Avoid crashing on variadic functions when producing arg-mismatch errors

compiler/rustc_hir_typeck/src/fn_ctxt/checks.rs

@@ -2619,9 +2619,9 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
         is_method: bool,
     ) -> Option<Vec<(Option<&hir::GenericParam<'_>>, &hir::Param<'_>)>> {
         let fn_node = self.tcx.hir().get_if_local(def_id)?;
+        let fn_decl = fn_node.fn_decl()?;
 
-        let generic_params: Vec<Option<&hir::GenericParam<'_>>> = fn_node
-            .fn_decl()?
+        let generic_params: Vec<Option<&hir::GenericParam<'_>>> = fn_decl
             .inputs
             .into_iter()
             .skip(if is_method { 1 } else { 0 })
@@ -2642,7 +2642,7 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             })
             .collect();
 
-        let params: Vec<&hir::Param<'_>> = self
+        let mut params: Vec<&hir::Param<'_>> = self
             .tcx
             .hir()
             .body(fn_node.body_id()?)
@@ -2651,7 +2651,16 @@ impl<'a, 'tcx> FnCtxt<'a, 'tcx> {
             .skip(if is_method { 1 } else { 0 })
             .collect();
 
-        Some(generic_params.into_iter().zip_eq(params).collect())
+        // The surrounding code expects variadic functions to not have a parameter representing
+        // the "..." parameter. This is already true of the FnDecl but not of the body params, so
+        // we drop it if it exists.
+
+        if fn_decl.c_variadic {
+            params.pop();
+        }
+
+        debug_assert_eq!(params.len(), generic_params.len());
+        Some(generic_params.into_iter().zip(params).collect())
     }
 }
 

tests/ui/mismatched_types/mismatch-args-vargs-issue-130372.rs

@@ -0,0 +1,12 @@
+#![feature(c_variadic)]
+
+// Regression test that covers all 3 cases of https://github.com/rust-lang/rust/issues/130372
+
+unsafe extern "C" fn test_va_copy(_: u64, mut ap: ...) {}
+
+pub fn main() {
+    unsafe {
+        test_va_copy();
+        //~^ ERROR this function takes at least 1 argument but 0 arguments were supplied
+    }
+}

tests/ui/mismatched_types/mismatch-args-vargs-issue-130372.stderr

@@ -0,0 +1,19 @@
+error[E0060]: this function takes at least 1 argument but 0 arguments were supplied
+  --> $DIR/mismatch-args-vargs-issue-130372.rs:9:9
+   |
+LL |         test_va_copy();
+   |         ^^^^^^^^^^^^-- argument #1 of type `u64` is missing
+   |
+note: function defined here
+  --> $DIR/mismatch-args-vargs-issue-130372.rs:5:22
+   |
+LL | unsafe extern "C" fn test_va_copy(_: u64, mut ap: ...) {}
+   |                      ^^^^^^^^^^^^ ------
+help: provide the argument
+   |
+LL |         test_va_copy(/* u64 */);
+   |                     ~~~~~~~~~~~
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0060`.