fix ICE in const_c_variadic when passing ZSTs

Author: folkertdev

compiler/rustc_const_eval/src/interpret/stack.rs

@@ -638,6 +638,9 @@ impl<'a, 'tcx: 'a, M: Machine<'tcx>> InterpCx<'tcx, M> {
         I: Iterator<Item = (&'a FnArg<'tcx, M::Provenance>, &'a ArgAbi<'tcx, Ty<'tcx>>)>,
         J: Iterator<Item = (usize, &'a ArgAbi<'tcx, Ty<'tcx>>)>,
     {
+        // Arguments that are ignored (ZSTs) are not passed by the caller.
+        let mut callee_abis = callee_abis.filter(|(_, abi)| !abi.is_ignore());
+
         // Consume the remaining arguments and store them in fresh allocations.
         let mut varargs = Vec::new();
         for (fn_arg, caller_abi) in caller_args {

tests/ui/consts/const-eval/c-variadic-ignored-argument.rs

@@ -0,0 +1,19 @@
+//@ compile-flags: --emit=obj
+#![feature(c_variadic)]
+#![feature(const_c_variadic)]
+#![feature(const_destruct)]
+#![crate_type = "lib"]
+
+// Regression test for when a c-variadic argument is `mode: Ignore`. The caller won't pass the
+// argument, but the callee ABI does have the argument. Ensure that const-eval is able to handle
+// this case, even though it only comes up in programs that would error anyway because ZSTs do not
+// implement VaArgSafe.
+
+const unsafe extern "C" fn read_n<const N: usize>(_: ...) {}
+
+unsafe fn read_too_many() {
+    const { read_n::<0>(read_as::<u32>, 1i32) }
+    //~^ ERROR can't pass a function item to a variadic function
+}
+
+fn read_as<T>() -> () {}

tests/ui/consts/const-eval/c-variadic-ignored-argument.stderr

@@ -0,0 +1,16 @@
+error[E0617]: can't pass a function item to a variadic function
+  --> $DIR/c-variadic-ignored-argument.rs:15:25
+   |
+LL |     const { read_n::<0>(read_as::<u32>, 1i32) }
+   |                         ^^^^^^^^^^^^^^
+   |
+   = help: a function item is zero-sized and needs to be cast into a function pointer to be used in FFI
+   = note: for more information on function items, visit https://doc.rust-lang.org/reference/types/function-item.html
+help: use a function pointer instead
+   |
+LL |     const { read_n::<0>(read_as::<u32> as fn(), 1i32) }
+   |                                        +++++++
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0617`.