Implement RFC 3525.

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -1,5 +1,6 @@
 use rustc_ast::{ast, attr, MetaItemKind, NestedMetaItem};
 use rustc_attr::{list_contains_name, InlineAttr, InstructionSetAttr, OptimizeAttr};
+use rustc_data_structures::fx::FxHashSet;
 use rustc_errors::codes::*;
 use rustc_errors::{struct_span_code_err, DiagMessage, SubdiagMessage};
 use rustc_hir as hir;
@@ -16,8 +17,10 @@ use rustc_middle::ty::{self as ty, TyCtxt};
 use rustc_session::lint;
 use rustc_session::parse::feature_err;
 use rustc_span::symbol::Ident;
-use rustc_span::{sym, Span};
+use rustc_span::{sym, Span, Symbol};
+use rustc_target::abi::VariantIdx;
 use rustc_target::spec::{abi, SanitizerSet};
+use rustc_type_ir::inherent::*;
 
 use crate::errors;
 use crate::target_features::{check_target_feature_trait_unsafe, from_target_feature};
@@ -78,23 +81,26 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
     let mut link_ordinal_span = None;
     let mut no_sanitize_span = None;
 
+    let fn_sig_outer = || {
+        use DefKind::*;
+
+        let def_kind = tcx.def_kind(did);
+        if let Fn | AssocFn | Variant | Ctor(..) = def_kind { Some(tcx.fn_sig(did)) } else { None }
+    };
+
     for attr in attrs.iter() {
         // In some cases, attribute are only valid on functions, but it's the `check_attr`
         // pass that check that they aren't used anywhere else, rather this module.
         // In these cases, we bail from performing further checks that are only meaningful for
         // functions (such as calling `fn_sig`, which ICEs if given a non-function). We also
         // report a delayed bug, just in case `check_attr` isn't doing its job.
         let fn_sig = || {
-            use DefKind::*;
-
-            let def_kind = tcx.def_kind(did);
-            if let Fn | AssocFn | Variant | Ctor(..) = def_kind {
-                Some(tcx.fn_sig(did))
-            } else {
+            let sig = fn_sig_outer();
+            if sig.is_none() {
                 tcx.dcx()
                     .span_delayed_bug(attr.span, "this attribute can only be applied to functions");
-                None
             }
+            sig
         };
 
         let Some(Ident { name, .. }) = attr.ident() else {
@@ -613,6 +619,57 @@ fn codegen_fn_attrs(tcx: TyCtxt<'_>, did: LocalDefId) -> CodegenFnAttrs {
         }
     }
 
+    if tcx.features().struct_target_features
+        && let Some(sig) = fn_sig_outer()
+    {
+        // Collect target features from types reachable from arguments.
+        // We define a type as "reachable" if:
+        //  - it is a function argument
+        //  - it is a field of a reachable struct
+        //  - there is a reachable reference to it
+        // FIXME: we may want to cache the result of this computation.
+        let mut visited_types = FxHashSet::default();
+        let mut reachable_types: Vec<_> = sig.skip_binder().inputs().skip_binder().to_owned();
+        let mut additional_tf = vec![];
+
+        while let Some(ty) = reachable_types.pop() {
+            if visited_types.contains(&ty) {
+                continue;
+            }
+            visited_types.insert(ty);
+            if ty.is_ref() {
+                reachable_types.push(ty.builtin_deref(false).unwrap());
+            } else if matches!(ty.kind(), ty::Tuple(_)) {
+                reachable_types.extend(ty.tuple_fields().iter());
+            } else if let ty::Adt(adt_def, args) = ty.kind() {
+                additional_tf.extend_from_slice(tcx.struct_target_features(adt_def.did()));
+                if adt_def.is_struct() {
+                    reachable_types.extend(
+                        adt_def
+                            .variant(VariantIdx::from_usize(0))
+                            .fields
+                            .iter()
+                            .map(|field| field.ty(tcx, args)),
+                    );
+                }
+            }
+        }
+
+        if !additional_tf.is_empty() && !sig.skip_binder().abi().is_rust() {
+            tcx.dcx().span_err(
+                tcx.hir().span(tcx.local_def_id_to_hir_id(did)),
+                "cannot use a struct with target features in a function with non-Rust ABI",
+            );
+        }
+        if !additional_tf.is_empty() && codegen_fn_attrs.inline == InlineAttr::Always {
+            tcx.dcx().span_err(
+                tcx.hir().span(tcx.local_def_id_to_hir_id(did)),
+                "cannot use a struct with target features in a #[inline(always)] function",
+            );
+        }
+        codegen_fn_attrs.target_features.extend_from_slice(&additional_tf);
+    }
+
     // If a function uses #[target_feature] it can't be inlined into general
     // purpose functions as they wouldn't have the right target features
     // enabled. For that reason we also forbid #[inline(always)] as it can't be
@@ -758,6 +815,20 @@ fn check_link_name_xor_ordinal(
     }
 }
 
+fn struct_target_features(tcx: TyCtxt<'_>, def_id: DefId) -> Vec<Symbol> {
+    let mut features = vec![];
+    let supported_features = tcx.supported_target_features(LOCAL_CRATE);
+    for attr in tcx.get_attrs(def_id, sym::target_feature) {
+        from_target_feature(tcx, attr, supported_features, &mut features);
+    }
+    features
+}
+
 pub fn provide(providers: &mut Providers) {
-    *providers = Providers { codegen_fn_attrs, should_inherit_track_caller, ..*providers };
+    *providers = Providers {
+        codegen_fn_attrs,
+        should_inherit_track_caller,
+        struct_target_features,
+        ..*providers
+    };
 }

compiler/rustc_feature/src/unstable.rs

@@ -605,6 +605,8 @@ declare_features! (
     (unstable, strict_provenance, "1.61.0", Some(95228)),
     /// Allows string patterns to dereference values to match them.
     (unstable, string_deref_patterns, "1.67.0", Some(87121)),
+    /// Allows structs to carry target_feature information.
+    (unstable, struct_target_features, "CURRENT_RUSTC_VERSION", Some(871212)) /*FIXME*/,
     /// Allows the use of `#[target_feature]` on safe functions.
     (unstable, target_feature_11, "1.45.0", Some(69098)),
     /// Allows using `#[thread_local]` on `static` items.

compiler/rustc_middle/src/query/mod.rs

@@ -1246,6 +1246,11 @@ rustc_queries! {
         feedable
     }
 
+    query struct_target_features(def_id: DefId) -> &'tcx Vec<Symbol> {
+        arena_cache
+        desc { |tcx| "computing target features for struct `{}`", tcx.def_path_str(def_id) }
+    }
+
     query asm_target_features(def_id: DefId) -> &'tcx FxIndexSet<Symbol> {
         desc { |tcx| "computing target features for inline asm of `{}`", tcx.def_path_str(def_id) }
     }

compiler/rustc_mir_build/messages.ftl

@@ -125,6 +125,17 @@ mir_build_initializing_type_with_requires_unsafe_unsafe_op_in_unsafe_fn_allowed
     .note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
     .label = initializing type with `rustc_layout_scalar_valid_range` attr
 
+mir_build_initializing_type_with_target_feature_requires_unsafe =
+    initializing type with `target_feature` attr is unsafe and requires unsafe block
+    .note = this struct can only be constructed if the corresponding `target_feature`s are available
+    .label = initializing type with `target_feature` attr
+
+mir_build_initializing_type_with_target_feature_requires_unsafe_unsafe_op_in_unsafe_fn_allowed =
+    initializing type with `target_feature` attr is unsafe and requires unsafe function or block
+    .note = this struct can only be constructed if the corresponding `target_feature`s are available
+    .label = initializing type with `target_feature` attr
+
+
 mir_build_inline_assembly_requires_unsafe =
     use of inline assembly is unsafe and requires unsafe block
     .note = inline assembly is entirely unchecked and can cause undefined behavior
@@ -384,6 +395,11 @@ mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_requires_unsafe =
     .note = initializing a layout restricted type's field with a value outside the valid range is undefined behavior
     .label = initializing type with `rustc_layout_scalar_valid_range` attr
 
+mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_target_feature_requires_unsafe =
+    initializing type with `target_feature` attr is unsafe and requires unsafe block
+    .note = this struct can only be constructed if the corresponding `target_feature`s are available
+    .label = initializing type with `target_feature` attr
+
 mir_build_unsafe_op_in_unsafe_fn_inline_assembly_requires_unsafe =
     use of inline assembly is unsafe and requires unsafe block
     .note = inline assembly is entirely unchecked and can cause undefined behavior

compiler/rustc_mir_build/src/check_unsafety.rs

@@ -510,10 +510,16 @@ impl<'a, 'tcx> Visitor<'a, 'tcx> for UnsafetyVisitor<'a, 'tcx> {
                 user_ty: _,
                 fields: _,
                 base: _,
-            }) => match self.tcx.layout_scalar_valid_range(adt_def.did()) {
-                (Bound::Unbounded, Bound::Unbounded) => {}
-                _ => self.requires_unsafe(expr.span, InitializingTypeWith),
-            },
+            }) => {
+                match self.tcx.layout_scalar_valid_range(adt_def.did()) {
+                    (Bound::Unbounded, Bound::Unbounded) => {}
+                    _ => self.requires_unsafe(expr.span, InitializingTypeWith),
+                }
+                if !self.tcx.struct_target_features(adt_def.did()).is_empty() {
+                    self.requires_unsafe(expr.span, ConstructingTargetFeaturesType)
+                }
+            }
+
             ExprKind::Closure(box ClosureExpr {
                 closure_id,
                 args: _,
@@ -615,6 +621,7 @@ enum UnsafeOpKind {
     CallToUnsafeFunction(Option<DefId>),
     UseOfInlineAssembly,
     InitializingTypeWith,
+    ConstructingTargetFeaturesType,
     UseOfMutableStatic,
     UseOfExternStatic,
     DerefOfRawPointer,
@@ -696,6 +703,15 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 },
             ),
+            ConstructingTargetFeaturesType => tcx.emit_node_span_lint(
+                UNSAFE_OP_IN_UNSAFE_FN,
+                hir_id,
+                span,
+                UnsafeOpInUnsafeFnInitializingTypeWithTargetFeatureRequiresUnsafe {
+                    span,
+                    unsafe_not_inherited_note,
+                },
+            ),
             UseOfMutableStatic => tcx.emit_node_span_lint(
                 UNSAFE_OP_IN_UNSAFE_FN,
                 hir_id,
@@ -853,6 +869,20 @@ impl UnsafeOpKind {
                     unsafe_not_inherited_note,
                 });
             }
+            ConstructingTargetFeaturesType if unsafe_op_in_unsafe_fn_allowed => {
+                dcx.emit_err(
+                    InitializingTypeWithTargetFeatureRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+                        span,
+                        unsafe_not_inherited_note,
+                    },
+                );
+            }
+            ConstructingTargetFeaturesType => {
+                dcx.emit_err(InitializingTypeWithTargetFeatureRequiresUnsafe {
+                    span,
+                    unsafe_not_inherited_note,
+                });
+            }
             UseOfMutableStatic if unsafe_op_in_unsafe_fn_allowed => {
                 dcx.emit_err(UseOfMutableStaticRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
                     span,

compiler/rustc_mir_build/src/errors.rs

@@ -87,6 +87,16 @@ pub(crate) struct UnsafeOpInUnsafeFnInitializingTypeWithRequiresUnsafe {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
 }
 
+#[derive(LintDiagnostic)]
+#[diag(mir_build_unsafe_op_in_unsafe_fn_initializing_type_with_target_feature_requires_unsafe, code = E0133)]
+#[note]
+pub(crate) struct UnsafeOpInUnsafeFnInitializingTypeWithTargetFeatureRequiresUnsafe {
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedLintNote>,
+}
+
 #[derive(LintDiagnostic)]
 #[diag(mir_build_unsafe_op_in_unsafe_fn_mutable_static_requires_unsafe, code = E0133)]
 #[note]
@@ -251,6 +261,17 @@ pub(crate) struct InitializingTypeWithRequiresUnsafe {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
 }
 
+#[derive(Diagnostic)]
+#[diag(mir_build_initializing_type_with_target_feature_requires_unsafe, code = E0133)]
+#[note]
+pub(crate) struct InitializingTypeWithTargetFeatureRequiresUnsafe {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
 #[derive(Diagnostic)]
 #[diag(
     mir_build_initializing_type_with_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
@@ -265,6 +286,20 @@ pub(crate) struct InitializingTypeWithRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
     pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
 }
 
+#[derive(Diagnostic)]
+#[diag(
+    mir_build_initializing_type_with_target_feature_requires_unsafe_unsafe_op_in_unsafe_fn_allowed,
+    code = E0133
+)]
+#[note]
+pub(crate) struct InitializingTypeWithTargetFeatureRequiresUnsafeUnsafeOpInUnsafeFnAllowed {
+    #[primary_span]
+    #[label]
+    pub(crate) span: Span,
+    #[subdiagnostic]
+    pub(crate) unsafe_not_inherited_note: Option<UnsafeNotInheritedNote>,
+}
+
 #[derive(Diagnostic)]
 #[diag(mir_build_mutable_static_requires_unsafe, code = E0133)]
 #[note]

compiler/rustc_passes/messages.ftl

@@ -659,6 +659,10 @@ passes_should_be_applied_to_fn =
         *[false] not a function definition
     }
 
+passes_should_be_applied_to_fn_or_unit_struct =
+    attribute should be applied to a function definition or unit struct
+    .label = not a function definition or a unit struct
+
 passes_should_be_applied_to_static =
     attribute should be applied to a static
     .label = not a static

compiler/rustc_passes/src/check_attr.rs

@@ -737,12 +737,35 @@ impl<'tcx> CheckAttrVisitor<'tcx> {
             Target::Field | Target::Arm | Target::MacroDef => {
                 self.inline_attr_str_error_with_macro_def(hir_id, attr, "target_feature");
             }
+            Target::Struct if self.tcx.features().struct_target_features => {
+                let ty = self.tcx.hir_node(hir_id).expect_item();
+                match ty.kind {
+                    ItemKind::Struct(data, _) => {
+                        if data.fields().len() != 0 {
+                            self.dcx().emit_err(errors::AttrShouldBeAppliedToFnOrUnitStruct {
+                                attr_span: attr.span,
+                                defn_span: span,
+                            });
+                        }
+                    }
+                    _ => {
+                        panic!("Target::Struct for a non-struct");
+                    }
+                }
+            }
             _ => {
-                self.dcx().emit_err(errors::AttrShouldBeAppliedToFn {
-                    attr_span: attr.span,
-                    defn_span: span,
-                    on_crate: hir_id == CRATE_HIR_ID,
-                });
+                if self.tcx.features().struct_target_features {
+                    self.dcx().emit_err(errors::AttrShouldBeAppliedToFnOrUnitStruct {
+                        attr_span: attr.span,
+                        defn_span: span,
+                    });
+                } else {
+                    self.dcx().emit_err(errors::AttrShouldBeAppliedToFn {
+                        attr_span: attr.span,
+                        defn_span: span,
+                        on_crate: hir_id == CRATE_HIR_ID,
+                    });
+                }
             }
         }
     }

compiler/rustc_passes/src/errors.rs

@@ -82,6 +82,15 @@ pub struct AttrShouldBeAppliedToFn {
     pub on_crate: bool,
 }
 
+#[derive(Diagnostic)]
+#[diag(passes_should_be_applied_to_fn_or_unit_struct)]
+pub struct AttrShouldBeAppliedToFnOrUnitStruct {
+    #[primary_span]
+    pub attr_span: Span,
+    #[label]
+    pub defn_span: Span,
+}
+
 #[derive(Diagnostic)]
 #[diag(passes_should_be_applied_to_fn, code = E0739)]
 pub struct TrackedCallerWrongLocation {

compiler/rustc_span/src/symbol.rs

@@ -1845,6 +1845,7 @@ symbols! {
         stringify,
         struct_field_attributes,
         struct_inherit,
+        struct_target_features,
         struct_variant,
         structural_match,
         structural_peq,