + let len = me.len; > + let cap = me.cap; > + (ptr, buf, len, cap) > + } > + > + /// Same as `Iterator::collect` but specialized for `Vec`'s `IntoIter`. > + /// > + /// # Examples > + /// > + /// ``` > + /// let v = kernel::kvec![1, 2, 3]?; > + /// let mut it = v.into_iter(); > + /// > + /// assert_eq!(it.next(), Some(1)); > + /// > + /// let v = it.collect(GFP_KERNEL); > + /// assert_eq!(v, [2, 3]); > + /// > + /// # Ok::<(), Error>(()) > + /// ``` > + /// # Implementation Details > + /// > + /// Currently, we can't implement `FromIterator`. There are a couple of issues with this trait > + /// in the kernel, namely: > + /// > + /// - Rust's specialization feature is unstable. This prevents us to optimze for the special > + /// case where `I::IntoIter` equals `Vec`'s `IntoIter` type. > + /// - We also can't use `I::IntoIter`'s type ID either to work around this, since `FromIterator` > + /// doesn't require this type to be `'static`. > + /// - `FromIterator::from_iter` does return `Self` instead of `Result`, hence > + /// we can't properly handle allocation failures. > + /// - Neither `Iterator::collect` nor `FromIterator::from_iter` can handle additional allocation > + /// flags. > + /// > + /// Instead, provide `IntoIter::collect`, such that we can at least convert a `IntoIter` into a > + /// `Vec` again. > + /// > + /// Note that `IntoIter::collect` doesn't require `Flags`, since it re-uses the existing backing > + /// buffer. However, this backing buffer may be shrunk to the actual count of elements. > + pub fn collect(self, flags: Flags) -> Vec { > + let (mut ptr, buf, len, mut cap) = self.into_raw_parts(); > + let has_advanced = ptr != buf.as_ptr(); > + > + if has_advanced { > + // Copy the contents we have advanced to at the beginning of the buffer. > + // > + // SAFETY: > + // - `ptr` is valid for reads of `len * size_of::()` bytes, > + // - `buf.as_ptr()` is valid for writes of `len * size_of::()` bytes, > + // - `ptr` and `buf.as_ptr()` are not be subject to aliasing restrictions relative to > + // each other, > + // - both `ptr` and `buf.ptr()` are properly aligned. > + unsafe { ptr::copy(ptr, buf.as_ptr(), len) }; > + ptr = buf.as_ptr(); > + } > + > + // This can never fail, `len` is guaranteed to be smaller than `cap`. > + let layout = core::alloc::Layout::array::(len).unwrap(); nit: could be `unwrap_unchecked()`. Although feel free to leave it to avoid an additional unsafe. > + > + // SAFETY: `buf` points to the start of the backing buffer and `len` is guaranteed to be > + // smaller than `cap`. Depending on `alloc` this operation may shrink the buffer or leaves > + // it as it is. > + ptr = match unsafe { A::realloc(Some(buf.cast()), layout, flags) } { > + // If we fail to shrink, which likely can't even happen, continue with the existing > + // buffer. > + Err(_) => ptr, > + Ok(ptr) => { > + cap = len; > + ptr.as_ptr().cast() > + } > + }; This should be moved to `Vec::shrink_to_fit`. And then this function can just `Vec::from_raw_parts(...)` and then `vec.shrink_to_fit`. But my question would be why this function needs to shrink in the first place. > + > + // SAFETY: If the iterator has been advanced, the advanced elements have been copied to > + // the beginning of the buffer and `len` has been adjusted accordingly. > + // > + // - `ptr` is guaranteed to point to the start of the backing buffer. > + // - `cap` is either the original capacity or, after shrinking the buffer, equal to `len`. > + // - `alloc` is guaranteed to be unchanged since `into_iter` has been called on the original > + // `Vec`. > + unsafe { Vec::from_raw_parts(ptr, len, cap) } > + } > } > > impl Iterator for IntoIter