stabilize c-style varargs for system, sysv64, win64, efiapi, aapcs

compiler/rustc_abi/src/extern_abi.rs

@@ -235,22 +235,33 @@ impl ExternAbi {
         matches!(self, Rust | RustCall | RustCold)
     }
 
-    pub fn supports_varargs(self) -> bool {
+    /// Returns whether the ABI supports C variadics.
+    ///
+    /// Note that this is insta-stable if the ABI is stable! If you add a new unstable ABI that
+    /// supports variadics, it is fine to just add it here. But if you want to equip an existing
+    /// *stable* ABI with support for variadics, you cannot just add it here. Instead, add a feature
+    /// gate check in `require_supported_abi_if_c_variadic`, and only move it here once variadic
+    /// support os stable.
+    pub fn supports_c_variadic(self) -> bool {
         // * 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,
+            | Self::EfiApi
+            | Self::System { .. } => true,
             _ => false,
         }
     }

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.
@@ -212,6 +215,8 @@ declare_features! (
     (accepted, extern_crate_self, "1.34.0", Some(56409)),
     /// Allows access to crate names passed via `--extern` through prelude.
     (accepted, extern_prelude, "1.30.0", Some(44660)),
+    /// Allows using `system` as a calling convention with varargs.
+    (accepted, extern_system_varargs, "CURRENT_RUSTC_VERSION", Some(136946)),
     /// Allows using F16C intrinsics from `core::arch::{x86, x86_64}`.
     (accepted, f16c_target_feature, "1.68.0", Some(44839)),
     /// Allows field shorthands (`x` meaning `x: x`) in struct literal expressions.

compiler/rustc_feature/src/unstable.rs

@@ -491,11 +491,6 @@ 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.
     (unstable, extern_types, "1.23.0", Some(43467)),
     /// Allow using 128-bit (quad precision) floating point numbers.

compiler/rustc_hir_analysis/src/check/check.rs

@@ -977,7 +977,7 @@ 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);

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) {

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)]
@@ -2403,7 +2403,7 @@ 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,

compiler/rustc_hir_analysis/src/lib.rs

@@ -98,8 +98,6 @@ use rustc_middle::middle;
 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 +106,23 @@ 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 {
+    if abi.supports_c_variadic() {
+        // Variadics are stable.
         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)
-        }
-        abi if abi.supports_varargs() => {
-            feature_err(&tcx.sess, sym::extended_varargs_abi_support, span, unstable_explain)
-        }
-        _ => tcx.dcx().create_err(errors::VariadicFunctionCompatibleConvention {
+    // Everything else is unstable.
+    tcx.dcx()
+        .create_err(errors::VariadicFunctionCompatibleConvention {
             span,
             convention: &format!("{abi}"),
-        }),
-    }
-    .emit();
+        })
+        .emit();
 }
 
 /// Adds query implementations to the [Providers] vtable, see [`rustc_middle::query`]

library/std/src/lib.rs

@@ -289,7 +289,6 @@
 #![feature(doc_masked)]
 #![feature(doc_notable_trait)]
 #![feature(dropck_eyepatch)]
-#![feature(extended_varargs_abi_support)]
 #![feature(f16)]
 #![feature(f128)]
 #![feature(ffi_const)]

tests/codegen/cffi/c-variadic-ffi.rs

@@ -0,0 +1,85 @@
+//! Test calling variadic functions with various ABIs.
+//@ add-core-stubs
+//@ compile-flags: -Z merge-functions=disabled
+//@ revisions: x86_32 x86_32_win x86_64 aarch64 arm32
+//@[x86_64] compile-flags: --target x86_64-unknown-linux-gnu
+//@[x86_64] needs-llvm-components: x86
+//@[x86_32_win] compile-flags: --target i686-pc-windows-msvc
+//@[x86_32_win] needs-llvm-components: x86
+//@[x86_32] compile-flags: --target i686-unknown-linux-gnu
+//@[x86_32] needs-llvm-components: x86
+//@[aarch64] compile-flags: --target aarch64-unknown-linux-gnu
+//@[aarch64] needs-llvm-components: aarch64
+//@[arm32] compile-flags: --target armv7-unknown-linux-gnueabihf
+//@[arm32] needs-llvm-components: arm
+#![crate_type = "lib"]
+#![feature(no_core)]
+#![no_core]
+
+extern crate minicore;
+
+// CHECK-LABEL: @c
+#[unsafe(no_mangle)]
+fn c(f: extern "C" fn(i32, ...)) {
+    // CHECK: call void (i32, ...)
+    f(22, 44);
+}
+
+// CHECK-LABEL: @system
+#[unsafe(no_mangle)]
+fn system(f: extern "system" fn(i32, ...)) {
+    // Crucially, this is *always* the C calling convention, even on Windows.
+    // CHECK: call void (i32, ...)
+    f(22, 44);
+}
+
+// x86_32-LABEL: @cdecl
+#[unsafe(no_mangle)]
+#[cfg(target_arch = "x86")]
+fn cdecl(f: extern "cdecl" fn(i32, ...)) {
+    // x86_32: call void (i32, ...)
+    f(22, 44);
+}
+
+// x86_64-LABEL: @sysv
+#[unsafe(no_mangle)]
+#[cfg(target_arch = "x86_64")]
+fn sysv(f: extern "sysv64" fn(i32, ...)) {
+    // x86_64: call x86_64_sysvcc void (i32, ...)
+    f(22, 44);
+}
+
+// x86_64-LABEL: @win
+#[unsafe(no_mangle)]
+#[cfg(target_arch = "x86_64")]
+fn win(f: extern "win64" fn(i32, ...)) {
+    // x86_64: call win64cc void (i32, ...)
+    f(22, 44);
+}
+
+// CHECK-LABEL: @efiapi
+#[unsafe(no_mangle)]
+#[cfg(any(
+    target_arch = "arm",
+    target_arch = "aarch64",
+    target_arch = "riscv32",
+    target_arch = "riscv64",
+    target_arch = "x86",
+    target_arch = "x86_64"
+))]
+fn efiapi(f: extern "efiapi" fn(i32, ...)) {
+    // x86_32: call void (i32, ...)
+    // x86_32_win: call void (i32, ...)
+    // x86_64: call win64cc void (i32, ...)
+    // aarch64: call void (i32, ...)
+    // arm32: call arm_aapcscc void (i32, ...)
+    f(22, 44);
+}
+
+// arm32-LABEL: @aapcs
+#[unsafe(no_mangle)]
+#[cfg(target_arch = "arm")]
+fn aapcs(f: extern "aapcs" fn(i32, ...)) {
+    // arm32: call arm_aapcscc void (i32, ...)
+    f(22, 44);
+}

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
 

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, ...)) {
    |                    ^^^^^^^

tests/ui/c-variadic/variadic-ffi-1.rs

@@ -12,6 +12,11 @@ extern "stdcall" {
     //~^ ERROR: C-variadic functions with the "stdcall" calling convention are not supported
 }
 
+fn baz(f: extern "Rust" fn(usize, ...)) {
+    //~^ ERROR: C-variadic functions with the "Rust" calling convention are not supported
+    f(22, 44);
+}
+
 extern "C" {
     fn foo(f: isize, x: u8, ...);
 }