fix #35501 part of #35233

Author: mikhail-m1

src/librustc_typeck/lib.rs

@@ -181,8 +181,10 @@ fn require_c_abi_if_variadic(tcx: TyCtxt,
                              abi: Abi,
                              span: Span) {
     if decl.variadic && abi != Abi::C {
-        span_err!(tcx.sess, span, E0045,
-                  "variadic function must have C calling convention");
+        struct_span_err!(tcx.sess, span, E0045,
+                  "variadic function must have C calling convention")
+            .span_label(span, &format!("variadics require C calling conventions"))
+            .emit()
     }
 }
 

src/test/compile-fail/E0045.rs

@@ -9,6 +9,7 @@
 // except according to those terms.
 
 extern "Rust" { fn foo(x: u8, ...); } //~ ERROR E0045
+//~^ NOTE variadics require C calling conventions
 
 fn main() {
 }