Auto merge of #69241 - shahn:checked_add_revert, r=Mark-Simulacrum,lqd

Revert "Remove `checked_add` in `Layout::repeat`"

This fixes a a segfault in safe code, a stable regression. Reported in #69225.

This reverts commit a983e0590a.

src/test/ui/issues/issue-69225-layout-repeated-checked-add.rs

@@ -0,0 +1,31 @@
+// Ensure we appropriately error instead of overflowing a calculation when creating a new Alloc
+// Layout
+
+// run-fail
+// compile-flags: -C opt-level=3
+// error-pattern: index out of bounds: the len is 0 but the index is 16777216
+// ignore-wasm no panic or subprocess support
+// ignore-emscripten no panic or subprocess support
+
+fn do_test(x: usize) {
+    let arr = vec![vec![0u8; 3]];
+
+    let mut z = Vec::new();
+    for arr_ref in arr {
+        for y in 0..x {
+            for _ in 0..1 {
+                z.extend(std::iter::repeat(0).take(x));
+                let a = y * x;
+                let b = (y + 1) * x - 1;
+                let slice = &arr_ref[a..b];
+                eprintln!("{} {} {} {}", a, b, arr_ref.len(), slice.len());
+                eprintln!("{:?}", slice[1 << 24]);
+            }
+        }
+    }
+}
+
+fn main() {
+    do_test(1);
+    do_test(2);
+}