Revert "Remove checked_add in Layout::repeat"

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

This reverts commit a983e05.

Also adds a test for the expected behaviour.

src/libcore/alloc.rs

@@ -241,11 +241,13 @@ impl Layout {
     #[unstable(feature = "alloc_layout_extra", issue = "55724")]
     #[inline]
     pub fn repeat(&self, n: usize) -> Result<(Self, usize), LayoutErr> {
-        // This cannot overflow. Quoting from the invariant of Layout:
-        // > `size`, when rounded up to the nearest multiple of `align`,
-        // > must not overflow (i.e., the rounded value must be less than
-        // > `usize::MAX`)
-        let padded_size = self.size() + self.padding_needed_for(self.align());
+        // Warning, removing the checked_add here led to segfaults in #67174. Further
+        // analysis in #69225 seems to indicate that this is an LTO-related
+        // miscompilation, so #67174 might be able to be reapplied in the future.
+        let padded_size = self
+            .size()
+            .checked_add(self.padding_needed_for(self.align()))
+            .ok_or(LayoutErr { private: () })?;
         let alloc_size = padded_size.checked_mul(n).ok_or(LayoutErr { private: () })?;
 
         unsafe {

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);
+}