Rollup merge of #153398 - folkertdev:const-c-variadic-trailing-zst, r=RalfJung

fix ICE in `const_c_variadic` when passing ZSTs

fixes https://github.com/rust-lang/rust/issues/153351
r? RalfJung

There was a mismatch between the caller and callee ABI where the caller does not pass ZST arguments, but the callee does expect them. Because ZSTs don't implement `VaArgSafe` the program must already be invalid if this comes up.

compiler/rustc_const_eval/src/interpret/call.rs

@@ -12,7 +12,7 @@
 use rustc_middle::ty::layout::{IntegerExt, TyAndLayout};
 use rustc_middle::ty::{self, AdtDef, Instance, Ty, VariantDef};
 use rustc_middle::{bug, mir, span_bug};
-use rustc_target::callconv::{ArgAbi, FnAbi, PassMode};
+use rustc_target::callconv::{ArgAbi, FnAbi};
 use tracing::field::Empty;
 use tracing::{info, instrument, trace};
 
@@ -284,7 +284,7 @@ fn pass_argument<'x, 'y>(
         'tcx: 'y,
     {
         assert_eq!(callee_ty, callee_abi.layout.ty);
-        if callee_abi.mode == PassMode::Ignore {
+        if callee_abi.is_ignore() {
             // This one is skipped. Still must be made live though!
             if !already_live {
                 self.storage_live(callee_arg.as_local().unwrap())?;
@@ -450,7 +450,7 @@ pub fn init_stack_frame(
             let mut caller_args = args
                 .iter()
                 .zip(caller_fn_abi.args.iter())
-                .filter(|arg_and_abi| !matches!(arg_and_abi.1.mode, PassMode::Ignore));
+                .filter(|arg_and_abi| !arg_and_abi.1.is_ignore());
 
             // Now we have to spread them out across the callee's locals,
             // taking into account the `spread_arg`. If we could write
@@ -480,7 +480,12 @@ pub fn init_stack_frame(
 
                     // Consume the remaining arguments by putting them into the variable argument
                     // list.
-                    let varargs = self.allocate_varargs(&mut caller_args, &mut callee_args_abis)?;
+                    let varargs = self.allocate_varargs(
+                        &mut caller_args,
+                        // "Ignored" arguments aren't actually passed, so the callee should also
+                        // ignore them. (`pass_argument` does this for regular arguments.)
+                        (&mut callee_args_abis).filter(|(_, abi)| !abi.is_ignore()),
+                    )?;
                     // When the frame is dropped, these variable arguments are deallocated.
                     self.frame_mut().va_list = varargs.clone();
                     let key = self.va_list_ptr(varargs.into());

compiler/rustc_const_eval/src/interpret/stack.rs

@@ -631,8 +631,8 @@ impl<'a, 'tcx: 'a, M: Machine<'tcx>> InterpCx<'tcx, M> {
     /// of variadic arguments. Return a list of the places that hold those arguments.
     pub(crate) fn allocate_varargs<I, J>(
         &mut self,
-        caller_args: &mut I,
-        callee_abis: &mut J,
+        caller_args: I,
+        mut callee_abis: J,
     ) -> InterpResult<'tcx, Vec<MPlaceTy<'tcx, M::Provenance>>>
     where
         I: Iterator<Item = (&'a FnArg<'tcx, M::Provenance>, &'a ArgAbi<'tcx, Ty<'tcx>>)>,

src/tools/miri/tests/pass/c-variadic-ignored-argument.rs

@@ -0,0 +1,21 @@
+//@ ignore-target: windows # does not ignore ZST arguments
+//@ ignore-target: powerpc # does not ignore ZST arguments
+//@ ignore-target: s390x # does not ignore ZST arguments
+//@ ignore-target: sparc # does not ignore ZST arguments
+#![feature(c_variadic)]
+
+// Some platforms ignore ZSTs, meaning that the argument is not passed, even though it is part
+// of the callee's ABI. Test that this doesn't trip any asserts.
+//
+// NOTE: this  test only succeeds when the `()` argument uses `Passmode::Ignore`. For some targets,
+// notably msvc, such arguments are not ignored, which would cause UB when attempting to read the
+// second `i32` argument while the next item in the variable argument list is `()`.
+
+fn main() {
+    unsafe extern "C" fn variadic(mut ap: ...) {
+        ap.arg::<i32>();
+        ap.arg::<i32>();
+    }
+
+    unsafe { variadic(0i32, (), 1i32) }
+}

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

@@ -0,0 +1,18 @@
+//@ build-pass
+//@ 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 `PassMode::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 without tripping any asserts.
+
+const unsafe extern "C" fn read_n<const N: usize>(_: ...) {}
+
+unsafe fn read_too_many() {
+    const { read_n::<0>((), 1i32) }
+}
+
+fn read_as<T>() -> () {}