Assert that the #[track_caller] ABI is correct before relying on it.

src/librustc_codegen_ssa/mir/block.rs

@@ -765,6 +765,10 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
         let needs_location =
             instance.map(|i| i.def.requires_caller_location(self.cx.tcx())).unwrap_or_default();
         if needs_location {
+            assert_eq!(
+                fn_abi.args.len(), args.len() + 1,
+                "#[track_caller] fn's must have 1 more argument in their ABI than in their MIR",
+            );
             let location = self.get_caller_location(&mut bx, span);
             let last_arg = &fn_abi.args.last().unwrap();
             self.codegen_argument(&mut bx, location, &mut llargs, last_arg);

src/librustc_codegen_ssa/mir/mod.rs

@@ -431,9 +431,13 @@ fn arg_local_refs<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             bx.store_fn_arg(arg, &mut llarg_idx, tmp);
             LocalRef::Place(tmp)
         }
-    }).collect();
+    }).collect::<Vec<_>>();
 
     if fx.instance.def.requires_caller_location(bx.tcx()) {
+        assert_eq!(
+            fx.fn_abi.args.len(), args.len() + 1,
+            "#[track_caller] fn's must have 1 more argument in their ABI than in their MIR",
+        );
         let arg = &fx.fn_abi.args.last().unwrap();
         let place = PlaceRef::alloca(bx, arg.layout);
         bx.store_fn_arg(arg, &mut llarg_idx, place);