Add regression test for CString::clone_into unwind safety

tests/pass/c_str_clone_into_alloc_error.rs

@@ -0,0 +1,55 @@
+//@compile-flags: -Zmiri-ignore-leaks
+#![feature(alloc_error_hook)]
+
+use std::alloc::{GlobalAlloc, Layout, System, set_alloc_error_hook};
+use std::ffi::CString;
+use std::panic::{self, AssertUnwindSafe, catch_unwind};
+use std::sync::atomic::{AtomicBool, Ordering};
+
+// Once armed, the first allocation of 8 bytes or more fails and disarms, so the
+// reallocation inside `clone_into`'s grow path fails while the runtime's own
+// smaller allocations keep succeeding.
+struct OneShotFailingAlloc;
+
+static ARMED: AtomicBool = AtomicBool::new(false);
+
+unsafe impl GlobalAlloc for OneShotFailingAlloc {
+    unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+        if layout.size() >= 8 && ARMED.swap(false, Ordering::SeqCst) {
+            return core::ptr::null_mut();
+        }
+        unsafe { System.alloc(layout) }
+    }
+
+    unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+        unsafe { System.dealloc(ptr, layout) }
+    }
+
+    unsafe fn realloc(&self, ptr: *mut u8, layout: Layout, new_size: usize) -> *mut u8 {
+        if new_size >= 8 && ARMED.swap(false, Ordering::SeqCst) {
+            return core::ptr::null_mut();
+        }
+        unsafe { System.realloc(ptr, layout, new_size) }
+    }
+}
+
+#[global_allocator]
+static ALLOC: OneShotFailingAlloc = OneShotFailingAlloc;
+
+// `<CStr as ToOwned>::clone_into` must leave the target `CString` valid even when
+// the reallocation for its grow path unwinds. See rust-lang/rust#155707.
+fn main() {
+    set_alloc_error_hook(|_| panic!("alloc error"));
+    panic::set_hook(Box::new(|_| {})); // keep the caught panic quiet
+
+    let src = CString::new("a fairly long value").unwrap();
+    let mut target = CString::new("x").unwrap();
+
+    ARMED.store(true, Ordering::SeqCst);
+    let res = catch_unwind(AssertUnwindSafe(|| src.as_c_str().clone_into(&mut target)));
+    ARMED.store(false, Ordering::SeqCst);
+
+    assert!(res.is_err(), "clone_into should have unwound on the alloc failure");
+    // `target` must still end in its nul terminator; before the fix it was left empty.
+    assert_eq!(target.as_bytes_with_nul().last(), Some(&0));
+}