Auto merge of #52206 - RalfJung:zst-slices, r=<try>

slices: fix ZST slice iterators making up pointers; debug_assert alignment in from_raw_parts

This fixes the problem that we are fabricating pointers out of thin air. I also managed to share more code between the mutable and shared iterators, while reducing the amount of macros.

I am not sure how useful it really is to add a `debug_assert!` in libcore. Everybody gets a release version of that anyway, right? Is there at least a CI job that runs the test suite with a debug version?

Fixes #42789

src/libcore/benches/slice.rs

@@ -65,3 +65,49 @@ fn binary_search_l2_with_dups(b: &mut Bencher) {
 fn binary_search_l3_with_dups(b: &mut Bencher) {
     binary_search(b, Cache::L3, |i| i / 16 * 16);
 }
+
+fn iter<T>(b: &mut Bencher, mapper: impl Fn(usize) -> T) {
+    let size = 1024;
+    let mut v = (0..size).map(mapper).collect::<Vec<T>>();
+    b.iter(move || {
+        let mut v = &mut v[..];
+        black_box(&mut v);
+        for i in v.iter() {
+            black_box(i);
+        }
+    });
+}
+
+fn iter_nth<T>(b: &mut Bencher, mapper: impl Fn(usize) -> T) {
+    let size = 1024;
+    let mut v = (0..size).map(mapper).collect::<Vec<T>>();
+    b.iter(move || {
+        let mut v = &mut v[..];
+        black_box(&mut v);
+        let mut it = v.iter();
+        for i in 0..size {
+            black_box(it.nth(0).unwrap());
+            black_box(v.iter().nth(i).unwrap());
+        }
+    });
+}
+
+#[bench]
+fn iter_usize(b: &mut Bencher) {
+    iter(b, |i| i);
+}
+
+#[bench]
+fn iter_zst(b: &mut Bencher) {
+    iter(b, |_i| ());
+}
+
+#[bench]
+fn iter_nth_usize(b: &mut Bencher) {
+    iter_nth(b, |i| i);
+}
+
+#[bench]
+fn iter_nth_zst(b: &mut Bencher) {
+    iter_nth(b, |_i| ());
+}