Auto merge of rust-lang#147353 - the8472:nondrop-array-iter, r=scottmcm

only call polymorphic array iter drop machinery when the type requires it

I saw a bunch of dead, empty  `<[core::mem::maybe_uninit::MaybeUninit<T>; N] as core::array::iter::iter_inner::PartialDrop>::partial_drop` functions when compiling with more than 1 CGU.

Let's see if we can help optimizations to eliminate stuff earlier.

r? ghost

Author: bors

library/core/src/array/iter.rs

@@ -2,9 +2,9 @@
 
 use crate::intrinsics::transmute_unchecked;
 use crate::iter::{FusedIterator, TrustedLen, TrustedRandomAccessNoCoerce};
-use crate::mem::MaybeUninit;
+use crate::mem::{ManuallyDrop, MaybeUninit};
 use crate::num::NonZero;
-use crate::ops::{IndexRange, Range, Try};
+use crate::ops::{Deref as _, DerefMut as _, IndexRange, Range, Try};
 use crate::{fmt, ptr};
 
 mod iter_inner;
@@ -18,17 +18,17 @@ type InnerUnsized<T> = iter_inner::PolymorphicIter<[MaybeUninit<T>]>;
 #[rustc_diagnostic_item = "ArrayIntoIter"]
 #[derive(Clone)]
 pub struct IntoIter<T, const N: usize> {
-    inner: InnerSized<T, N>,
+    inner: ManuallyDrop<InnerSized<T, N>>,
 }
 
 impl<T, const N: usize> IntoIter<T, N> {
     #[inline]
     fn unsize(&self) -> &InnerUnsized<T> {
-        &self.inner
+        self.inner.deref()
     }
     #[inline]
     fn unsize_mut(&mut self) -> &mut InnerUnsized<T> {
-        &mut self.inner
+        self.inner.deref_mut()
     }
 }
 
@@ -69,7 +69,7 @@ impl<T, const N: usize> IntoIterator for [T; N] {
         // SAFETY: The original array was entirely initialized and the the alive
         // range we're passing here represents that fact.
         let inner = unsafe { InnerSized::new_unchecked(IndexRange::zero_to(N), data) };
-        IntoIter { inner }
+        IntoIter { inner: ManuallyDrop::new(inner) }
     }
 }
 
@@ -146,7 +146,7 @@ impl<T, const N: usize> IntoIter<T, N> {
         let alive = unsafe { IndexRange::new_unchecked(initialized.start, initialized.end) };
         // SAFETY: one of our safety condition is that these items are initialized.
         let inner = unsafe { InnerSized::new_unchecked(alive, buffer) };
-        IntoIter { inner }
+        IntoIter { inner: ManuallyDrop::new(inner) }
     }
 
     /// Creates an iterator over `T` which returns no elements.
@@ -205,7 +205,7 @@ impl<T, const N: usize> IntoIter<T, N> {
     #[inline]
     pub const fn empty() -> Self {
         let inner = InnerSized::empty();
-        IntoIter { inner }
+        IntoIter { inner: ManuallyDrop::new(inner) }
     }
 
     /// Returns an immutable slice of all elements that have not been yielded
@@ -320,11 +320,20 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
 }
 
 #[stable(feature = "array_value_iter_impls", since = "1.40.0")]
+// Even though all the Drop logic could be completely handled by
+// PolymorphicIter, this impl still serves two purposes:
+// - Drop has been part of the public API, so we can't remove it
+// - the partial_drop function doesn't always get fully optimized away
+//   for !Drop types and ends up as dead code in the final binary.
+//   Branching on needs_drop higher in the call-tree allows it to be
+//   removed by earlier optimization passes.
 impl<T, const N: usize> Drop for IntoIter<T, N> {
     #[inline]
     fn drop(&mut self) {
-        // `inner` now handles this, but it'd technically be a breaking change
-        // to remove this `impl`, even though it's useless.
+        if crate::mem::needs_drop::<T>() {
+            // SAFETY: This is the only place where we drop this field.
+            unsafe { ManuallyDrop::drop(&mut self.inner) }
+        }
     }
 }