Rollup merge of #95376 - WaffleLapkin:drain_keep_rest, r=dtolnay

Add `vec::Drain{,Filter}::keep_rest`

This PR adds `keep_rest` methods to `vec::Drain` and `vec::DrainFilter` under `drain_keep_rest` feature gate:
```rust
// mod alloc::vec

impl<T, A: Allocator> Drain<'_, T, A> {
    pub fn keep_rest(self);
}

impl<T, F, A: Allocator> DrainFilter<'_, T, F, A>
where
    F: FnMut(&mut T) -> bool,
{
    pub fn keep_rest(self);
}
```

Both these methods cancel draining of elements that were not yet yielded from the iterators. While this needs more testing & documentation, I want at least start the discussion. This may be a potential way out of the "should `DrainFilter` exhaust itself on drop?" argument.

library/alloc/src/vec/drain.rs

@@ -1,7 +1,7 @@
 use crate::alloc::{Allocator, Global};
 use core::fmt;
 use core::iter::{FusedIterator, TrustedLen};
-use core::mem;
+use core::mem::{self, ManuallyDrop};
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
@@ -65,6 +65,77 @@ impl<'a, T, A: Allocator> Drain<'a, T, A> {
     pub fn allocator(&self) -> &A {
         unsafe { self.vec.as_ref().allocator() }
     }
+
+    /// Keep unyielded elements in the source `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(drain_keep_rest)]
+    ///
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain(..);
+    ///
+    /// assert_eq!(drain.next().unwrap(), 'a');
+    ///
+    /// // This call keeps 'b' and 'c' in the vec.
+    /// drain.keep_rest();
+    ///
+    /// // If we wouldn't call `keep_rest()`,
+    /// // `vec` would be empty.
+    /// assert_eq!(vec, ['b', 'c']);
+    /// ```
+    #[unstable(feature = "drain_keep_rest", issue = "101122")]
+    pub fn keep_rest(self) {
+        // At this moment layout looks like this:
+        //
+        // [head] [yielded by next] [unyielded] [yielded by next_back] [tail]
+        //        ^-- start         \_________/-- unyielded_len        \____/-- self.tail_len
+        //                          ^-- unyielded_ptr                  ^-- tail
+        //
+        // Normally `Drop` impl would drop [unyielded] and then move [tail] to the `start`.
+        // Here we want to
+        // 1. Move [unyielded] to `start`
+        // 2. Move [tail] to a new start at `start + len(unyielded)`
+        // 3. Update length of the original vec to `len(head) + len(unyielded) + len(tail)`
+        //    a. In case of ZST, this is the only thing we want to do
+        // 4. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
+        let mut this = ManuallyDrop::new(self);
+
+        unsafe {
+            let source_vec = this.vec.as_mut();
+
+            let start = source_vec.len();
+            let tail = this.tail_start;
+
+            let unyielded_len = this.iter.len();
+            let unyielded_ptr = this.iter.as_slice().as_ptr();
+
+            // ZSTs have no identity, so we don't need to move them around.
+            let needs_move = mem::size_of::<T>() != 0;
+
+            if needs_move {
+                let start_ptr = source_vec.as_mut_ptr().add(start);
+
+                // memmove back unyielded elements
+                if unyielded_ptr != start_ptr {
+                    let src = unyielded_ptr;
+                    let dst = start_ptr;
+
+                    ptr::copy(src, dst, unyielded_len);
+                }
+
+                // memmove back untouched tail
+                if tail != (start + unyielded_len) {
+                    let src = source_vec.as_ptr().add(tail);
+                    let dst = start_ptr.add(unyielded_len);
+                    ptr::copy(src, dst, this.tail_len);
+                }
+            }
+
+            source_vec.set_len(start + unyielded_len + this.tail_len);
+        }
+    }
 }
 
 #[stable(feature = "vec_drain_as_slice", since = "1.46.0")]

library/alloc/src/vec/drain_filter.rs

@@ -1,6 +1,7 @@
 use crate::alloc::{Allocator, Global};
-use core::ptr::{self};
-use core::slice::{self};
+use core::mem::{self, ManuallyDrop};
+use core::ptr;
+use core::slice;
 
 use super::Vec;
 
@@ -54,6 +55,61 @@ where
     pub fn allocator(&self) -> &A {
         self.vec.allocator()
     }
+
+    /// Keep unyielded elements in the source `Vec`.
+    ///
+    /// # Examples
+    ///
+    /// ```
+    /// #![feature(drain_filter)]
+    /// #![feature(drain_keep_rest)]
+    ///
+    /// let mut vec = vec!['a', 'b', 'c'];
+    /// let mut drain = vec.drain_filter(|_| true);
+    ///
+    /// assert_eq!(drain.next().unwrap(), 'a');
+    ///
+    /// // This call keeps 'b' and 'c' in the vec.
+    /// drain.keep_rest();
+    ///
+    /// // If we wouldn't call `keep_rest()`,
+    /// // `vec` would be empty.
+    /// assert_eq!(vec, ['b', 'c']);
+    /// ```
+    #[unstable(feature = "drain_keep_rest", issue = "101122")]
+    pub fn keep_rest(self) {
+        // At this moment layout looks like this:
+        //
+        //  _____________________/-- old_len
+        // /                     \
+        // [kept] [yielded] [tail]
+        //        \_______/ ^-- idx
+        //                \-- del
+        //
+        // Normally `Drop` impl would drop [tail] (via .for_each(drop), ie still calling `pred`)
+        //
+        // 1. Move [tail] after [kept]
+        // 2. Update length of the original vec to `old_len - del`
+        //    a. In case of ZST, this is the only thing we want to do
+        // 3. Do *not* drop self, as everything is put in a consistent state already, there is nothing to do
+        let mut this = ManuallyDrop::new(self);
+
+        unsafe {
+            // ZSTs have no identity, so we don't need to move them around.
+            let needs_move = mem::size_of::<T>() != 0;
+
+            if needs_move && this.idx < this.old_len && this.del > 0 {
+                let ptr = this.vec.as_mut_ptr();
+                let src = ptr.add(this.idx);
+                let dst = src.sub(this.del);
+                let tail_len = this.old_len - this.idx;
+                src.copy_to(dst, tail_len);
+            }
+
+            let new_len = this.old_len - this.del;
+            this.vec.set_len(new_len);
+        }
+    }
 }
 
 #[unstable(feature = "drain_filter", reason = "recently added", issue = "43244")]

library/alloc/tests/lib.rs

@@ -44,6 +44,7 @@
 #![feature(bench_black_box)]
 #![feature(strict_provenance)]
 #![feature(once_cell)]
+#![feature(drain_keep_rest)]
 
 use std::collections::hash_map::DefaultHasher;
 use std::hash::{Hash, Hasher};

library/alloc/tests/vec.rs

@@ -839,6 +839,36 @@ fn test_drain_leak() {
     assert_eq!(v, vec![D(0, false), D(1, false), D(6, false),]);
 }
 
+#[test]
+fn test_drain_keep_rest() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    let mut drain = v.drain(1..6);
+    assert_eq!(drain.next(), Some(1));
+    assert_eq!(drain.next_back(), Some(5));
+    assert_eq!(drain.next(), Some(2));
+
+    drain.keep_rest();
+    assert_eq!(v, &[0, 3, 4, 6]);
+}
+
+#[test]
+fn test_drain_keep_rest_all() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    v.drain(1..6).keep_rest();
+    assert_eq!(v, &[0, 1, 2, 3, 4, 5, 6]);
+}
+
+#[test]
+fn test_drain_keep_rest_none() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    let mut drain = v.drain(1..6);
+
+    drain.by_ref().for_each(drop);
+
+    drain.keep_rest();
+    assert_eq!(v, &[0, 6]);
+}
+
 #[test]
 fn test_splice() {
     let mut v = vec![1, 2, 3, 4, 5];
@@ -1533,6 +1563,35 @@ fn drain_filter_unconsumed() {
     assert_eq!(vec, [2, 4]);
 }
 
+#[test]
+fn test_drain_filter_keep_rest() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    let mut drain = v.drain_filter(|&mut x| x % 2 == 0);
+    assert_eq!(drain.next(), Some(0));
+    assert_eq!(drain.next(), Some(2));
+
+    drain.keep_rest();
+    assert_eq!(v, &[1, 3, 4, 5, 6]);
+}
+
+#[test]
+fn test_drain_filter_keep_rest_all() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    v.drain_filter(|_| true).keep_rest();
+    assert_eq!(v, &[0, 1, 2, 3, 4, 5, 6]);
+}
+
+#[test]
+fn test_drain_filter_keep_rest_none() {
+    let mut v = vec![0, 1, 2, 3, 4, 5, 6];
+    let mut drain = v.drain_filter(|_| true);
+
+    drain.by_ref().for_each(drop);
+
+    drain.keep_rest();
+    assert_eq!(v, &[]);
+}
+
 #[test]
 fn test_reserve_exact() {
     // This is all the same as test_reserve