Externally implementable items

compiler/rustc_ast/src/ast.rs

@@ -2090,6 +2090,19 @@ pub struct MacroDef {
     pub body: Box<DelimArgs>,
     /// `true` if macro was defined with `macro_rules`.
     pub macro_rules: bool,
+
+    /// If this is a macro used for externally implementable items,
+    /// it refers to an extern item which is its "target". This requires
+    /// name resolution so can't just be an attribute, so we store it in this field.
+    pub eii_extern_target: Option<EiiExternTarget>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic, Walkable)]
+pub struct EiiExternTarget {
+    /// path to the extern item we're targetting
+    pub extern_item_path: Path,
+    pub impl_unsafe: bool,
+    pub span: Span,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug, Copy, Hash, Eq, PartialEq)]
@@ -3729,6 +3742,21 @@ pub struct Fn {
     pub contract: Option<Box<FnContract>>,
     pub define_opaque: Option<ThinVec<(NodeId, Path)>>,
     pub body: Option<Box<Block>>,
+
+    /// This function is an implementation of an externally implementable item (EII).
+    /// This means, there was an EII declared somewhere and this function is the
+    /// implementation that should be run when the declaration is called.
+    pub eii_impls: ThinVec<EiiImpl>,
+}
+
+#[derive(Clone, Encodable, Decodable, Debug, Walkable)]
+pub struct EiiImpl {
+    pub node_id: NodeId,
+    pub eii_macro_path: Path,
+    pub impl_safety: Safety,
+    pub span: Span,
+    pub inner_span: Span,
+    pub is_default: bool,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug, Walkable)]
@@ -4095,7 +4123,7 @@ mod size_asserts {
     static_assert_size!(Block, 32);
     static_assert_size!(Expr, 72);
     static_assert_size!(ExprKind, 40);
-    static_assert_size!(Fn, 184);
+    static_assert_size!(Fn, 192);
     static_assert_size!(ForeignItem, 80);
     static_assert_size!(ForeignItemKind, 16);
     static_assert_size!(GenericArg, 24);

compiler/rustc_ast/src/visit.rs

@@ -393,6 +393,7 @@ macro_rules! common_visitor_and_walkers {
             ThinVec<Pat>,
             ThinVec<Box<Ty>>,
             ThinVec<TyPat>,
+            ThinVec<EiiImpl>,
         );
 
         // This macro generates `impl Visitable` and `impl MutVisitable` that forward to `Walkable`
@@ -485,6 +486,8 @@ macro_rules! common_visitor_and_walkers {
             WhereEqPredicate,
             WhereRegionPredicate,
             YieldKind,
+            EiiExternTarget,
+            EiiImpl,
         );
 
         /// Each method of this trait is a hook to be potentially
@@ -919,13 +922,13 @@ macro_rules! common_visitor_and_walkers {
                     _ctxt,
                     // Visibility is visited as a part of the item.
                     _vis,
-                    Fn { defaultness, ident, sig, generics, contract, body, define_opaque },
+                    Fn { defaultness, ident, sig, generics, contract, body, define_opaque, eii_impls },
                 ) => {
                     let FnSig { header, decl, span } = sig;
                     visit_visitable!($($mut)? vis,
                         defaultness, ident, header, generics, decl,
-                        contract, body, span, define_opaque
-                    )
+                        contract, body, span, define_opaque, eii_impls
+                    );
                 }
                 FnKind::Closure(binder, coroutine_kind, decl, body) =>
                     visit_visitable!($($mut)? vis, binder, coroutine_kind, decl, body),

compiler/rustc_ast_lowering/src/item.rs

@@ -2,7 +2,7 @@ use rustc_abi::ExternAbi;
 use rustc_ast::visit::AssocCtxt;
 use rustc_ast::*;
 use rustc_errors::{E0570, ErrorGuaranteed, struct_span_code_err};
-use rustc_hir::attrs::AttributeKind;
+use rustc_hir::attrs::{AttributeKind, EiiDecl};
 use rustc_hir::def::{DefKind, PerNS, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, LocalDefId};
 use rustc_hir::{
@@ -11,6 +11,7 @@ use rustc_hir::{
 use rustc_index::{IndexSlice, IndexVec};
 use rustc_middle::span_bug;
 use rustc_middle::ty::{ResolverAstLowering, TyCtxt};
+use rustc_span::def_id::DefId;
 use rustc_span::edit_distance::find_best_match_for_name;
 use rustc_span::{DUMMY_SP, DesugaringKind, Ident, Span, Symbol, kw, sym};
 use smallvec::{SmallVec, smallvec};
@@ -133,17 +134,103 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    fn generate_extra_attrs_for_item_kind(
+        &mut self,
+        id: NodeId,
+        i: &ItemKind,
+    ) -> Vec<hir::Attribute> {
+        match i {
+            ItemKind::Fn(box Fn { eii_impls, .. }) if eii_impls.is_empty() => Vec::new(),
+            ItemKind::Fn(box Fn { eii_impls, .. }) => {
+                vec![hir::Attribute::Parsed(AttributeKind::EiiImpls(
+                    eii_impls
+                        .iter()
+                        .flat_map(
+                            |EiiImpl {
+                                 node_id,
+                                 eii_macro_path,
+                                 impl_safety,
+                                 span,
+                                 inner_span,
+                                 is_default,
+                             }| {
+                                self.lower_path_simple_eii(*node_id, eii_macro_path).map(|did| {
+                                    hir::attrs::EiiImpl {
+                                        eii_macro: did,
+                                        span: self.lower_span(*span),
+                                        inner_span: self.lower_span(*inner_span),
+                                        impl_marked_unsafe: self
+                                            .lower_safety(*impl_safety, hir::Safety::Safe)
+                                            .is_unsafe(),
+                                        is_default: *is_default,
+                                    }
+                                })
+                            },
+                        )
+                        .collect(),
+                ))]
+            }
+            ItemKind::MacroDef(
+                _,
+                MacroDef {
+                    eii_extern_target: Some(EiiExternTarget { extern_item_path, impl_unsafe, span }),
+                    ..
+                },
+            ) => self
+                .lower_path_simple_eii(id, extern_item_path)
+                .map(|did| {
+                    vec![hir::Attribute::Parsed(AttributeKind::EiiExternTarget(EiiDecl {
+                        eii_extern_target: did,
+                        impl_unsafe: *impl_unsafe,
+                        span: self.lower_span(*span),
+                    }))]
+                })
+                .unwrap_or_default(),
+            ItemKind::ExternCrate(..)
+            | ItemKind::Use(..)
+            | ItemKind::Static(..)
+            | ItemKind::Const(..)
+            | ItemKind::Mod(..)
+            | ItemKind::ForeignMod(..)
+            | ItemKind::GlobalAsm(..)
+            | ItemKind::TyAlias(..)
+            | ItemKind::Enum(..)
+            | ItemKind::Struct(..)
+            | ItemKind::Union(..)
+            | ItemKind::Trait(..)
+            | ItemKind::TraitAlias(..)
+            | ItemKind::Impl(..)
+            | ItemKind::MacCall(..)
+            | ItemKind::MacroDef(..)
+            | ItemKind::Delegation(..)
+            | ItemKind::DelegationMac(..) => Vec::new(),
+        }
+    }
+
     fn lower_item(&mut self, i: &Item) -> &'hir hir::Item<'hir> {
         let vis_span = self.lower_span(i.vis.span);
         let hir_id = hir::HirId::make_owner(self.current_hir_id_owner.def_id);
-        let attrs = self.lower_attrs(hir_id, &i.attrs, i.span, Target::from_ast_item(i));
+
+        let extra_hir_attributes = self.generate_extra_attrs_for_item_kind(i.id, &i.kind);
+        let attrs = self.lower_attrs_with_extra(
+            hir_id,
+            &i.attrs,
+            i.span,
+            Target::from_ast_item(i),
+            &extra_hir_attributes,
+        );
+
         let kind = self.lower_item_kind(i.span, i.id, hir_id, attrs, vis_span, &i.kind);
         let item = hir::Item {
             owner_id: hir_id.expect_owner(),
             kind,
             vis_span,
             span: self.lower_span(i.span),
             has_delayed_lints: !self.delayed_lints.is_empty(),
+            eii: find_attr!(
+                attrs,
+                AttributeKind::EiiImpls(..) | AttributeKind::EiiExternTarget(..)
+            ),
         };
         self.arena.alloc(item)
     }
@@ -435,7 +522,7 @@ impl<'hir> LoweringContext<'_, 'hir> {
                 );
                 hir::ItemKind::TraitAlias(constness, ident, generics, bounds)
             }
-            ItemKind::MacroDef(ident, MacroDef { body, macro_rules }) => {
+            ItemKind::MacroDef(ident, MacroDef { body, macro_rules, eii_extern_target: _ }) => {
                 let ident = self.lower_ident(*ident);
                 let body = Box::new(self.lower_delim_args(body));
                 let def_id = self.local_def_id(id);
@@ -446,7 +533,11 @@ impl<'hir> LoweringContext<'_, 'hir> {
                         def_kind.descr(def_id.to_def_id())
                     );
                 };
-                let macro_def = self.arena.alloc(ast::MacroDef { body, macro_rules: *macro_rules });
+                let macro_def = self.arena.alloc(ast::MacroDef {
+                    body,
+                    macro_rules: *macro_rules,
+                    eii_extern_target: None,
+                });
                 hir::ItemKind::Macro(ident, macro_def, macro_kinds)
             }
             ItemKind::Delegation(box delegation) => {
@@ -465,6 +556,16 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    fn lower_path_simple_eii(&mut self, id: NodeId, path: &Path) -> Option<DefId> {
+        let res = self.resolver.get_partial_res(id)?;
+        let Some(did) = res.expect_full_res().opt_def_id() else {
+            self.dcx().span_delayed_bug(path.span, "should have errored in resolve");
+            return None;
+        };
+
+        Some(did)
+    }
+
     #[instrument(level = "debug", skip(self))]
     fn lower_use_tree(
         &mut self,
@@ -573,6 +674,10 @@ impl<'hir> LoweringContext<'_, 'hir> {
                             vis_span,
                             span: this.lower_span(use_tree.span),
                             has_delayed_lints: !this.delayed_lints.is_empty(),
+                            eii: find_attr!(
+                                attrs,
+                                AttributeKind::EiiImpls(..) | AttributeKind::EiiExternTarget(..)
+                            ),
                         };
                         hir::OwnerNode::Item(this.arena.alloc(item))
                     });

compiler/rustc_ast_lowering/src/lib.rs

@@ -958,11 +958,23 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         target_span: Span,
         target: Target,
     ) -> &'hir [hir::Attribute] {
-        if attrs.is_empty() {
+        self.lower_attrs_with_extra(id, attrs, target_span, target, &[])
+    }
+
+    fn lower_attrs_with_extra(
+        &mut self,
+        id: HirId,
+        attrs: &[Attribute],
+        target_span: Span,
+        target: Target,
+        extra_hir_attributes: &[hir::Attribute],
+    ) -> &'hir [hir::Attribute] {
+        if attrs.is_empty() && extra_hir_attributes.is_empty() {
             &[]
         } else {
-            let lowered_attrs =
+            let mut lowered_attrs =
                 self.lower_attrs_vec(attrs, self.lower_span(target_span), id, target);
+            lowered_attrs.extend(extra_hir_attributes.iter().cloned());
 
             assert_eq!(id.owner, self.current_hir_id_owner);
             let ret = self.arena.alloc_from_iter(lowered_attrs);

compiler/rustc_ast_passes/src/ast_validation.rs

@@ -1171,11 +1171,16 @@ impl<'a> Visitor<'a> for AstValidator<'a> {
                     contract: _,
                     body,
                     define_opaque: _,
+                    eii_impls,
                 },
             ) => {
                 self.visit_attrs_vis_ident(&item.attrs, &item.vis, ident);
                 self.check_defaultness(item.span, *defaultness);
 
+                for EiiImpl { eii_macro_path, .. } in eii_impls {
+                    self.visit_path(eii_macro_path);
+                }
+
                 let is_intrinsic = item.attrs.iter().any(|a| a.has_name(sym::rustc_intrinsic));
                 if body.is_none() && !is_intrinsic && !self.is_sdylib_interface {
                     self.dcx().emit_err(errors::FnWithoutBody {

compiler/rustc_ast_pretty/src/pprust/state.rs

@@ -852,6 +852,17 @@ pub trait PrintState<'a>: std::ops::Deref<Target = pp::Printer> + std::ops::Dere
         sp: Span,
         print_visibility: impl FnOnce(&mut Self),
     ) {
+        if let Some(eii_extern_target) = &macro_def.eii_extern_target {
+            self.word("#[eii_extern_target(");
+            self.print_path(&eii_extern_target.extern_item_path, false, 0);
+            if eii_extern_target.impl_unsafe {
+                self.word(",");
+                self.space();
+                self.word("unsafe");
+            }
+            self.word(")]");
+            self.hardbreak();
+        }
         let (kw, has_bang) = if macro_def.macro_rules {
             ("macro_rules", true)
         } else {
@@ -2148,6 +2159,15 @@ impl<'a> State<'a> {
 
     fn print_meta_item(&mut self, item: &ast::MetaItem) {
         let ib = self.ibox(INDENT_UNIT);
+
+        match item.unsafety {
+            ast::Safety::Unsafe(_) => {
+                self.word("unsafe");
+                self.popen();
+            }
+            ast::Safety::Default | ast::Safety::Safe(_) => {}
+        }
+
         match &item.kind {
             ast::MetaItemKind::Word => self.print_path(&item.path, false, 0),
             ast::MetaItemKind::NameValue(value) => {
@@ -2163,6 +2183,12 @@ impl<'a> State<'a> {
                 self.pclose();
             }
         }
+
+        match item.unsafety {
+            ast::Safety::Unsafe(_) => self.pclose(),
+            ast::Safety::Default | ast::Safety::Safe(_) => {}
+        }
+
         self.end(ib);
     }
 

compiler/rustc_ast_pretty/src/pprust/state/item.rs

@@ -1,6 +1,6 @@
 use ast::StaticItem;
 use itertools::{Itertools, Position};
-use rustc_ast::{self as ast, ModKind, TraitAlias};
+use rustc_ast::{self as ast, EiiImpl, ModKind, Safety, TraitAlias};
 use rustc_span::Ident;
 
 use crate::pp::BoxMarker;
@@ -671,10 +671,25 @@ impl<'a> State<'a> {
     }
 
     fn print_fn_full(&mut self, vis: &ast::Visibility, attrs: &[ast::Attribute], func: &ast::Fn) {
-        let ast::Fn { defaultness, ident, generics, sig, contract, body, define_opaque } = func;
+        let ast::Fn { defaultness, ident, generics, sig, contract, body, define_opaque, eii_impls } =
+            func;
 
         self.print_define_opaques(define_opaque.as_deref());
 
+        for EiiImpl { eii_macro_path, impl_safety, .. } in eii_impls {
+            self.word("#[");
+            if let Safety::Unsafe(..) = impl_safety {
+                self.word("unsafe");
+                self.popen();
+            }
+            self.print_path(eii_macro_path, false, 0);
+            if let Safety::Unsafe(..) = impl_safety {
+                self.pclose();
+            }
+            self.word("]");
+            self.hardbreak();
+        }
+
         let body_cb_ib = body.as_ref().map(|body| (body, self.head("")));
 
         self.print_visibility(vis);

compiler/rustc_builtin_macros/messages.ftl

@@ -152,6 +152,17 @@ builtin_macros_derive_path_args_value = traits in `#[derive(...)]` don't accept
 
 builtin_macros_duplicate_macro_attribute = duplicated attribute
 
+builtin_macros_eii_extern_target_expected_list = `#[eii_extern_target(...)]` expects a list of one or two elements
+builtin_macros_eii_extern_target_expected_macro = `#[eii_extern_target(...)]` is only valid on macros
+builtin_macros_eii_extern_target_expected_unsafe = expected this argument to be "unsafe"
+    .note = the second argument is optional
+
+builtin_macros_eii_only_once = `#[{$name}]` can only be specified once
+    .note = specified again here
+
+builtin_macros_eii_shared_macro_expected_function = `#[{$name}]` is only valid on functions
+builtin_macros_eii_shared_macro_expected_max_one_argument = `#[{$name}]` expected no arguments or a single argument: `#[{$name}(default)]`
+
 builtin_macros_env_not_defined = environment variable `{$var}` not defined at compile time
     .cargo = Cargo sets build script variables at run time. Use `std::env::var({$var_expr})` instead
     .cargo_typo = there is a similar Cargo environment variable: `{$suggested_var}`