Rollup merge of #146434 - folkertdev:c-variadic-inherent-methods, r=workingjubilee

c-variadic: allow c-variadic inherent and trait methods

tracking issue: https://github.com/rust-lang/rust/issues/44930

Continuing the work of https://github.com/rust-lang/rust/pull/146342, allow inherent and trait methods to be c-variadic. However, a trait that contains a c-variadic method is no longer dyn-compatible.

There is, presumably, some way to make c-variadic methods dyn-compatible. However currently, we don't have confidence that it'll work reliably: when methods from a `dyn` object are cast to a function pointer, a `ReifyShim` is created. If that shim is c-variadic, it would need to forward the C variable argument list.

That does appear to work, because the `va_list` is not represented in MIR at all in this case, so the registers from the call site are untouched by the shim and can be read by the actual implementation. That just does not seem like a solid implementation.

Also, intuitively, why would c-variadic function, primarily needed for FFI, need to be used with `dyn` objects at all? We can revisit this limitation if a need arises.

r? `@workingjubilee`

1 files changed, 9 insertions, 9 deletions

diff --git a/tests/ui/parser/variadic-ffi-semantic-restrictions.rs b/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
index 4db056f15a5..415472176d9 100644
--- a/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
+++ b/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
@@ -53,30 +53,30 @@ struct X;
 
 impl X {
     fn i_f1(x: isize, ...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn i_f2(...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn i_f3(..., x: isize, ...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     //~| ERROR `...` must be the last argument of a C-variadic function
     fn i_f4(..., x: isize, ...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     //~| ERROR `...` must be the last argument of a C-variadic function
     const fn i_f5(x: isize, ...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     //~| ERROR functions cannot be both `const` and C-variadic
     //~| ERROR destructor of `VaListImpl<'_>` cannot be evaluated at compile-time
 }
 
 trait T {
     fn t_f1(x: isize, ...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn t_f2(x: isize, ...);
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn t_f3(...) {}
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn t_f4(...);
-    //~^ ERROR associated functions cannot have a C variable argument list
+    //~^ ERROR `...` is not supported for non-extern functions
     fn t_f5(..., x: isize) {}
     //~^ ERROR `...` must be the last argument of a C-variadic function
     fn t_f6(..., x: isize);