ensure we catch incorrectly unwinding calls

src/shims/panic.rs

@@ -45,7 +45,9 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
 
         trace!("miri_start_panic: {:?}", this.frame().instance);
         // Make sure we only start unwinding when this matches our panic strategy.
-        assert_eq!(this.tcx.sess.panic_strategy(), PanicStrategy::Unwind);
+        if this.tcx.sess.panic_strategy() != PanicStrategy::Unwind {
+            throw_ub_format!("unwinding despite panic=abort");
+        }
 
         // Get the raw pointer stored in arg[0] (the panic payload).
         let &[ref payload] = check_arg_count(args)?;

tests/compile-fail/panic/bad_unwind.rs

@@ -0,0 +1,16 @@
+// error-pattern: calling a function with ABI C-unwind using caller ABI C
+#![feature(c_unwind)]
+
+//! Unwinding when the caller ABI is "C" (without "-unwind") is UB.
+//! Currently we detect the ABI mismatch; we could probably allow such calls in principle one day
+//! but then we have to detect the unexpected unwinding.
+
+extern "C-unwind" fn unwind() {
+    panic!();
+}
+
+fn main() {
+    let unwind: extern "C-unwind" fn() = unwind;
+    let unwind: extern "C" fn() = unsafe { std::mem::transmute(unwind) };
+    std::panic::catch_unwind(|| unwind()).unwrap_err();
+}

tests/compile-fail/panic/unwind_panic_abort.rs

@@ -0,0 +1,11 @@
+// compile-flags: -Cpanic=abort
+
+//! Unwinding despite `-C panic=abort` is an error.
+
+extern "Rust" {
+    fn miri_start_panic(payload: *mut u8) -> !;
+}
+
+fn main() {
+    unsafe { miri_start_panic(&mut 0); } //~ ERROR unwinding despite panic=abort
+}