Auto merge of rust-lang#133662 - paolobarbolini:vec-extend-with-via-repeatn, r=<try>

Use `iter::repeat_n` to implement `Vec::extend_with`

This replaces the `Vec::extend_with` manual implementation, which is used by `Vec::resize` and `Vec` `SpecFromElem`, with `iter::repeat_n`.

I've compared the codegen output between:

1. the current `Vec::resize` impl
2. this branch
3. this branch + rust-lang#130887

3 gives the closest codegen output to 1, with some output improvements. 2 doesn't look good: https://rust.godbolt.org/z/Yrc83EhjY.
May also help rust-lang#120050?

---

WARNING: DO NOT MERGE - in order to run the perf run in rust-lang#133662 (comment) this PR currently also contains commits from rust-lang#130887

Author: bors

library/alloc/src/vec/mod.rs

@@ -3112,31 +3112,7 @@ impl<T: Clone, A: Allocator> Vec<T, A> {
     #[track_caller]
     /// Extend the vector by `n` clones of value.
     fn extend_with(&mut self, n: usize, value: T) {
-        self.reserve(n);
-
-        unsafe {
-            let mut ptr = self.as_mut_ptr().add(self.len());
-            // Use SetLenOnDrop to work around bug where compiler
-            // might not realize the store through `ptr` through self.set_len()
-            // don't alias.
-            let mut local_len = SetLenOnDrop::new(&mut self.len);
-
-            // Write all elements except the last one
-            for _ in 1..n {
-                ptr::write(ptr, value.clone());
-                ptr = ptr.add(1);
-                // Increment the length in every step in case clone() panics
-                local_len.increment_len(1);
-            }
-
-            if n > 0 {
-                // We can write the last element directly without cloning needlessly
-                ptr::write(ptr, value);
-                local_len.increment_len(1);
-            }
-
-            // len set by scope guard
-        }
+        self.extend_trusted(iter::repeat_n(value, n));
     }
 }
 

library/core/src/iter/sources/repeat_n.rs

@@ -1,6 +1,5 @@
 use crate::fmt;
 use crate::iter::{FusedIterator, TrustedLen, UncheckedIterator};
-use crate::mem::{self, MaybeUninit};
 use crate::num::NonZero;
 
 /// Creates a new iterator that repeats a single element a given number of times.
@@ -57,78 +56,49 @@ use crate::num::NonZero;
 #[inline]
 #[stable(feature = "iter_repeat_n", since = "1.82.0")]
 pub fn repeat_n<T: Clone>(element: T, count: usize) -> RepeatN<T> {
-    let element = if count == 0 {
-        // `element` gets dropped eagerly.
-        MaybeUninit::uninit()
-    } else {
-        MaybeUninit::new(element)
-    };
-
-    RepeatN { element, count }
+    RepeatN { inner: RepeatNInner::new(element, count) }
+}
+
+#[derive(Clone, Copy)]
+#[repr(C)] // keep the layout consistent for codegen tests
+struct RepeatNInner<T> {
+    count: NonZero<usize>,
+    element: T,
+}
+
+impl<T> RepeatNInner<T> {
+    fn new(element: T, count: usize) -> Option<Self> {
+        let count = NonZero::<usize>::new(count)?;
+        Some(Self { element, count })
+    }
 }
 
 /// An iterator that repeats an element an exact number of times.
 ///
 /// This `struct` is created by the [`repeat_n()`] function.
 /// See its documentation for more.
 #[stable(feature = "iter_repeat_n", since = "1.82.0")]
+#[derive(Clone)]
 pub struct RepeatN<A> {
-    count: usize,
-    // Invariant: uninit iff count == 0.
-    element: MaybeUninit<A>,
+    inner: Option<RepeatNInner<A>>,
 }
 
 impl<A> RepeatN<A> {
-    /// Returns the element if it hasn't been dropped already.
-    fn element_ref(&self) -> Option<&A> {
-        if self.count > 0 {
-            // SAFETY: The count is non-zero, so it must be initialized.
-            Some(unsafe { self.element.assume_init_ref() })
-        } else {
-            None
-        }
-    }
     /// If we haven't already dropped the element, return it in an option.
-    ///
-    /// Clears the count so it won't be dropped again later.
     #[inline]
     fn take_element(&mut self) -> Option<A> {
-        if self.count > 0 {
-            self.count = 0;
-            let element = mem::replace(&mut self.element, MaybeUninit::uninit());
-            // SAFETY: We just set count to zero so it won't be dropped again,
-            // and it used to be non-zero so it hasn't already been dropped.
-            unsafe { Some(element.assume_init()) }
-        } else {
-            None
-        }
-    }
-}
-
-#[stable(feature = "iter_repeat_n", since = "1.82.0")]
-impl<A: Clone> Clone for RepeatN<A> {
-    fn clone(&self) -> RepeatN<A> {
-        RepeatN {
-            count: self.count,
-            element: self.element_ref().cloned().map_or_else(MaybeUninit::uninit, MaybeUninit::new),
-        }
+        self.inner.take().map(|inner| inner.element)
     }
 }
 
 #[stable(feature = "iter_repeat_n", since = "1.82.0")]
 impl<A: fmt::Debug> fmt::Debug for RepeatN<A> {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        f.debug_struct("RepeatN")
-            .field("count", &self.count)
-            .field("element", &self.element_ref())
-            .finish()
-    }
-}
-
-#[stable(feature = "iter_repeat_n", since = "1.82.0")]
-impl<A> Drop for RepeatN<A> {
-    fn drop(&mut self) {
-        self.take_element();
+        let (count, element) = match self.inner.as_ref() {
+            Some(inner) => (inner.count.get(), Some(&inner.element)),
+            None => (0, None),
+        };
+        f.debug_struct("RepeatN").field("count", &count).field("element", &element).finish()
     }
 }
 
@@ -138,12 +108,17 @@ impl<A: Clone> Iterator for RepeatN<A> {
 
     #[inline]
     fn next(&mut self) -> Option<A> {
-        if self.count > 0 {
-            // SAFETY: Just checked it's not empty
-            unsafe { Some(self.next_unchecked()) }
-        } else {
-            None
+        let inner = self.inner.as_mut()?;
+        let count = inner.count.get();
+
+        if let Some(decremented) = NonZero::<usize>::new(count - 1) {
+            // Order of these is important for optimization
+            let tmp = inner.element.clone();
+            inner.count = decremented;
+            return Some(tmp);
         }
+
+        return self.take_element();
     }
 
     #[inline]
@@ -154,19 +129,19 @@ impl<A: Clone> Iterator for RepeatN<A> {
 
     #[inline]
     fn advance_by(&mut self, skip: usize) -> Result<(), NonZero<usize>> {
-        let len = self.count;
+        let Some(inner) = self.inner.as_mut() else {
+            return NonZero::<usize>::new(skip).map(Err).unwrap_or(Ok(()));
+        };
 
-        if skip >= len {
-            self.take_element();
-        }
+        let len = inner.count.get();
 
-        if skip > len {
-            // SAFETY: we just checked that the difference is positive
-            Err(unsafe { NonZero::new_unchecked(skip - len) })
-        } else {
-            self.count = len - skip;
-            Ok(())
+        if let Some(new_len) = len.checked_sub(skip).and_then(NonZero::<usize>::new) {
+            inner.count = new_len;
+            return Ok(());
         }
+
+        self.inner = None;
+        return NonZero::<usize>::new(skip - len).map(Err).unwrap_or(Ok(()));
     }
 
     #[inline]
@@ -183,7 +158,7 @@ impl<A: Clone> Iterator for RepeatN<A> {
 #[stable(feature = "iter_repeat_n", since = "1.82.0")]
 impl<A: Clone> ExactSizeIterator for RepeatN<A> {
     fn len(&self) -> usize {
-        self.count
+        self.inner.as_ref().map(|inner| inner.count.get()).unwrap_or(0)
     }
 }
 
@@ -211,20 +186,4 @@ impl<A: Clone> FusedIterator for RepeatN<A> {}
 #[unstable(feature = "trusted_len", issue = "37572")]
 unsafe impl<A: Clone> TrustedLen for RepeatN<A> {}
 #[stable(feature = "iter_repeat_n", since = "1.82.0")]
-impl<A: Clone> UncheckedIterator for RepeatN<A> {
-    #[inline]
-    unsafe fn next_unchecked(&mut self) -> Self::Item {
-        // SAFETY: The caller promised the iterator isn't empty
-        self.count = unsafe { self.count.unchecked_sub(1) };
-        if self.count == 0 {
-            // SAFETY: the check above ensured that the count used to be non-zero,
-            // so element hasn't been dropped yet, and we just lowered the count to
-            // zero so it won't be dropped later, and thus it's okay to take it here.
-            unsafe { mem::replace(&mut self.element, MaybeUninit::uninit()).assume_init() }
-        } else {
-            // SAFETY: the count is non-zero, so it must have not been dropped yet.
-            let element = unsafe { self.element.assume_init_ref() };
-            A::clone(element)
-        }
-    }
-}
+impl<A: Clone> UncheckedIterator for RepeatN<A> {}

tests/codegen/iter-repeat-n-trivial-drop.rs

@@ -26,9 +26,9 @@ pub fn iter_repeat_n_next(it: &mut std::iter::RepeatN<NotCopy>) -> Option<NotCop
 
     // CHECK: [[NOT_EMPTY]]:
     // CHECK-NEXT: %[[DEC:.+]] = add i64 %[[COUNT]], -1
-    // CHECK-NEXT: store i64 %[[DEC]]
     // CHECK-NOT: br
     // CHECK: %[[VAL:.+]] = load i16
+    // CHECK-NEXT: store i64 %[[DEC]]
     // CHECK-NEXT: br label %[[EMPTY]]
 
     // CHECK: [[EMPTY]]: