don't ICE when we unwind despite panic=abort

src/shims/panic.rs

@@ -45,7 +45,9 @@ fn handle_miri_start_panic(
 
         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/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
+}