Support C23's Variadics Without a Named Parameter

compiler/rustc_ast_passes/messages.ftl

@@ -97,9 +97,6 @@ ast_passes_fn_body_extern = incorrect function inside `extern` block
 ast_passes_fn_param_c_var_args_not_last =
     `...` must be the last argument of a C-variadic function
 
-ast_passes_fn_param_c_var_args_only =
-    C-variadic function must be declared with at least one named argument
-
 ast_passes_fn_param_doc_comment =
     documentation comments cannot be applied to function parameters
     .label = doc comments are not allowed here

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -365,7 +365,7 @@ impl<'a> AstValidator<'a> {
 
     fn check_fn_decl(&self, fn_decl: &FnDecl, self_semantic: SelfSemantic) {
         self.check_decl_num_args(fn_decl);
-        self.check_decl_cvaradic_pos(fn_decl);
+        self.check_decl_cvariadic_pos(fn_decl);
         self.check_decl_attrs(fn_decl);
         self.check_decl_self_param(fn_decl, self_semantic);
     }
@@ -380,13 +380,11 @@ impl<'a> AstValidator<'a> {
         }
     }
 
-    fn check_decl_cvaradic_pos(&self, fn_decl: &FnDecl) {
+    /// Emits an error if a function declaration has a variadic parameter in the
+    /// beginning or middle of parameter list.
+    /// Example: `fn foo(..., x: i32)` will emit an error.
+    fn check_decl_cvariadic_pos(&self, fn_decl: &FnDecl) {
         match &*fn_decl.inputs {
-            [Param { ty, span, .. }] => {
-                if let TyKind::CVarArgs = ty.kind {
-                    self.dcx().emit_err(errors::FnParamCVarArgsOnly { span: *span });
-                }
-            }
             [ps @ .., _] => {
                 for Param { ty, span, .. } in ps {
                     if let TyKind::CVarArgs = ty.kind {

compiler/rustc_ast_passes/src/errors.rs

@@ -92,13 +92,6 @@ pub struct FnParamTooMany {
     pub max_num_args: usize,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_fn_param_c_var_args_only)]
-pub struct FnParamCVarArgsOnly {
-    #[primary_span]
-    pub span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_fn_param_c_var_args_not_last)]
 pub struct FnParamCVarArgsNotLast {

tests/ui/c-variadic/variadic-ffi-no-fixed-args.rs

@@ -1,6 +1,9 @@
+//@ build-pass
+
+// Supported since C23
+// https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2975.pdf
 extern "C" {
     fn foo(...);
-//~^ ERROR C-variadic function must be declared with at least one named argument
 }
 
 fn main() {}

tests/ui/parser/variadic-ffi-semantic-restrictions.rs

@@ -8,14 +8,12 @@ fn f1_1(x: isize, ...) {}
 
 fn f1_2(...) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-//~| ERROR C-variadic function must be declared with at least one named argument
 
 extern "C" fn f2_1(x: isize, ...) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
 
 extern "C" fn f2_2(...) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-//~| ERROR C-variadic function must be declared with at least one named argument
 
 extern "C" fn f2_3(..., x: isize) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
@@ -26,7 +24,6 @@ extern "C" fn f3_1(x: isize, ...) {}
 
 extern "C" fn f3_2(...) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-//~| ERROR C-variadic function must be declared with at least one named argument
 
 extern "C" fn f3_3(..., x: isize) {}
 //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
@@ -47,8 +44,6 @@ const extern "C" fn f4_3(..., x: isize, ...) {}
 //~| ERROR `...` must be the last argument of a C-variadic function
 
 extern "C" {
-    fn e_f1(...);
-    //~^ ERROR C-variadic function must be declared with at least one named argument
     fn e_f2(..., x: isize);
     //~^ ERROR `...` must be the last argument of a C-variadic function
 }
@@ -60,7 +55,6 @@ impl X {
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     fn i_f2(...) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-    //~| ERROR C-variadic function must be declared with at least one named argument
     fn i_f3(..., x: isize, ...) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     //~| ERROR `...` must be the last argument of a C-variadic function
@@ -80,10 +74,8 @@ trait T {
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     fn t_f3(...) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-    //~| ERROR C-variadic function must be declared with at least one named argument
     fn t_f4(...);
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
-    //~| ERROR C-variadic function must be declared with at least one named argument
     fn t_f5(..., x: isize) {}
     //~^ ERROR only foreign or `unsafe extern "C"` functions may be C-variadic
     //~| ERROR `...` must be the last argument of a C-variadic function