Allow "C-unwind" fn to have C variadics

Author: workingjubilee

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -637,6 +637,7 @@ impl<'a> AstValidator<'a> {
             (Some(FnCtxt::Foreign), _) => return,
             (Some(FnCtxt::Free), Some(header)) => match header.ext {
                 Extern::Explicit(StrLit { symbol_unescaped: sym::C, .. }, _)
+                | Extern::Explicit(StrLit { symbol_unescaped: sym::C_dash_unwind, .. }, _)
                 | Extern::Implicit(_)
                     if matches!(header.safety, Safety::Unsafe(_)) =>
                 {

compiler/rustc_span/src/symbol.rs

@@ -167,6 +167,7 @@ symbols! {
         Break,
         C,
         CStr,
+        C_dash_unwind: "C-unwind",
         CallOnceFuture,
         CallRefFuture,
         Capture,

tests/ui/abi/variadic-ffi.rs

@@ -14,6 +14,10 @@ pub unsafe extern "C" fn test_valist_forward(n: u64, mut ap: ...) -> f64 {
     rust_valist_interesting_average(n, ap.as_va_list())
 }
 
+pub unsafe extern "C-unwind" fn c_unwind_can_forward(n: u64, mut ap: ...) -> f64 {
+    rust_valist_interesting_average(n, ap.as_va_list())
+}
+
 pub unsafe extern "C" fn test_va_copy(_: u64, mut ap: ...) {
     let mut ap2 = ap.clone();
     assert_eq!(rust_valist_interesting_average(2, ap2.as_va_list()) as i64, 30);
@@ -72,6 +76,10 @@ pub fn main() {
         assert_eq!(test_valist_forward(2, 10i64, 10f64, 20i64, 20f64) as i64, 30);
     }
 
+    unsafe {
+        assert_eq!(c_unwind_can_forward(2, 10i64, 10f64, 20i64, 20f64) as i64, 30);
+    }
+
     unsafe {
         test_va_copy(4, 10i64, 10f64, 20i64, 20f64, 30i64, 30f64, 40i64, 40f64);
     }