Rollup merge of rust-lang#124048 - veera-sivarajan:bugfix-123773-c23-variadics, r=compiler-errors

Support C23's Variadics Without a Named Parameter

Fixes rust-lang#123773

This PR removes the static check that disallowed extern functions
with ellipsis (varargs) as the only parameter since this is now
valid in C23.

This will not break any existing code as mentioned in the proposal
document: https://www.open-std.org/jtc1/sc22/wg14/www/docs/n2975.pdf.

Also, adds a doc comment for `check_decl_cvariadic_pos()` and
fixes the name of the function (`varadic` -> `variadic`).

Author: workingjubilee

compiler/rustc_ast_passes/messages.ftl

@@ -92,9 +92,6 @@ 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

@@ -364,7 +364,7 @@ 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);
     }
@@ -379,13 +379,11 @@ 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/c-variadic/variadic-ffi-no-fixed-args.stderr

@@ -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