Auto merge of #1744 - rust-lang:bad-unwind, r=RalfJung

ensure we catch incorrectly unwinding calls

Fixes #1740

Author: bors

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();
+}