rustc_ast_passes: allow c-variadic associated fns

 - Allow c-variadic associated functions
 - Add test for c-variadic functions in the Impl context

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -537,7 +537,8 @@ impl<'a> AstValidator<'a> {
     fn check_c_varadic_type(&self, fk: FnKind<'a>) {
         match (fk.ctxt(), fk.header()) {
             (Some(FnCtxt::Foreign), _) => return,
-            (Some(FnCtxt::Free), Some(header)) => match header.ext {
+            (Some(FnCtxt::Free), Some(header))
+            | (Some(FnCtxt::Assoc(AssocCtxt::Impl)), Some(header)) => match header.ext {
                 Extern::Explicit(StrLit { symbol_unescaped: sym::C, .. }) | Extern::Implicit
                     if matches!(header.unsafety, Unsafe::Yes(_)) =>
                 {

src/test/ui/c-variadic/variadic-ffi-6.rs

@@ -11,3 +11,19 @@ pub unsafe extern "C" fn use_vararg_lifetime(
 pub unsafe extern "C" fn use_normal_arg_lifetime(x: &usize, y: ...) -> &usize { // OK
     x
 }
+
+#[repr(C)]
+pub struct Foo(usize);
+
+
+impl Foo {
+    #[no_mangle]
+    pub unsafe extern "C" fn assoc_fn(_format: *const i8, ap: ...) -> usize { // OK
+        ap.arg()
+    }
+
+    #[no_mangle]
+    pub unsafe extern "C" fn method(&self, _format: *const i8, ap: ...) -> usize { // OK
+        ap.arg()
+    }
+}

src/test/ui/c-variadic/variadic-ffi-7.rs

@@ -0,0 +1,55 @@
+// run-pass
+#![feature(c_variadic)]
+
+macro_rules! variadic_tests {
+    (generate) => {
+        pub unsafe extern "C" fn sum(n: usize, mut ap: ...) -> usize {
+            (0..n).fold(0usize, |acc, _| {
+                acc + ap.arg::<usize>()
+            })
+        }
+
+        pub unsafe extern "C" fn mixed_types(_: usize, mut ap: ...) {
+            use std::ffi::{CString, CStr};
+
+            assert_eq!(ap.arg::<f64>().floor(), 12.0f64.floor());
+            assert_eq!(ap.arg::<u32>(), 0x10000001);
+            assert_eq!(ap.arg::<i8>(), 0x42);
+            let cstr = CStr::from_ptr(ap.arg::<*const i8>());
+            assert!(cstr == &*CString::new("Hello").unwrap());
+        }
+    };
+
+    (test_sum $p:path) => {
+        unsafe {
+            assert_eq!($p(5, 2, 4, 8, 16, 32), 62);
+            assert_eq!($p(4, 10, 8, 8, 16, 2), 42);
+        }
+    };
+
+    (test_mixed_types $p:path) => {
+        unsafe {
+            use std::ffi::CString;
+
+            $p(0, 12.56f64, 0x10000001, 0x42, CString::new("Hello").unwrap().as_ptr());
+        }
+    }
+}
+
+mod foo {
+    variadic_tests!(generate);
+}
+
+struct Foo;
+
+impl Foo {
+    variadic_tests!(generate);
+}
+
+fn main() {
+    variadic_tests!(test_sum foo::sum);
+    variadic_tests!(test_sum Foo::sum);
+
+    variadic_tests!(test_mixed_types foo::mixed_types);
+    variadic_tests!(test_mixed_types Foo::mixed_types);
+}