Rework c_variadic

compiler/rustc_abi/src/layout/ty.rs

@@ -172,6 +172,8 @@ pub trait TyAbiInterface<'a, C>: Sized + std::fmt::Debug {
     fn is_tuple(this: TyAndLayout<'a, Self>) -> bool;
     fn is_unit(this: TyAndLayout<'a, Self>) -> bool;
     fn is_transparent(this: TyAndLayout<'a, Self>) -> bool;
+    /// See [`TyAndLayout::pass_indirectly_in_non_rustic_abis`] for details.
+    fn is_pass_indirectly_in_non_rustic_abis_flag_set(this: TyAndLayout<'a, Self>) -> bool;
 }
 
 impl<'a, Ty> TyAndLayout<'a, Ty> {
@@ -269,6 +271,30 @@ impl<'a, Ty> TyAndLayout<'a, Ty> {
         Ty::is_transparent(self)
     }
 
+    /// If this method returns `true`, then this type should always have a `PassMode` of
+    /// `Indirect { on_stack: false, .. }` when being used as the argument type of a function with a
+    /// non-Rustic ABI (this is true for structs annotated with the
+    /// `#[rustc_pass_indirectly_in_non_rustic_abis]` attribute). Currently this is only used by
+    /// `VaList`, so this only needs to be handled on architectures where `VaList` requires it:
+    /// specifically there is a `support_architectures` array in the
+    /// `check_pass_indirectly_in_non_rustic_abis` function in
+    /// `compiler/rustc_passes/src/check_attr.rs` which lists all the architectures this needs to be
+    /// handled on. See the comment near the top of `library/core/src/ffi/va_list.rs` for more
+    /// details.
+    ///
+    /// This function handles transparent types automatically.
+    pub fn pass_indirectly_in_non_rustic_abis<C>(mut self, cx: &C) -> bool
+    where
+        Ty: TyAbiInterface<'a, C> + Copy,
+    {
+        while self.is_transparent()
+            && let Some((_, field)) = self.non_1zst_field(cx)
+        {
+            self = field;
+        }
+        Ty::is_pass_indirectly_in_non_rustic_abis_flag_set(self)
+    }
+
     /// Finds the one field that is not a 1-ZST.
     /// Returns `None` if there are multiple non-1-ZST fields or only 1-ZST-fields.
     pub fn non_1zst_field<C>(&self, cx: &C) -> Option<(FieldIdx, Self)>

compiler/rustc_abi/src/lib.rs

@@ -86,14 +86,17 @@ bitflags! {
         const IS_C               = 1 << 0;
         const IS_SIMD            = 1 << 1;
         const IS_TRANSPARENT     = 1 << 2;
-        // Internal only for now. If true, don't reorder fields.
-        // On its own it does not prevent ABI optimizations.
+        /// Internal only for now. If true, don't reorder fields.
+        /// On its own it does not prevent ABI optimizations.
         const IS_LINEAR          = 1 << 3;
-        // If true, the type's crate has opted into layout randomization.
-        // Other flags can still inhibit reordering and thus randomization.
-        // The seed stored in `ReprOptions.field_shuffle_seed`.
+        /// If true, the type's crate has opted into layout randomization.
+        /// Other flags can still inhibit reordering and thus randomization.
+        /// The seed stored in `ReprOptions.field_shuffle_seed`.
         const RANDOMIZE_LAYOUT   = 1 << 4;
-        // Any of these flags being set prevent field reordering optimisation.
+        /// If true, the type is always passed indirectly by non-Rustic ABIs.
+        /// See [`TyAndLayout::pass_indirectly_in_non_rustic_abis`] for details.
+        const PASS_INDIRECTLY_IN_NON_RUSTIC_ABIS = 1 << 5;
+        /// Any of these flags being set prevent field reordering optimisation.
         const FIELD_ORDER_UNOPTIMIZABLE   = ReprFlags::IS_C.bits()
                                  | ReprFlags::IS_SIMD.bits()
                                  | ReprFlags::IS_LINEAR.bits();

compiler/rustc_attr_data_structures/src/attributes.rs

@@ -315,6 +315,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_object_lifetime_default]`.
     RustcObjectLifetimeDefault,
 
+    /// Represents `#[rustc_pass_indirectly_in_non_rustic_abis]`
+    RustcPassIndirectlyInNonRusticAbis(Span),
+
     /// Represents `#[rustc_skip_during_method_dispatch]`.
     SkipDuringMethodDispatch { array: bool, boxed_slice: bool, span: Span },
 

compiler/rustc_attr_data_structures/src/encode_cross_crate.rs

@@ -45,6 +45,7 @@ impl AttributeKind {
             RustcLayoutScalarValidRangeEnd(..) => Yes,
             RustcLayoutScalarValidRangeStart(..) => Yes,
             RustcObjectLifetimeDefault => No,
+            RustcPassIndirectlyInNonRusticAbis(..) => No,
             SkipDuringMethodDispatch { .. } => No,
             Stability { .. } => Yes,
             TargetFeature(..) => No,

compiler/rustc_attr_parsing/src/attributes/codegen_attrs.rs

@@ -330,3 +330,10 @@ impl<S: Stage> CombineAttributeParser<S> for TargetFeatureParser {
         features
     }
 }
+
+pub(crate) struct RustcPassIndirectlyInNonRusticAbisParser;
+impl<S: Stage> NoArgsAttributeParser<S> for RustcPassIndirectlyInNonRusticAbisParser {
+    const PATH: &[Symbol] = &[sym::rustc_pass_indirectly_in_non_rustic_abis];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::RustcPassIndirectlyInNonRusticAbis;
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -16,8 +16,8 @@ use rustc_span::{DUMMY_SP, ErrorGuaranteed, Span, Symbol, sym};
 
 use crate::attributes::allow_unstable::{AllowConstFnUnstableParser, AllowInternalUnstableParser};
 use crate::attributes::codegen_attrs::{
-    ColdParser, ExportNameParser, NakedParser, NoMangleParser, OptimizeParser, TargetFeatureParser,
-    TrackCallerParser, UsedParser,
+    ColdParser, ExportNameParser, NakedParser, NoMangleParser, OptimizeParser,
+    RustcPassIndirectlyInNonRusticAbisParser, TargetFeatureParser, TrackCallerParser, UsedParser,
 };
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::deprecation::DeprecationParser;
@@ -150,6 +150,7 @@ attribute_parsers!(
         Single<WithoutArgs<NonExhaustiveParser>>,
         Single<WithoutArgs<PassByValueParser>>,
         Single<WithoutArgs<PubTransparentParser>>,
+        Single<WithoutArgs<RustcPassIndirectlyInNonRusticAbisParser>>,
         Single<WithoutArgs<TrackCallerParser>>,
         // tidy-alphabetical-end
     ];

compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

@@ -1450,7 +1450,7 @@ fn codegen_regular_intrinsic_call<'tcx>(
         }
 
         // FIXME implement variadics in cranelift
-        sym::va_copy | sym::va_arg | sym::va_end => {
+        sym::va_copy | sym::va_arg => {
             fx.tcx.dcx().span_fatal(
                 source_info.span,
                 "Defining variadic functions is not yet supported by Cranelift",

compiler/rustc_codegen_ssa/src/mir/intrinsic.rs

@@ -131,8 +131,6 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                 return Ok(());
             }
 
-            sym::va_start => bx.va_start(args[0].immediate()),
-            sym::va_end => bx.va_end(args[0].immediate()),
             sym::size_of_val => {
                 let tp_ty = fn_args.type_at(0);
                 let (_, meta) = args[0].val.pointer_parts();

compiler/rustc_codegen_ssa/src/traits/intrinsic.rs

@@ -30,10 +30,10 @@ pub trait IntrinsicCallBuilderMethods<'tcx>: BackendTypes {
         vtable_byte_offset: u64,
         typeid: Self::Metadata,
     ) -> Self::Value;
-    /// Trait method used to inject `va_start` on the "spoofed" `VaListImpl` in
+    /// Trait method used to inject `va_start` on the "spoofed" `VaList` in
     /// Rust defined C-variadic functions.
     fn va_start(&mut self, val: Self::Value) -> Self::Value;
-    /// Trait method used to inject `va_end` on the "spoofed" `VaListImpl` before
+    /// Trait method used to inject `va_end` on the "spoofed" `VaList` before
     /// Rust defined C-variadic functions return.
     fn va_end(&mut self, val: Self::Value) -> Self::Value;
 }

compiler/rustc_feature/src/builtin_attrs.rs

@@ -497,6 +497,12 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ungated!(used, Normal, template!(Word, List: "compiler|linker"), WarnFollowing, EncodeCrossCrate::No),
     ungated!(link_ordinal, Normal, template!(List: "ordinal"), ErrorPreceding, EncodeCrossCrate::Yes),
     ungated!(unsafe naked, Normal, template!(Word), WarnFollowing, EncodeCrossCrate::No),
+    // See `TyAndLayout::pass_indirectly_in_non_rustic_abis` for details.
+    rustc_attr!(
+        rustc_pass_indirectly_in_non_rustic_abis, Normal, template!(Word), ErrorFollowing,
+        EncodeCrossCrate::No,
+        "types marked with `#[rustc_pass_indirectly_in_non_rustic_abis]` are always passed indirectly by non-Rustic abis."
+    ),
 
     // Limits:
     ungated!(

compiler/rustc_hir_analysis/src/check/intrinsic.rs

@@ -506,10 +506,6 @@ pub(crate) fn check_intrinsic_type(
             )
         }
 
-        sym::va_start | sym::va_end => {
-            (0, 0, vec![mk_va_list_ty(hir::Mutability::Mut).0], tcx.types.unit)
-        }
-
         sym::va_copy => {
             let (va_list_ref_ty, va_list_ty) = mk_va_list_ty(hir::Mutability::Not);
             let va_list_ptr_ty = Ty::new_mut_ptr(tcx, va_list_ty);

compiler/rustc_middle/src/ty/layout.rs

@@ -3,7 +3,7 @@ use std::{cmp, fmt};
 
 use rustc_abi::{
     AddressSpace, Align, ExternAbi, FieldIdx, FieldsShape, HasDataLayout, LayoutData, PointeeInfo,
-    PointerKind, Primitive, ReprOptions, Scalar, Size, TagEncoding, TargetDataLayout,
+    PointerKind, Primitive, ReprFlags, ReprOptions, Scalar, Size, TagEncoding, TargetDataLayout,
     TyAbiInterface, VariantIdx, Variants,
 };
 use rustc_error_messages::DiagMessage;
@@ -1138,6 +1138,11 @@ where
     fn is_transparent(this: TyAndLayout<'tcx>) -> bool {
         matches!(this.ty.kind(), ty::Adt(def, _) if def.repr().transparent())
     }
+
+    /// See [`TyAndLayout::pass_indirectly_in_non_rustic_abis`] for details.
+    fn is_pass_indirectly_in_non_rustic_abis_flag_set(this: TyAndLayout<'tcx>) -> bool {
+        matches!(this.ty.kind(), ty::Adt(def, _) if def.repr().flags.contains(ReprFlags::PASS_INDIRECTLY_IN_NON_RUSTIC_ABIS))
+    }
 }
 
 /// Calculates whether a function's ABI can unwind or not.

compiler/rustc_middle/src/ty/mod.rs

@@ -1589,6 +1589,14 @@ impl<'tcx> TyCtxt<'tcx> {
             flags.insert(ReprFlags::IS_LINEAR);
         }
 
+        // See `TyAndLayout::pass_indirectly_in_non_rustic_abis` for details.
+        if attr::find_attr!(
+            self.get_all_attrs(did),
+            AttributeKind::RustcPassIndirectlyInNonRusticAbis(..)
+        ) {
+            flags.insert(ReprFlags::PASS_INDIRECTLY_IN_NON_RUSTIC_ABIS);
+        }
+
         ReprOptions { int: size, align: max_align, pack: min_pack, flags, field_shuffle_seed }
     }
 

compiler/rustc_passes/messages.ftl

@@ -579,6 +579,10 @@ passes_pass_by_value =
     `pass_by_value` attribute should be applied to a struct, enum or type alias
     .label = is not a struct, enum or type alias
 
+passes_pass_indirectly_not_a_struct =
+    `#[rustc_pass_indirectly_in_non_rustic_abis]` can only be applied to `struct`s
+    .label = is not a `struct`
+
 passes_proc_macro_bad_sig = {$kind} has incorrect signature
 
 passes_remove_fields =

compiler/rustc_passes/src/check_attr.rs

@@ -230,6 +230,9 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                 &Attribute::Parsed(AttributeKind::PassByValue(attr_span)) => {
                     self.check_pass_by_value(attr_span, span, target)
                 }
+                Attribute::Parsed(AttributeKind::RustcPassIndirectlyInNonRusticAbis(attr_span)) => {
+                    self.check_pass_indirectly_in_non_rustic_abis(*attr_span, span, target);
+                }
                 Attribute::Unparsed(attr_item) => {
                     style = Some(attr_item.style);
                     match attr.path().as_slice() {
@@ -2106,6 +2109,29 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
         }
     }
 
+    fn check_pass_indirectly_in_non_rustic_abis(
+        &self,
+        attr_span: Span,
+        span: Span,
+        target: Target,
+    ) {
+        if target != Target::Struct {
+            self.dcx().emit_err(errors::PassIndirectlyNotAStruct { attr_span, span });
+        }
+        // Currently this attribute is only supported on architectures where it is required for the
+        // `VaList` type. See the comment near the top of `library/core/src/ffi/va_list.rs` for
+        // details.
+        let supported_architectures = ["aarch64", "powerpc", "s390x", "x86_64", "xtensa"];
+        let arch = &*self.tcx.sess.target.arch;
+        if !supported_architectures.contains(&arch) {
+            span_bug!(
+                attr_span,
+                "support for `#[rustc_pass_indirectly_in_non_rustic_abis]` on `{arch}` has not yet been implemented; \
+                see the comment near the top of `library/core/src/ffi/va_list.rs` for details"
+            );
+        }
+    }
+
     fn check_align_value(&self, align: Align, span: Span) {
         if align.bytes() > 2_u64.pow(29) {
             // for values greater than 2^29, a different error will be emitted, make sure that happens

compiler/rustc_passes/src/errors.rs

@@ -425,6 +425,15 @@ pub(crate) struct PassByValue {
     pub span: Span,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_pass_indirectly_not_a_struct)]
+pub(crate) struct PassIndirectlyNotAStruct {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_allow_incoherent_impl)]
 pub(crate) struct AllowIncoherentImpl {

compiler/rustc_span/src/symbol.rs

@@ -1882,6 +1882,7 @@ symbols! {
         rustc_partition_codegened,
         rustc_partition_reused,
         rustc_pass_by_value,
+        rustc_pass_indirectly_in_non_rustic_abis,
         rustc_peek,
         rustc_peek_liveness,
         rustc_peek_maybe_init,
@@ -2309,9 +2310,7 @@ symbols! {
         v8plus,
         va_arg,
         va_copy,
-        va_end,
         va_list,
-        va_start,
         val,
         validity,
         values,

compiler/rustc_target/src/callconv/aarch64.rs

@@ -114,6 +114,10 @@ where
         // Not touching this...
         return;
     }
+    if arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
+        arg.make_indirect();
+        return;
+    }
     if !arg.layout.is_aggregate() {
         if kind == AbiKind::DarwinPCS {
             // On Darwin, when passing an i8/i16, it must be sign-extended to 32 bits,

compiler/rustc_target/src/callconv/mod.rs

@@ -703,6 +703,16 @@ impl<'a, Ty> FnAbi<'a, Ty> {
             "bpf" => bpf::compute_abi_info(self),
             arch => panic!("no lowering implemented for {arch}"),
         }
+        if cfg!(debug_assertions) {
+            // When debug assertions are enabled, double check that any argument types annotated
+            // with the `#[rustc_pass_indirectly_in_non_rustic_abis]` attribute are passed
+            // indirectly.
+            for arg in &self.args {
+                if arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
+                    assert!(matches!(arg.mode, PassMode::Indirect { on_stack: false, .. }));
+                }
+            }
+        }
     }
 
     pub fn adjust_for_rust_abi<C>(&mut self, cx: &C)

compiler/rustc_target/src/callconv/powerpc.rs

@@ -1,3 +1,5 @@
+use rustc_abi::TyAbiInterface;
+
 use crate::callconv::{ArgAbi, FnAbi};
 use crate::spec::HasTargetSpec;
 
@@ -9,7 +11,10 @@ fn classify_ret<Ty>(ret: &mut ArgAbi<'_, Ty>) {
     }
 }
 
-fn classify_arg<Ty>(cx: &impl HasTargetSpec, arg: &mut ArgAbi<'_, Ty>) {
+fn classify_arg<'a, Ty, C: HasTargetSpec>(cx: &C, arg: &mut ArgAbi<'a, Ty>)
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
     if arg.is_ignore() {
         // powerpc-unknown-linux-{gnu,musl,uclibc} doesn't ignore ZSTs.
         if cx.target_spec().os == "linux"
@@ -20,14 +25,17 @@ fn classify_arg<Ty>(cx: &impl HasTargetSpec, arg: &mut ArgAbi<'_, Ty>) {
         }
         return;
     }
-    if arg.layout.is_aggregate() {
+    if arg.layout.pass_indirectly_in_non_rustic_abis(cx) || arg.layout.is_aggregate() {
         arg.make_indirect();
     } else {
         arg.extend_integer_width_to(32);
     }
 }
 
-pub(crate) fn compute_abi_info<Ty>(cx: &impl HasTargetSpec, fn_abi: &mut FnAbi<'_, Ty>) {
+pub(crate) fn compute_abi_info<'a, Ty, C: HasTargetSpec>(cx: &C, fn_abi: &mut FnAbi<'a, Ty>)
+where
+    Ty: TyAbiInterface<'a, C> + Copy,
+{
     if !fn_abi.ret.is_ignore() {
         classify_ret(&mut fn_abi.ret);
     }

compiler/rustc_target/src/callconv/s390x.rs

@@ -37,6 +37,10 @@ where
         }
         return;
     }
+    if arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
+        arg.make_indirect();
+        return;
+    }
 
     let size = arg.layout.size;
     if size.bits() <= 128 {

compiler/rustc_target/src/callconv/x86_64.rs

@@ -186,6 +186,11 @@ where
             // Not touching this...
             return;
         }
+        if is_arg && arg.layout.pass_indirectly_in_non_rustic_abis(cx) {
+            int_regs = int_regs.saturating_sub(1);
+            arg.make_indirect();
+            return;
+        }
         let mut cls_or_mem = classify_arg(cx, arg);
 
         if is_arg {