c-variadic: allow trait methods to be c-variadic

but a C-variadic method makes a trait dyn-incompatible. That is because
methods from dyn traits, when cast to a function pointer, create a shim.
That shim can't really forward the c-variadic arguments.

1 files changed, 6 insertions, 0 deletions

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>),
 }