From a093372e5e560b1a768971c2b87b306755f2c17d Mon Sep 17 00:00:00 2001
From: Folkert de Vries <folkert@folkertdev.nl>
Date: Sat, 6 Sep 2025 15:13:02 +0200
Subject: disallow c-variadic associated functions (for now)

there is no reason this should not work, really, we're just cutting some scope for now
---
 compiler/rustc_ast_passes/messages.ftl          |  2 ++
 compiler/rustc_ast_passes/src/ast_validation.rs | 14 +++++++++-----
 compiler/rustc_ast_passes/src/errors.rs         |  7 +++++++
 3 files changed, 18 insertions(+), 5 deletions(-)

(limited to 'compiler')

diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index 68ac2a05a48..6eb4e4bc452 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -66,6 +66,8 @@ ast_passes_body_in_extern = incorrect `{$kind}` inside `extern` block
 
 ast_passes_bound_in_context = bounds on `type`s in {$ctx} have no effect
 
+ast_passes_c_variadic_associated_function = associated functions cannot have a C variable argument list
+
 ast_passes_const_and_c_variadic = functions cannot be both `const` and C-variadic
     .const = `const` because of this
     .variadic = C-variadic because of this
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index 672c1e94418..c5c23c47e74 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -704,12 +704,16 @@ impl<'a> AstValidator<'a> {
                 {
                     return;
                 }
-                _ => {}
+                _ => {
+                    self.dcx().emit_err(errors::BadCVariadic { span: variadic_param.span });
+                }
             },
-            FnCtxt::Assoc(_) => {}
-        };
-
-        self.dcx().emit_err(errors::BadCVariadic { span: variadic_param.span });
+            FnCtxt::Assoc(_) => {
+                // For now, C variable argument lists are unsupported in associated functions.
+                let err = errors::CVariadicAssociatedFunction { span: variadic_param.span };
+                self.dcx().emit_err(err);
+            }
+        }
     }
 
     fn check_item_named(&self, ident: Ident, kind: &str) {
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index f8806ef21c1..a085e09fe94 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -318,6 +318,13 @@ pub(crate) struct ExternItemAscii {
     pub block: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(ast_passes_c_variadic_associated_function)]
+pub(crate) struct CVariadicAssociatedFunction {
+    #[primary_span]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(ast_passes_bad_c_variadic)]
 pub(crate) struct BadCVariadic {
-- 
cgit 1.4.1-3-g733a5