Support C23's Variadics Without a Named Parameter

This PR removes the static check that disallowed extern functions
with ellipsis (varargs) as the only parameter since this is now
valid in C23.
Also, adds a doc comment for `check_decl_cvariadic_pos()` and
fixes the name of the function (`varadic` -> `variadic`).

Author: veera-sivarajan

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> {
 
 fncheck_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> {
 }
 }
 
-fncheck_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.
+fncheck_decl_cvariadic_pos(&self,fn_decl:&FnDecl){
 match&*fn_decl.inputs{
-[Param{ ty, span, .. }] => {
-ifletTyKind::CVarArgs = ty.kind{
-self.dcx().emit_err(errors::FnParamCVarArgsOnly{span:*span });
-}
-}
 [ps @ .., _] => {
 forParam{ ty, span, .. }in ps {
 ifletTyKind::CVarArgs = ty.kind{

compiler/rustc_ast_passes/src/errors.rs

@@ -92,13 +92,6 @@ pub struct FnParamTooMany {
 pubmax_num_args:usize,
 }
 
-#[derive(Diagnostic)]
-#[diag(ast_passes_fn_param_c_var_args_only)]
-pubstructFnParamCVarArgsOnly{
-#[primary_span]
-pubspan:Span,
-}
-
 #[derive(Diagnostic)]
 #[diag(ast_passes_fn_param_c_var_args_not_last)]
 pubstructFnParamCVarArgsNotLast{