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 +271,29 @@ 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).
+    ///
+    /// This is used to replicate some of the behaviour of C array-to-pointer decay; however unlike
+    /// C any changes the caller makes to the passed value will not be reflected in the callee, so
+    /// the attribute is only useful for types where observing the value in the caller after the
+    /// function call isn't allowed (a.k.a. `va_list`).
+    ///
+    /// 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

@@ -88,14 +88,17 @@ 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_parsing/src/attributes/codegen_attrs.rs

@@ -676,3 +676,12 @@ impl<S: Stage> SingleAttributeParser<S> for SanitizeParser {
         Some(AttributeKind::Sanitize { on_set, off_set, span: cx.attr_span })
     }
 }
+
+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 ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Struct)]);
+    const CREATE: fn(Span) -> AttributeKind = AttributeKind::RustcPassIndirectlyInNonRusticAbis;
+}

compiler/rustc_attr_parsing/src/context.rs

@@ -20,8 +20,9 @@ use crate::attributes::allow_unstable::{
 use crate::attributes::body::CoroutineParser;
 use crate::attributes::codegen_attrs::{
     ColdParser, CoverageParser, ExportNameParser, ForceTargetFeatureParser, NakedParser,
-    NoMangleParser, ObjcClassParser, ObjcSelectorParser, OptimizeParser, SanitizeParser,
-    TargetFeatureParser, TrackCallerParser, UsedParser,
+    NoMangleParser, ObjcClassParser, ObjcSelectorParser, OptimizeParser,
+    RustcPassIndirectlyInNonRusticAbisParser, SanitizeParser, TargetFeatureParser,
+    TrackCallerParser, UsedParser,
 };
 use crate::attributes::confusables::ConfusablesParser;
 use crate::attributes::crate_level::{
@@ -241,6 +242,7 @@ attribute_parsers!(
         Single<WithoutArgs<PubTransparentParser>>,
         Single<WithoutArgs<RustcCoherenceIsCoreParser>>,
         Single<WithoutArgs<RustcMainParser>>,
+        Single<WithoutArgs<RustcPassIndirectlyInNonRusticAbisParser>>,
         Single<WithoutArgs<SpecializationTraitParser>>,
         Single<WithoutArgs<StdInternalSymbolParser>>,
         Single<WithoutArgs<TrackCallerParser>>,

compiler/rustc_feature/src/builtin_attrs.rs

@@ -657,6 +657,12 @@ pub static BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
         template!(Word, "https://doc.rust-lang.org/reference/attributes/codegen.html#the-naked-attribute"),
         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/src/attrs/data_structures.rs

@@ -679,6 +679,9 @@ pub enum AttributeKind {
     /// Represents `#[rustc_object_lifetime_default]`.
     RustcObjectLifetimeDefault,
 
+    /// Represents `#[rustc_pass_indirectly_in_non_rustic_abis]`
+    RustcPassIndirectlyInNonRusticAbis(Span),
+
     /// Represents `#[rustc_simd_monomorphize_lane_limit = "N"]`.
     RustcSimdMonomorphizeLaneLimit(Limit),
 

compiler/rustc_hir/src/attrs/encode_cross_crate.rs

@@ -91,6 +91,7 @@ impl AttributeKind {
             RustcLayoutScalarValidRangeStart(..) => Yes,
             RustcMain => No,
             RustcObjectLifetimeDefault => No,
+            RustcPassIndirectlyInNonRusticAbis(..) => No,
             RustcSimdMonomorphizeLaneLimit(..) => Yes, // Affects layout computation, which needs to work cross-crate
             Sanitize { .. } => No,
             ShouldPanic { .. } => No,

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;
@@ -1173,6 +1173,11 @@ 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

@@ -1520,6 +1520,14 @@ impl<'tcx> TyCtxt<'tcx> {
             flags.insert(ReprFlags::IS_LINEAR);
         }
 
+        // See `TyAndLayout::pass_indirectly_in_non_rustic_abis` for details.
+        if 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/src/check_attr.rs

@@ -284,6 +284,7 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
                     | AttributeKind::RustcCoherenceIsCore(..)
                     | AttributeKind::DebuggerVisualizer(..)
                     | AttributeKind::RustcMain
+                    | AttributeKind::RustcPassIndirectlyInNonRusticAbis(..)
                     | AttributeKind::PinV2(..),
                 ) => { /* do nothing  */ }
                 Attribute::Unparsed(attr_item) => {