5 files changed, 10 insertions, 15 deletions
diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index e5405a7ad91..5e10c5a77d9 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -64,8 +64,6 @@ 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_c_variadic_bad_extern = `...` is not supported for `extern "{$abi}"` functions
     .label = `extern "{$abi}"` because of this
     .help = only `extern "C"` and `extern "C-unwind"` functions may have a C variable argument list
diff --git a/compiler/rustc_ast_passes/src/ast_validation.rs b/compiler/rustc_ast_passes/src/ast_validation.rs
index dc221c2fb1a..f773b02058e 100644
--- a/compiler/rustc_ast_passes/src/ast_validation.rs
+++ b/compiler/rustc_ast_passes/src/ast_validation.rs
@@ -696,7 +696,7 @@ impl<'a> AstValidator<'a> {
 
         match fn_ctxt {
             FnCtxt::Foreign => return,
-            FnCtxt::Free => match sig.header.ext {
+            FnCtxt::Free | FnCtxt::Assoc(_) => match sig.header.ext {
                 Extern::Implicit(_) => {
                     if !matches!(sig.header.safety, Safety::Unsafe(_)) {
                         self.dcx().emit_err(errors::CVariadicMustBeUnsafe {
@@ -726,11 +726,6 @@ impl<'a> AstValidator<'a> {
                     self.dcx().emit_err(err);
                 }
             },
-            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);
-            }
         }
     }
 
diff --git a/compiler/rustc_ast_passes/src/errors.rs b/compiler/rustc_ast_passes/src/errors.rs
index e09ca5b81c8..fd75e999d13 100644
--- a/compiler/rustc_ast_passes/src/errors.rs
+++ b/compiler/rustc_ast_passes/src/errors.rs
@@ -319,13 +319,6 @@ pub(crate) struct ExternItemAscii {
 }
 
 #[derive(Diagnostic)]
-#[diag(ast_passes_c_variadic_associated_function)]
-pub(crate) struct CVariadicAssociatedFunction {
-    #[primary_span]
-    pub span: Span,
-}
-
-#[derive(Diagnostic)]
 #[diag(ast_passes_c_variadic_no_extern)]
 #[help]
 pub(crate) struct CVariadicNoExtern {
diff --git a/compiler/rustc_middle/src/traits/mod.rs b/compiler/rustc_middle/src/traits/mod.rs
index ab8a3142953..0c7bddf60d9 100644
--- a/compiler/rustc_middle/src/traits/mod.rs
+++ b/compiler/rustc_middle/src/traits/mod.rs
@@ -823,6 +823,9 @@ impl DynCompatibilityViolation {
             DynCompatibilityViolation::Method(name, MethodViolationCode::AsyncFn, _) => {
                 format!("method `{name}` is `async`").into()
             }
+            DynCompatibilityViolation::Method(name, MethodViolationCode::CVariadic, _) => {
+                format!("method `{name}` is C-variadic").into()
+            }
             DynCompatibilityViolation::Method(
                 name,
                 MethodViolationCode::WhereClauseReferencesSelf,
@@ -977,6 +980,9 @@ pub enum MethodViolationCode {
     /// e.g., `fn foo<A>()`
     Generic,
 
+    /// e.g., `fn (mut ap: ...)`
+    CVariadic,
+
     /// the method's receiver (`self` argument) can't be dispatched on
     UndispatchableReceiver(Option<Span>),
 }
diff --git a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
index bcd11d6918d..3260dd712b9 100644
--- a/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
+++ b/compiler/rustc_trait_selection/src/traits/dyn_compatibility.rs
@@ -426,6 +426,9 @@ fn virtual_call_violations_for_method<'tcx>(
     if let Some(code) = contains_illegal_impl_trait_in_trait(tcx, method.def_id, sig.output()) {
         errors.push(code);
     }
+    if sig.skip_binder().c_variadic {
+        errors.push(MethodViolationCode::CVariadic);
+    }
 
     // We can't monomorphize things like `fn foo<A>(...)`.
     let own_counts = tcx.generics_of(method.def_id).own_counts();