stabilize extended_varargs_abi_support

28 files changed, 128 insertions, 162 deletions

diff --git a/compiler/rustc_abi/src/extern_abi.rs b/compiler/rustc_abi/src/extern_abi.rs
index 41d744e1946..e3b2b1eff72 100644
--- a/compiler/rustc_abi/src/extern_abi.rs
+++ b/compiler/rustc_abi/src/extern_abi.rs
@@ -6,6 +6,8 @@ use std::hash::{Hash, Hasher};
 use rustc_data_structures::stable_hasher::{HashStable, StableHasher, StableOrd};
 #[cfg(feature = "nightly")]
 use rustc_macros::{Decodable, Encodable};
+#[cfg(feature = "nightly")]
+use rustc_span::Symbol;
 
 use crate::AbiFromStrErr;
 
@@ -226,6 +228,13 @@ impl StableOrd for ExternAbi {
 #[cfg(feature = "nightly")]
 rustc_error_messages::into_diag_arg_using_display!(ExternAbi);
 
+#[cfg(feature = "nightly")]
+pub enum CVariadicStatus {
+    NotSupported,
+    Stable,
+    Unstable { feature: Symbol },
+}
+
 impl ExternAbi {
     /// An ABI "like Rust"
     ///
@@ -238,23 +247,33 @@ impl ExternAbi {
         matches!(self, Rust | RustCall | RustCold)
     }
 
-    pub fn supports_varargs(self) -> bool {
+    /// Returns whether the ABI supports C variadics. This only controls whether we allow *imports*
+    /// of such functions via `extern` blocks; there's a separate check during AST construction
+    /// guarding *definitions* of variadic functions.
+    #[cfg(feature = "nightly")]
+    pub fn supports_c_variadic(self) -> CVariadicStatus {
         // * C and Cdecl obviously support varargs.
         // * C can be based on Aapcs, SysV64 or Win64, so they must support varargs.
         // * EfiApi is based on Win64 or C, so it also supports it.
+        // * System automatically falls back to C when used with variadics, therefore supports it.
         //
         // * Stdcall does not, because it would be impossible for the callee to clean
         //   up the arguments. (callee doesn't know how many arguments are there)
         // * Same for Fastcall, Vectorcall and Thiscall.
         // * Other calling conventions are related to hardware or the compiler itself.
+        //
+        // All of the supported ones must have a test in `tests/codegen/cffi/c-variadic-ffi.rs`.
         match self {
             Self::C { .. }
             | Self::Cdecl { .. }
             | Self::Aapcs { .. }
             | Self::Win64 { .. }
             | Self::SysV64 { .. }
-            | Self::EfiApi => true,
-            _ => false,
+            | Self::EfiApi => CVariadicStatus::Stable,
+            Self::System { .. } => {
+                CVariadicStatus::Unstable { feature: rustc_span::sym::extern_system_varargs }
+            }
+            _ => CVariadicStatus::NotSupported,
         }
     }
 }
diff --git a/compiler/rustc_abi/src/lib.rs b/compiler/rustc_abi/src/lib.rs
index 14e256b8045..22aba1fac0f 100644
--- a/compiler/rustc_abi/src/lib.rs
+++ b/compiler/rustc_abi/src/lib.rs
@@ -63,6 +63,8 @@ mod tests;
 
 pub use callconv::{Heterogeneous, HomogeneousAggregate, Reg, RegKind};
 pub use canon_abi::{ArmCall, CanonAbi, InterruptKind, X86Call};
+#[cfg(feature = "nightly")]
+pub use extern_abi::CVariadicStatus;
 pub use extern_abi::{ExternAbi, all_names};
 #[cfg(feature = "nightly")]
 pub use layout::{FIRST_VARIANT, FieldIdx, Layout, TyAbiInterface, TyAndLayout, VariantIdx};
diff --git a/compiler/rustc_ast_passes/messages.ftl b/compiler/rustc_ast_passes/messages.ftl
index a95f1443968..e5f1fcdc4b4 100644
--- a/compiler/rustc_ast_passes/messages.ftl
+++ b/compiler/rustc_ast_passes/messages.ftl
@@ -57,7 +57,7 @@ ast_passes_auto_super_lifetime = auto traits cannot have super traits or lifetim
     .label = {ast_passes_auto_super_lifetime}
     .suggestion = remove the super traits or lifetime bounds
 
-ast_passes_bad_c_variadic = only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+ast_passes_bad_c_variadic = defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 ast_passes_body_in_extern = incorrect `{$kind}` inside `extern` block
     .cannot_have = cannot have a body
diff --git a/compiler/rustc_feature/src/accepted.rs b/compiler/rustc_feature/src/accepted.rs
index 9fe55216f93..6af4cfb0e56 100644
--- a/compiler/rustc_feature/src/accepted.rs
+++ b/compiler/rustc_feature/src/accepted.rs
@@ -203,6 +203,9 @@ declare_features! (
     (accepted, expr_fragment_specifier_2024, "1.83.0", Some(123742)),
     /// Allows arbitrary expressions in key-value attributes at parse time.
     (accepted, extended_key_value_attributes, "1.54.0", Some(78835)),
+    /// Allows using `aapcs`, `efiapi`, `sysv64` and `win64` as calling conventions
+    /// for functions with varargs.
+    (accepted, extended_varargs_abi_support, "CURRENT_RUSTC_VERSION", Some(100189)),
     /// Allows resolving absolute paths as paths from other crates.
     (accepted, extern_absolute_paths, "1.30.0", Some(44660)),
     /// Allows `extern crate foo as bar;`. This puts `bar` into extern prelude.
diff --git a/compiler/rustc_feature/src/unstable.rs b/compiler/rustc_feature/src/unstable.rs
index 92b435b4b01..573e9aab055 100644
--- a/compiler/rustc_feature/src/unstable.rs
+++ b/compiler/rustc_feature/src/unstable.rs
@@ -492,9 +492,6 @@ declare_features! (
     (incomplete, explicit_tail_calls, "1.72.0", Some(112788)),
     /// Allows using `#[export_stable]` which indicates that an item is exportable.
     (incomplete, export_stable, "1.88.0", Some(139939)),
-    /// Allows using `aapcs`, `efiapi`, `sysv64` and `win64` as calling conventions
-    /// for functions with varargs.
-    (unstable, extended_varargs_abi_support, "1.65.0", Some(100189)),
     /// Allows using `system` as a calling convention with varargs.
     (unstable, extern_system_varargs, "1.86.0", Some(136946)),
     /// Allows defining `extern type`s.
diff --git a/compiler/rustc_hir_analysis/src/check/check.rs b/compiler/rustc_hir_analysis/src/check/check.rs
index eccb88a938f..08b344638dd 100644
--- a/compiler/rustc_hir_analysis/src/check/check.rs
+++ b/compiler/rustc_hir_analysis/src/check/check.rs
@@ -978,7 +978,7 @@ pub(crate) fn check_item_type(tcx: TyCtxt<'_>, def_id: LocalDefId) -> Result<(),
                         tcx.ensure_ok().fn_sig(def_id);
                         let item = tcx.hir_foreign_item(item);
                         let hir::ForeignItemKind::Fn(sig, ..) = item.kind else { bug!() };
-                        require_c_abi_if_c_variadic(tcx, sig.decl, abi, item.span);
+                        check_c_variadic_abi(tcx, sig.decl, abi, item.span);
                     }
                     DefKind::Static { .. } => {
                         tcx.ensure_ok().codegen_fn_attrs(def_id);
diff --git a/compiler/rustc_hir_analysis/src/check/mod.rs b/compiler/rustc_hir_analysis/src/check/mod.rs
index 2e4b151d4dc..b5c0ca4727c 100644
--- a/compiler/rustc_hir_analysis/src/check/mod.rs
+++ b/compiler/rustc_hir_analysis/src/check/mod.rs
@@ -98,7 +98,7 @@ use tracing::debug;
 
 use self::compare_impl_item::collect_return_position_impl_trait_in_trait_tys;
 use self::region::region_scope_tree;
-use crate::{errors, require_c_abi_if_c_variadic};
+use crate::{check_c_variadic_abi, errors};
 
 /// Adds query implementations to the [Providers] vtable, see [`rustc_middle::query`]
 pub(super) fn provide(providers: &mut Providers) {
diff --git a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
index c7b984d9b25..c5e079fe89a 100644
--- a/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
+++ b/compiler/rustc_hir_analysis/src/hir_ty_lowering/mod.rs
@@ -52,11 +52,11 @@ use rustc_trait_selection::traits::{self, FulfillmentError};
 use tracing::{debug, instrument};
 
 use crate::check::check_abi;
+use crate::check_c_variadic_abi;
 use crate::errors::{AmbiguousLifetimeBound, BadReturnTypeNotation};
 use crate::hir_ty_lowering::errors::{GenericsArgsErrExtend, prohibit_assoc_item_constraint};
 use crate::hir_ty_lowering::generics::{check_generic_arg_count, lower_generic_args};
 use crate::middle::resolve_bound_vars as rbv;
-use crate::require_c_abi_if_c_variadic;
 
 /// A path segment that is semantically allowed to have generic arguments.
 #[derive(Debug)]
@@ -2412,7 +2412,7 @@ impl<'tcx> dyn HirTyLowerer<'tcx> + '_ {
                 Ty::new_tup_from_iter(tcx, fields.iter().map(|t| self.lower_ty(t)))
             }
             hir::TyKind::FnPtr(bf) => {
-                require_c_abi_if_c_variadic(tcx, bf.decl, bf.abi, hir_ty.span);
+                check_c_variadic_abi(tcx, bf.decl, bf.abi, hir_ty.span);
 
                 Ty::new_fn_ptr(
                     tcx,
diff --git a/compiler/rustc_hir_analysis/src/lib.rs b/compiler/rustc_hir_analysis/src/lib.rs
index 44a5ceed469..2562ab7542a 100644
--- a/compiler/rustc_hir_analysis/src/lib.rs
+++ b/compiler/rustc_hir_analysis/src/lib.rs
@@ -90,7 +90,7 @@ mod outlives;
 mod variance;
 
 pub use errors::NoVariantNamed;
-use rustc_abi::ExternAbi;
+use rustc_abi::{CVariadicStatus, ExternAbi};
 use rustc_hir::def::DefKind;
 use rustc_hir::lints::DelayedLint;
 use rustc_hir::{self as hir};
@@ -99,7 +99,6 @@ use rustc_middle::mir::interpret::GlobalId;
 use rustc_middle::query::Providers;
 use rustc_middle::ty::{self, Const, Ty, TyCtxt};
 use rustc_session::parse::feature_err;
-use rustc_span::symbol::sym;
 use rustc_span::{ErrorGuaranteed, Span};
 use rustc_trait_selection::traits;
 
@@ -108,46 +107,34 @@ use crate::hir_ty_lowering::{FeedConstTy, HirTyLowerer};
 
 rustc_fluent_macro::fluent_messages! { "../messages.ftl" }
 
-fn require_c_abi_if_c_variadic(
-    tcx: TyCtxt<'_>,
-    decl: &hir::FnDecl<'_>,
-    abi: ExternAbi,
-    span: Span,
-) {
-    // ABIs which can stably use varargs
-    if !decl.c_variadic || matches!(abi, ExternAbi::C { .. } | ExternAbi::Cdecl { .. }) {
+fn check_c_variadic_abi(tcx: TyCtxt<'_>, decl: &hir::FnDecl<'_>, abi: ExternAbi, span: Span) {
+    if !decl.c_variadic {
+        // Not even a variadic function.
         return;
     }
 
-    // ABIs with feature-gated stability
-    let extended_abi_support = tcx.features().extended_varargs_abi_support();
-    let extern_system_varargs = tcx.features().extern_system_varargs();
-
-    // If the feature gate has been enabled, we can stop here
-    if extern_system_varargs && let ExternAbi::System { .. } = abi {
-        return;
-    };
-    if extended_abi_support && abi.supports_varargs() {
-        return;
-    };
-
-    // Looks like we need to pick an error to emit.
-    // Is there any feature which we could have enabled to make this work?
-    let unstable_explain =
-        format!("C-variadic functions with the {abi} calling convention are unstable");
-    match abi {
-        ExternAbi::System { .. } => {
-            feature_err(&tcx.sess, sym::extern_system_varargs, span, unstable_explain)
+    match abi.supports_c_variadic() {
+        CVariadicStatus::Stable => {}
+        CVariadicStatus::NotSupported => {
+            tcx.dcx()
+                .create_err(errors::VariadicFunctionCompatibleConvention {
+                    span,
+                    convention: &format!("{abi}"),
+                })
+                .emit();
         }
-        abi if abi.supports_varargs() => {
-            feature_err(&tcx.sess, sym::extended_varargs_abi_support, span, unstable_explain)
+        CVariadicStatus::Unstable { feature } => {
+            if !tcx.features().enabled(feature) {
+                feature_err(
+                    &tcx.sess,
+                    feature,
+                    span,
+                    format!("C-variadic functions with the {abi} calling convention are unstable"),
+                )
+                .emit();
+            }
         }
-        _ => tcx.dcx().create_err(errors::VariadicFunctionCompatibleConvention {
-            span,
-            convention: &format!("{abi}"),
-        }),
     }
-    .emit();
 }
 
 /// Adds query implementations to the [Providers] vtable, see [`rustc_middle::query`]
diff --git a/library/std/src/lib.rs b/library/std/src/lib.rs
index f0de03c9a28..da8fda42cd1 100644
--- a/library/std/src/lib.rs
+++ b/library/std/src/lib.rs
@@ -291,7 +291,6 @@
 #![feature(doc_masked)]
 #![feature(doc_notable_trait)]
 #![feature(dropck_eyepatch)]
-#![feature(extended_varargs_abi_support)]
 #![feature(f16)]
 #![feature(f128)]
 #![feature(ffi_const)]
diff --git a/src/doc/unstable-book/src/language-features/extended-varargs-abi-support.md b/src/doc/unstable-book/src/language-features/extended-varargs-abi-support.md
deleted file mode 100644
index b20c30ec8f1..00000000000
--- a/src/doc/unstable-book/src/language-features/extended-varargs-abi-support.md
+++ /dev/null
@@ -1,10 +0,0 @@
-# `extended_varargs_abi_support`
-
-The tracking issue for this feature is: [#100189]
-
-[#100189]: https://github.com/rust-lang/rust/issues/100189
-
-------------------------
-
-This feature adds the possibility of using `sysv64`, `win64` or `efiapi` calling
-conventions on functions with varargs.
diff --git a/tests/codegen-llvm/cffi/c-variadic-ffi.rs b/tests/codegen-llvm/cffi/c-variadic-ffi.rs
index 3e99c9fb84e..1dee477e9ed 100644
--- a/tests/codegen-llvm/cffi/c-variadic-ffi.rs
+++ b/tests/codegen-llvm/cffi/c-variadic-ffi.rs
@@ -14,7 +14,7 @@
 //@[arm32] needs-llvm-components: arm
 #![crate_type = "lib"]
 #![feature(no_core)]
-#![feature(extended_varargs_abi_support, extern_system_varargs)]
+#![feature(extern_system_varargs)]
 #![no_core]
 
 extern crate minicore;
diff --git a/tests/ui/abi/unsupported-varargs-fnptr.rs b/tests/ui/abi/unsupported-varargs-fnptr.rs
index 1d23916d039..ac3913b3ef8 100644
--- a/tests/ui/abi/unsupported-varargs-fnptr.rs
+++ b/tests/ui/abi/unsupported-varargs-fnptr.rs
@@ -6,8 +6,6 @@
 // We have to use this flag to force ABI computation of an invalid ABI
 //@ compile-flags: -Clink-dead-code
 
-#![feature(extended_varargs_abi_support)]
-
 // sometimes fn ptrs with varargs make layout and ABI computation ICE
 // as found in https://github.com/rust-lang/rust/issues/142107
 
diff --git a/tests/ui/abi/unsupported-varargs-fnptr.stderr b/tests/ui/abi/unsupported-varargs-fnptr.stderr
index 238f2b31330..c05c55df82e 100644
--- a/tests/ui/abi/unsupported-varargs-fnptr.stderr
+++ b/tests/ui/abi/unsupported-varargs-fnptr.stderr
@@ -1,5 +1,5 @@
 error[E0570]: "aapcs" is not a supported ABI for the current target
-  --> $DIR/unsupported-varargs-fnptr.rs:14:20
+  --> $DIR/unsupported-varargs-fnptr.rs:12:20
    |
 LL | fn aapcs(f: extern "aapcs" fn(usize, ...)) {
    |                    ^^^^^^^
diff --git a/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.rs b/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.rs
deleted file mode 100644
index 1e0576b2186..00000000000
--- a/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.rs
+++ /dev/null
@@ -1,16 +0,0 @@
-//@ only-x86_64
-
-fn efiapi(f: extern "efiapi" fn(usize, ...)) {
-    //~^ ERROR: unstable
-    f(22, 44);
-}
-fn sysv(f: extern "sysv64" fn(usize, ...)) {
-    //~^ ERROR: unstable
-    f(22, 44);
-}
-fn win(f: extern "win64" fn(usize, ...)) {
-    //~^ ERROR: unstable
-    f(22, 44);
-}
-
-fn main() {}
diff --git a/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.stderr b/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.stderr
deleted file mode 100644
index 7ef54b639ad..00000000000
--- a/tests/ui/c-variadic/feature-gate-extended_varargs_abi_support.stderr
+++ /dev/null
@@ -1,33 +0,0 @@
-error[E0658]: C-variadic functions with the "efiapi" calling convention are unstable
-  --> $DIR/feature-gate-extended_varargs_abi_support.rs:3:14
-   |
-LL | fn efiapi(f: extern "efiapi" fn(usize, ...)) {
-   |              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: see issue #100189 <https://github.com/rust-lang/rust/issues/100189> for more information
-   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error[E0658]: C-variadic functions with the "sysv64" calling convention are unstable
-  --> $DIR/feature-gate-extended_varargs_abi_support.rs:7:12
-   |
-LL | fn sysv(f: extern "sysv64" fn(usize, ...)) {
-   |            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: see issue #100189 <https://github.com/rust-lang/rust/issues/100189> for more information
-   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error[E0658]: C-variadic functions with the "win64" calling convention are unstable
-  --> $DIR/feature-gate-extended_varargs_abi_support.rs:11:11
-   |
-LL | fn win(f: extern "win64" fn(usize, ...)) {
-   |           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
-   |
-   = note: see issue #100189 <https://github.com/rust-lang/rust/issues/100189> for more information
-   = help: add `#![feature(extended_varargs_abi_support)]` to the crate attributes to enable
-   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
-
-error: aborting due to 3 previous errors
-
-For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/c-variadic/issue-86053-1.rs b/tests/ui/c-variadic/issue-86053-1.rs
index 8eeb99a4cc1..537d0263adf 100644
--- a/tests/ui/c-variadic/issue-86053-1.rs
+++ b/tests/ui/c-variadic/issue-86053-1.rs
@@ -13,6 +13,6 @@ fn ordering4 < 'a , 'b     > ( a :            ,   self , self ,   self ,
     //~| ERROR unexpected `self` parameter in function
     //~| ERROR unexpected `self` parameter in function
     //~| ERROR `...` must be the last argument of a C-variadic function
-    //~| ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~| ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR cannot find type `F` in this scope
 }
diff --git a/tests/ui/c-variadic/issue-86053-1.stderr b/tests/ui/c-variadic/issue-86053-1.stderr
index dc323f9a234..472295c92b9 100644
--- a/tests/ui/c-variadic/issue-86053-1.stderr
+++ b/tests/ui/c-variadic/issue-86053-1.stderr
@@ -46,7 +46,7 @@ error: `...` must be the last argument of a C-variadic function
 LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
    |            ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/issue-86053-1.rs:11:12
    |
 LL |     self , ... ,   self ,   self , ... ) where F : FnOnce ( & 'a & 'b usize ) {
diff --git a/tests/ui/c-variadic/same-program-multiple-abis-arm.rs b/tests/ui/c-variadic/same-program-multiple-abis-arm.rs
index 1b0bdecabfb..1445bbb47bb 100644
--- a/tests/ui/c-variadic/same-program-multiple-abis-arm.rs
+++ b/tests/ui/c-variadic/same-program-multiple-abis-arm.rs
@@ -1,4 +1,3 @@
-#![feature(extended_varargs_abi_support)]
 //@ run-pass
 //@ only-arm
 //@ ignore-thumb (this test uses arm assembly)
diff --git a/tests/ui/c-variadic/same-program-multiple-abis-x86_64.rs b/tests/ui/c-variadic/same-program-multiple-abis-x86_64.rs
index b21accb999e..aff8ab613c4 100644
--- a/tests/ui/c-variadic/same-program-multiple-abis-x86_64.rs
+++ b/tests/ui/c-variadic/same-program-multiple-abis-x86_64.rs
@@ -1,4 +1,3 @@
-#![feature(extended_varargs_abi_support)]
 //@ run-pass
 //@ only-x86_64
 
diff --git a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.rs b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.rs
index 800dd580af2..ad4fca8252d 100644
--- a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.rs
+++ b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.rs
@@ -71,6 +71,6 @@ extern "cmse-nonsecure-entry" fn wrapped_trait_object(
 }
 
 extern "cmse-nonsecure-entry" fn c_variadic(_: u32, _: ...) {
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR requires `va_list` lang_item
 }
diff --git a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.stderr b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.stderr
index f0190671b5a..7aeb6969feb 100644
--- a/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.stderr
+++ b/tests/ui/cmse-nonsecure/cmse-nonsecure-entry/generics.stderr
@@ -1,4 +1,4 @@
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/generics.rs:73:53
    |
 LL | extern "cmse-nonsecure-entry" fn c_variadic(_: u32, _: ...) {
diff --git a/tests/ui/feature-gates/feature-gate-c_variadic.rs b/tests/ui/feature-gates/feature-gate-c_variadic.rs
index f189f02a26d..45c68842093 100644
--- a/tests/ui/feature-gates/feature-gate-c_variadic.rs
+++ b/tests/ui/feature-gates/feature-gate-c_variadic.rs
@@ -2,3 +2,9 @@
 
 pub unsafe extern "C" fn test(_: i32, ap: ...) { }
 //~^ ERROR C-variadic functions are unstable
+
+trait Trait {
+    unsafe extern "C" fn trait_test(_: i32, ap: ...) { }
+    //~^ ERROR C-variadic functions are unstable
+    //~| ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
+}
diff --git a/tests/ui/feature-gates/feature-gate-c_variadic.stderr b/tests/ui/feature-gates/feature-gate-c_variadic.stderr
index 1b6a8c92af5..e30a2f1ede3 100644
--- a/tests/ui/feature-gates/feature-gate-c_variadic.stderr
+++ b/tests/ui/feature-gates/feature-gate-c_variadic.stderr
@@ -1,3 +1,9 @@
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
+  --> $DIR/feature-gate-c_variadic.rs:7:45
+   |
+LL |     unsafe extern "C" fn trait_test(_: i32, ap: ...) { }
+   |                                             ^^^^^^^
+
 error[E0658]: C-variadic functions are unstable
   --> $DIR/feature-gate-c_variadic.rs:3:1
    |
@@ -8,6 +14,16 @@ LL | pub unsafe extern "C" fn test(_: i32, ap: ...) { }
    = help: add `#![feature(c_variadic)]` to the crate attributes to enable
    = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
 
-error: aborting due to 1 previous error
+error[E0658]: C-variadic functions are unstable
+  --> $DIR/feature-gate-c_variadic.rs:7:5
+   |
+LL |     unsafe extern "C" fn trait_test(_: i32, ap: ...) { }
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: see issue #44930 <https://github.com/rust-lang/rust/issues/44930> for more information
+   = help: add `#![feature(c_variadic)]` to the crate attributes to enable
+   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
+
+error: aborting due to 3 previous errors
 
 For more information about this error, try `rustc --explain E0658`.
diff --git a/tests/ui/mir/issue-83499-input-output-iteration-ice.rs b/tests/ui/mir/issue-83499-input-output-iteration-ice.rs
index 78e5c961802..9277994d9b3 100644
--- a/tests/ui/mir/issue-83499-input-output-iteration-ice.rs
+++ b/tests/ui/mir/issue-83499-input-output-iteration-ice.rs
@@ -5,6 +5,6 @@
 fn main() {}
 
 fn foo(_: Bar, ...) -> impl {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 //~| ERROR cannot find type `Bar` in this scope
 //~| ERROR at least one trait must be specified
diff --git a/tests/ui/mir/issue-83499-input-output-iteration-ice.stderr b/tests/ui/mir/issue-83499-input-output-iteration-ice.stderr
index 80a8a94aea4..4a1aa49eb6e 100644
--- a/tests/ui/mir/issue-83499-input-output-iteration-ice.stderr
+++ b/tests/ui/mir/issue-83499-input-output-iteration-ice.stderr
@@ -1,4 +1,4 @@
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/issue-83499-input-output-iteration-ice.rs:7:16
    |
 LL | fn foo(_: Bar, ...) -> impl {}
diff --git a/tests/ui/parser/variadic-ffi-semantic-restrictions.rs b/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
index 1cd6d13d56b..e7a0248cffa 100644
--- a/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
+++ b/tests/ui/parser/variadic-ffi-semantic-restrictions.rs
@@ -4,29 +4,29 @@
 fn main() {}
 
 fn f1_1(x: isize, ...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 fn f1_2(...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 extern "C" fn f2_1(x: isize, ...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 extern "C" fn f2_2(...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 extern "C" fn f2_3(..., x: isize) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 //~| ERROR `...` must be the last argument of a C-variadic function
 
 extern "C" fn f3_1(x: isize, ...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 extern "C" fn f3_2(...) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 
 extern "C" fn f3_3(..., x: isize) {}
-//~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 //~| ERROR `...` must be the last argument of a C-variadic function
 
 const unsafe extern "C" fn f4_1(x: isize, ...) {}
@@ -35,12 +35,12 @@ const unsafe extern "C" fn f4_1(x: isize, ...) {}
 
 const extern "C" fn f4_2(x: isize, ...) {}
 //~^ ERROR functions cannot be both `const` and C-variadic
-//~| ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~| ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 //~| ERROR destructor of `VaListImpl<'_>` cannot be evaluated at compile-time
 
 const extern "C" fn f4_3(..., x: isize, ...) {}
 //~^ ERROR functions cannot be both `const` and C-variadic
-//~| ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+//~| ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
 //~| ERROR `...` must be the last argument of a C-variadic function
 
 extern "C" {
@@ -52,34 +52,34 @@ struct X;
 
 impl X {
     fn i_f1(x: isize, ...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn i_f2(...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn i_f3(..., x: isize, ...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR `...` must be the last argument of a C-variadic function
     fn i_f4(..., x: isize, ...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR `...` must be the last argument of a C-variadic function
     const fn i_f5(x: isize, ...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| 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 only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn t_f2(x: isize, ...);
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn t_f3(...) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn t_f4(...);
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     fn t_f5(..., x: isize) {}
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR `...` must be the last argument of a C-variadic function
     fn t_f6(..., x: isize);
-    //~^ ERROR only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+    //~^ ERROR defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
     //~| ERROR `...` must be the last argument of a C-variadic function
 }
diff --git a/tests/ui/parser/variadic-ffi-semantic-restrictions.stderr b/tests/ui/parser/variadic-ffi-semantic-restrictions.stderr
index b740cef0200..5379045967a 100644
--- a/tests/ui/parser/variadic-ffi-semantic-restrictions.stderr
+++ b/tests/ui/parser/variadic-ffi-semantic-restrictions.stderr
@@ -1,22 +1,22 @@
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:6:19
    |
 LL | fn f1_1(x: isize, ...) {}
    |                   ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:9:9
    |
 LL | fn f1_2(...) {}
    |         ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:12:30
    |
 LL | extern "C" fn f2_1(x: isize, ...) {}
    |                              ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:15:20
    |
 LL | extern "C" fn f2_2(...) {}
@@ -28,19 +28,19 @@ error: `...` must be the last argument of a C-variadic function
 LL | extern "C" fn f2_3(..., x: isize) {}
    |                    ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:18:20
    |
 LL | extern "C" fn f2_3(..., x: isize) {}
    |                    ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:22:30
    |
 LL | extern "C" fn f3_1(x: isize, ...) {}
    |                              ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:25:20
    |
 LL | extern "C" fn f3_2(...) {}
@@ -52,7 +52,7 @@ error: `...` must be the last argument of a C-variadic function
 LL | extern "C" fn f3_3(..., x: isize) {}
    |                    ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:28:20
    |
 LL | extern "C" fn f3_3(..., x: isize) {}
@@ -70,7 +70,7 @@ error: functions cannot be both `const` and C-variadic
 LL | const extern "C" fn f4_2(x: isize, ...) {}
    | ^^^^^ `const` because of this      ^^^ C-variadic because of this
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:36:36
    |
 LL | const extern "C" fn f4_2(x: isize, ...) {}
@@ -91,7 +91,7 @@ LL | const extern "C" fn f4_3(..., x: isize, ...) {}
    | |                        C-variadic because of this
    | `const` because of this
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:41:26
    |
 LL | const extern "C" fn f4_3(..., x: isize, ...) {}
@@ -103,13 +103,13 @@ error: `...` must be the last argument of a C-variadic function
 LL |     fn e_f2(..., x: isize);
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:54:23
    |
 LL |     fn i_f1(x: isize, ...) {}
    |                       ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:56:13
    |
 LL |     fn i_f2(...) {}
@@ -121,7 +121,7 @@ error: `...` must be the last argument of a C-variadic function
 LL |     fn i_f3(..., x: isize, ...) {}
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:58:13
    |
 LL |     fn i_f3(..., x: isize, ...) {}
@@ -133,7 +133,7 @@ error: `...` must be the last argument of a C-variadic function
 LL |     fn i_f4(..., x: isize, ...) {}
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:61:13
    |
 LL |     fn i_f4(..., x: isize, ...) {}
@@ -147,31 +147,31 @@ LL |     const fn i_f5(x: isize, ...) {}
    |     |
    |     `const` because of this
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:64:29
    |
 LL |     const fn i_f5(x: isize, ...) {}
    |                             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:71:23
    |
 LL |     fn t_f1(x: isize, ...) {}
    |                       ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:73:23
    |
 LL |     fn t_f2(x: isize, ...);
    |                       ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:75:13
    |
 LL |     fn t_f3(...) {}
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:77:13
    |
 LL |     fn t_f4(...);
@@ -183,7 +183,7 @@ error: `...` must be the last argument of a C-variadic function
 LL |     fn t_f5(..., x: isize) {}
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:79:13
    |
 LL |     fn t_f5(..., x: isize) {}
@@ -195,7 +195,7 @@ error: `...` must be the last argument of a C-variadic function
 LL |     fn t_f6(..., x: isize);
    |             ^^^
 
-error: only foreign, `unsafe extern "C"`, or `unsafe extern "C-unwind"` functions may have a C-variadic arg
+error: defining functions with C-variadic arguments is only allowed for free functions with the "C" or "C-unwind" calling convention
   --> $DIR/variadic-ffi-semantic-restrictions.rs:82:13
    |
 LL |     fn t_f6(..., x: isize);