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

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

This reverts commit a983e0590a43ed8b0f60417828efd4e79b51f494.

Also adds a test for the expected behaviour.

1 files changed, 7 insertions, 5 deletions

diff --git a/src/libcore/alloc.rs b/src/libcore/alloc.rs
index 71f7f971eab..a04e75bc7ce 100644
--- a/src/libcore/alloc.rs
+++ b/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 {