Auto merge of #114205 - the8472:vec-iter-nonnull, r=scottmcm

mark vec::IntoIter pointers as `!nonnull`

This applies the same NonNull optimizations to `vec::IntoIter` as  #113344 did for `slice::Iter`

[Godbolt](https://rust.godbolt.org/z/n1cTea718) showing the test IR on current nightly, note the absence of `!nonnull` on the loads.

r? `@scottmcm`

6 files changed, 122 insertions, 50 deletions

diff --git a/library/alloc/src/lib.rs b/library/alloc/src/lib.rs
index 49352553486..78629b39d34 100644
--- a/library/alloc/src/lib.rs
+++ b/library/alloc/src/lib.rs
@@ -138,6 +138,7 @@
 #![feature(maybe_uninit_slice)]
 #![feature(maybe_uninit_uninit_array)]
 #![feature(maybe_uninit_uninit_array_transpose)]
+#![feature(non_null_convenience)]
 #![feature(panic_internals)]
 #![feature(pattern)]
 #![feature(ptr_internals)]
@@ -180,6 +181,7 @@
 #![feature(const_ptr_write)]
 #![feature(const_trait_impl)]
 #![feature(const_try)]
+#![feature(decl_macro)]
 #![feature(dropck_eyepatch)]
 #![feature(exclusive_range_pattern)]
 #![feature(fundamental)]
diff --git a/library/alloc/src/vec/in_place_collect.rs b/library/alloc/src/vec/in_place_collect.rs
index a6cbed092c0..0a2280545da 100644
--- a/library/alloc/src/vec/in_place_collect.rs
+++ b/library/alloc/src/vec/in_place_collect.rs
@@ -257,7 +257,7 @@ where
         // then the source pointer will stay in its initial position and we can't use it as reference
         if src.ptr != src_ptr {
             debug_assert!(
-                unsafe { dst_buf.add(len) as *const _ } <= src.ptr,
+                unsafe { dst_buf.add(len) as *const _ } <= src.ptr.as_ptr(),
                 "InPlaceIterable contract violation, write pointer advanced beyond read pointer"
             );
         }
diff --git a/library/alloc/src/vec/into_iter.rs b/library/alloc/src/vec/into_iter.rs
index b03e04b7c70..654ce09afcd 100644
--- a/library/alloc/src/vec/into_iter.rs
+++ b/library/alloc/src/vec/into_iter.rs
@@ -18,6 +18,17 @@ use core::ops::Deref;
 use core::ptr::{self, NonNull};
 use core::slice::{self};
 
+macro non_null {
+    (mut $place:expr, $t:ident) => {{
+        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
+        unsafe { &mut *(ptr::addr_of_mut!($place) as *mut NonNull<$t>) }
+    }},
+    ($place:expr, $t:ident) => {{
+        #![allow(unused_unsafe)] // we're sometimes used within an unsafe block
+        unsafe { *(ptr::addr_of!($place) as *const NonNull<$t>) }
+    }},
+}
+
 /// An iterator that moves out of a vector.
 ///
 /// This `struct` is created by the `into_iter` method on [`Vec`](super::Vec)
@@ -41,10 +52,12 @@ pub struct IntoIter<
     // the drop impl reconstructs a RawVec from buf, cap and alloc
     // to avoid dropping the allocator twice we need to wrap it into ManuallyDrop
     pub(super) alloc: ManuallyDrop<A>,
-    pub(super) ptr: *const T,
-    pub(super) end: *const T, // If T is a ZST, this is actually ptr+len. This encoding is picked so that
-                              // ptr == end is a quick test for the Iterator being empty, that works
-                              // for both ZST and non-ZST.
+    pub(super) ptr: NonNull<T>,
+    /// If T is a ZST, this is actually ptr+len. This encoding is picked so that
+    /// ptr == end is a quick test for the Iterator being empty, that works
+    /// for both ZST and non-ZST.
+    /// For non-ZSTs the pointer is treated as `NonNull<T>`
+    pub(super) end: *const T,
 }
 
 #[stable(feature = "vec_intoiter_debug", since = "1.13.0")]
@@ -68,7 +81,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
     /// ```
     #[stable(feature = "vec_into_iter_as_slice", since = "1.15.0")]
     pub fn as_slice(&self) -> &[T] {
-        unsafe { slice::from_raw_parts(self.ptr, self.len()) }
+        unsafe { slice::from_raw_parts(self.ptr.as_ptr(), self.len()) }
     }
 
     /// Returns the remaining items of this iterator as a mutable slice.
@@ -97,7 +110,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
     }
 
     fn as_raw_mut_slice(&mut self) -> *mut [T] {
-        ptr::slice_from_raw_parts_mut(self.ptr as *mut T, self.len())
+        ptr::slice_from_raw_parts_mut(self.ptr.as_ptr(), self.len())
     }
 
     /// Drops remaining elements and relinquishes the backing allocation.
@@ -124,7 +137,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
         // this creates less assembly
         self.cap = 0;
         self.buf = unsafe { NonNull::new_unchecked(RawVec::NEW.ptr()) };
-        self.ptr = self.buf.as_ptr();
+        self.ptr = self.buf;
         self.end = self.buf.as_ptr();
 
         // Dropping the remaining elements can panic, so this needs to be
@@ -136,9 +149,9 @@ impl<T, A: Allocator> IntoIter<T, A> {
 
     /// Forgets to Drop the remaining elements while still allowing the backing allocation to be freed.
     pub(crate) fn forget_remaining_elements(&mut self) {
-        // For th ZST case, it is crucial that we mutate `end` here, not `ptr`.
+        // For the ZST case, it is crucial that we mutate `end` here, not `ptr`.
         // `ptr` must stay aligned, while `end` may be unaligned.
-        self.end = self.ptr;
+        self.end = self.ptr.as_ptr();
     }
 
     #[cfg(not(no_global_oom_handling))]
@@ -160,7 +173,7 @@ impl<T, A: Allocator> IntoIter<T, A> {
                 // say that they're all at the beginning of the "allocation".
                 0..this.len()
             } else {
-                this.ptr.sub_ptr(buf)..this.end.sub_ptr(buf)
+                this.ptr.sub_ptr(this.buf)..this.end.sub_ptr(buf)
             };
             let cap = this.cap;
             let alloc = ManuallyDrop::take(&mut this.alloc);
@@ -187,29 +200,35 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 
     #[inline]
     fn next(&mut self) -> Option<T> {
-        if self.ptr == self.end {
-            None
-        } else if T::IS_ZST {
-            // `ptr` has to stay where it is to remain aligned, so we reduce the length by 1 by
-            // reducing the `end`.
-            self.end = self.end.wrapping_byte_sub(1);
-
-            // Make up a value of this ZST.
-            Some(unsafe { mem::zeroed() })
+        if T::IS_ZST {
+            if self.ptr.as_ptr() == self.end as *mut _ {
+                None
+            } else {
+                // `ptr` has to stay where it is to remain aligned, so we reduce the length by 1 by
+                // reducing the `end`.
+                self.end = self.end.wrapping_byte_sub(1);
+
+                // Make up a value of this ZST.
+                Some(unsafe { mem::zeroed() })
+            }
         } else {
-            let old = self.ptr;
-            self.ptr = unsafe { self.ptr.add(1) };
+            if self.ptr == non_null!(self.end, T) {
+                None
+            } else {
+                let old = self.ptr;
+                self.ptr = unsafe { old.add(1) };
 
-            Some(unsafe { ptr::read(old) })
+                Some(unsafe { ptr::read(old.as_ptr()) })
+            }
         }
     }
 
     #[inline]
     fn size_hint(&self) -> (usize, Option<usize>) {
         let exact = if T::IS_ZST {
-            self.end.addr().wrapping_sub(self.ptr.addr())
+            self.end.addr().wrapping_sub(self.ptr.as_ptr().addr())
         } else {
-            unsafe { self.end.sub_ptr(self.ptr) }
+            unsafe { non_null!(self.end, T).sub_ptr(self.ptr) }
         };
         (exact, Some(exact))
     }
@@ -217,7 +236,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
     #[inline]
     fn advance_by(&mut self, n: usize) -> Result<(), NonZeroUsize> {
         let step_size = self.len().min(n);
-        let to_drop = ptr::slice_from_raw_parts_mut(self.ptr as *mut T, step_size);
+        let to_drop = ptr::slice_from_raw_parts_mut(self.ptr.as_ptr(), step_size);
         if T::IS_ZST {
             // See `next` for why we sub `end` here.
             self.end = self.end.wrapping_byte_sub(step_size);
@@ -259,7 +278,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
             // Safety: `len` indicates that this many elements are available and we just checked that
             // it fits into the array.
             unsafe {
-                ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, len);
+                ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, len);
                 self.forget_remaining_elements();
                 return Err(array::IntoIter::new_unchecked(raw_ary, 0..len));
             }
@@ -268,7 +287,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // Safety: `len` is larger than the array size. Copy a fixed amount here to fully initialize
         // the array.
         return unsafe {
-            ptr::copy_nonoverlapping(self.ptr, raw_ary.as_mut_ptr() as *mut T, N);
+            ptr::copy_nonoverlapping(self.ptr.as_ptr(), raw_ary.as_mut_ptr() as *mut T, N);
             self.ptr = self.ptr.add(N);
             Ok(raw_ary.transpose().assume_init())
         };
@@ -286,7 +305,7 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
         // Also note the implementation of `Self: TrustedRandomAccess` requires
         // that `T: Copy` so reading elements from the buffer doesn't invalidate
         // them for `Drop`.
-        unsafe { if T::IS_ZST { mem::zeroed() } else { ptr::read(self.ptr.add(i)) } }
+        unsafe { if T::IS_ZST { mem::zeroed() } else { self.ptr.add(i).read() } }
     }
 }
 
@@ -294,18 +313,25 @@ impl<T, A: Allocator> Iterator for IntoIter<T, A> {
 impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
     #[inline]
     fn next_back(&mut self) -> Option<T> {
-        if self.end == self.ptr {
-            None
-        } else if T::IS_ZST {
-            // See above for why 'ptr.offset' isn't used
-            self.end = self.end.wrapping_byte_sub(1);
-
-            // Make up a value of this ZST.
-            Some(unsafe { mem::zeroed() })
+        if T::IS_ZST {
+            if self.end as *mut _ == self.ptr.as_ptr() {
+                None
+            } else {
+                // See above for why 'ptr.offset' isn't used
+                self.end = self.end.wrapping_byte_sub(1);
+
+                // Make up a value of this ZST.
+                Some(unsafe { mem::zeroed() })
+            }
         } else {
-            self.end = unsafe { self.end.sub(1) };
+            if non_null!(self.end, T) == self.ptr {
+                None
+            } else {
+                let new_end = unsafe { non_null!(self.end, T).sub(1) };
+                *non_null!(mut self.end, T) = new_end;
 
-            Some(unsafe { ptr::read(self.end) })
+                Some(unsafe { ptr::read(new_end.as_ptr()) })
+            }
         }
     }
 
@@ -331,7 +357,11 @@ impl<T, A: Allocator> DoubleEndedIterator for IntoIter<T, A> {
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T, A: Allocator> ExactSizeIterator for IntoIter<T, A> {
     fn is_empty(&self) -> bool {
-        self.ptr == self.end
+        if T::IS_ZST {
+            self.ptr.as_ptr() == self.end as *mut _
+        } else {
+            self.ptr == non_null!(self.end, T)
+        }
     }
 }
 
diff --git a/library/alloc/src/vec/mod.rs b/library/alloc/src/vec/mod.rs
index e8a096cac86..8aa0c6e7ed6 100644
--- a/library/alloc/src/vec/mod.rs
+++ b/library/alloc/src/vec/mod.rs
@@ -2825,14 +2825,8 @@ impl<T, A: Allocator> IntoIterator for Vec<T, A> {
                 begin.add(me.len()) as *const T
             };
             let cap = me.buf.capacity();
-            IntoIter {
-                buf: NonNull::new_unchecked(begin),
-                phantom: PhantomData,
-                cap,
-                alloc,
-                ptr: begin,
-                end,
-            }
+            let buf = NonNull::new_unchecked(begin);
+            IntoIter { buf, phantom: PhantomData, cap, alloc, ptr: buf, end }
         }
     }
 }
diff --git a/library/alloc/src/vec/spec_from_iter.rs b/library/alloc/src/vec/spec_from_iter.rs
index efa6868473e..e976552cf2b 100644
--- a/library/alloc/src/vec/spec_from_iter.rs
+++ b/library/alloc/src/vec/spec_from_iter.rs
@@ -44,12 +44,12 @@ impl<T> SpecFromIter<T, IntoIter<T>> for Vec<T> {
         // than creating it through the generic FromIterator implementation would. That limitation
         // is not strictly necessary as Vec's allocation behavior is intentionally unspecified.
         // But it is a conservative choice.
-        let has_advanced = iterator.buf.as_ptr() as *const _ != iterator.ptr;
+        let has_advanced = iterator.buf != iterator.ptr;
         if !has_advanced || iterator.len() >= iterator.cap / 2 {
             unsafe {
                 let it = ManuallyDrop::new(iterator);
                 if has_advanced {
-                    ptr::copy(it.ptr, it.buf.as_ptr(), it.len());
+                    ptr::copy(it.ptr.as_ptr(), it.buf.as_ptr(), it.len());
                 }
                 return Vec::from_raw_parts(it.buf.as_ptr(), it.len(), it.cap);
             }
diff --git a/tests/codegen/vec-iter.rs b/tests/codegen/vec-iter.rs
new file mode 100644
index 00000000000..0282791e9d1
--- /dev/null
+++ b/tests/codegen/vec-iter.rs
@@ -0,0 +1,46 @@
+// ignore-debug: the debug assertions get in the way
+// compile-flags: -O
+#![crate_type = "lib"]
+#![feature(exact_size_is_empty)]
+
+use std::vec;
+
+// CHECK-LABEL: @vec_iter_len_nonnull
+#[no_mangle]
+pub fn vec_iter_len_nonnull(it: &vec::IntoIter<u8>) -> usize {
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: sub nuw
+    // CHECK: ret
+    it.len()
+}
+
+// CHECK-LABEL: @vec_iter_is_empty_nonnull
+#[no_mangle]
+pub fn vec_iter_is_empty_nonnull(it: &vec::IntoIter<u8>) -> bool {
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: ret
+    it.is_empty()
+}
+
+// CHECK-LABEL: @vec_iter_next
+#[no_mangle]
+pub fn vec_iter_next(it: &mut vec::IntoIter<u8>) -> Option<u8> {
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: load ptr
+    // CHECK-SAME: !nonnull
+    // CHECK-SAME: !noundef
+    // CHECK: ret
+    it.next()
+}