Add #[rustc_pass_indirectly_in_non_rustic_abis]

Author: beetrees

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

@@ -305,6 +305,9 @@ 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

@@ -43,6 +43,7 @@ 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;
@@ -146,6 +146,7 @@ attribute_parsers!(
         Single<WithoutArgs<NoMangleParser>>,
         Single<WithoutArgs<PassByValueParser>>,
         Single<WithoutArgs<PubTransparentParser>>,
+        Single<WithoutArgs<RustcPassIndirectlyInNonRusticAbisParser>>,
         Single<WithoutArgs<TrackCallerParser>>,
         // tidy-alphabetical-end
     ];

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_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 +579,14 @@ 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_pass_indirectly_unsupported_arch =
+    support for `#[rustc_pass_indirectly_in_non_rustic_abis]` on `{$arch}` has not yet been implemented
+    .help = see the comment near the top of `library/core/src/ffi/va_list.rs` for details
+
 passes_proc_macro_bad_sig = {$kind} has incorrect signature
 
 passes_remove_fields =