Never inline C variadic functions

compiler/rustc_mir/src/transform/inline.rs

@@ -206,6 +206,11 @@ impl Inliner<'tcx> {
         debug!("should_inline({:?})", callsite);
         let tcx = self.tcx;
 
+        if callsite.fn_sig.c_variadic() {
+            debug!("callee is variadic - not inlining");
+            return false;
+        }
+
         let codegen_fn_attrs = tcx.codegen_fn_attrs(callsite.callee.def_id());
 
         let self_features = &self.codegen_fn_attrs.target_features;

src/test/mir-opt/inline/inline-compatibility.rs

@@ -5,6 +5,7 @@
 #![crate_type = "lib"]
 #![feature(no_sanitize)]
 #![feature(target_feature_11)]
+#![feature(c_variadic)]
 
 // EMIT_MIR inline_compatibility.inlined_target_feature.Inline.diff
 #[target_feature(enable = "sse2")]
@@ -35,3 +36,20 @@ pub unsafe fn target_feature() {}
 #[inline]
 #[no_sanitize(address)]
 pub unsafe fn no_sanitize() {}
+
+// EMIT_MIR inline_compatibility.not_inlined_c_variadic.Inline.diff
+pub unsafe fn not_inlined_c_variadic() {
+    let s = sum(4u32, 4u32, 30u32, 200u32, 1000u32);
+}
+
+#[no_mangle]
+#[inline(always)]
+unsafe extern "C" fn sum(n: u32, mut vs: ...) -> u32 {
+    let mut s = 0;
+    let mut i = 0;
+    while i != n {
+        s += vs.arg::<u32>();
+        i += 1;
+    }
+    s
+}