support c-variadic functions in rustc_const_eval

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -698,13 +698,11 @@ impl<'a> AstValidator<'a> {
             unreachable!("C variable argument list cannot be used in closures")
         };
 
-        // C-variadics are not yet implemented in const evaluation.
-        if let Const::Yes(const_span) = sig.header.constness {
-            self.dcx().emit_err(errors::ConstAndCVariadic {
-                spans: vec![const_span, variadic_param.span],
-                const_span,
-                variadic_span: variadic_param.span,
-            });
+        if let Const::Yes(_) = sig.header.constness
+            && !self.features.enabled(sym::const_c_variadic)
+        {
+            let msg = format!("c-variadic const function definitions are unstable");
+            feature_err(&self.sess, sym::const_c_variadic, sig.span, msg).emit();
         }
 
         if let Some(coroutine_kind) = sig.header.coroutine_kind {

compiler/rustc_ast_passes/src/errors.rs

@@ -823,17 +823,6 @@ pub(crate) struct ConstAndCoroutine {
     pub coroutine_kind: &'static str,
 }
 
-#[derive(Diagnostic)]
-#[diag("functions cannot be both `const` and C-variadic")]
-pub(crate) struct ConstAndCVariadic {
-    #[primary_span]
-    pub spans: Vec<Span>,
-    #[label("`const` because of this")]
-    pub const_span: Span,
-    #[label("C-variadic because of this")]
-    pub variadic_span: Span,
-}
-
 #[derive(Diagnostic)]
 #[diag("functions cannot be both `{$coroutine_kind}` and C-variadic")]
 pub(crate) struct CoroutineAndCVariadic {