Auto merge of #146348 - jdonszelmann:eiiv3, r=lcnr,oli-obk

Externally implementable items

Supersedes #140010
Tracking issue: #125418

Getting started:

```rust
#![feature(eii)]

#[eii(eii1)]
pub fn decl1(x: u64)
// body optional (it's the default)
{
    println!("default {x}");
}

// in another crate, maybe
#[eii1]
pub fn decl2(x: u64) {
    println!("explicit {x}");
}

fn main() {
    decl1(4);
}
```

- tiny perf regression, underlying issue makes multiple things in the compiler slow, not just EII, planning to solve those separately.
- No codegen_gcc support, they don't have bindings for weak symbols yet but could
- No windows support yet for weak definitions

This PR merges the implementation of EII for just llvm + not windows, doesn't yet contain like a new panic handler implementation or alloc handler. With this implementation, it would support implementing the panic handler in terms of EII already since it requires no default implementation so no weak symbols

The PR has been open in various forms for about a year now, but I feel that having some implementation merged to build upon

Author: bors

compiler/rustc_ast/src/ast.rs

@@ -2109,6 +2109,19 @@ 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)]
@@ -3748,6 +3761,21 @@ 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)]
@@ -4114,7 +4142,7 @@ 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

@@ -971,11 +971,23 @@ 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

@@ -1179,11 +1179,16 @@ 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

@@ -865,6 +865,17 @@ 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 {
@@ -2162,6 +2173,15 @@ 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) => {
@@ -2177,6 +2197,12 @@ 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_attr_parsing/src/attributes/codegen_attrs.rs

@@ -699,3 +699,12 @@ impl<S: Stage> NoArgsAttributeParser<S> for RustcPassIndirectlyInNonRusticAbisPa
     const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::Struct)]);
     const CREATE: fn(Span) -> AttributeKind = AttributeKind::RustcPassIndirectlyInNonRusticAbis;
 }
+
+pub(crate) struct EiiExternItemParser;
+
+impl<S: Stage> NoArgsAttributeParser<S> for EiiExternItemParser {
+    const PATH: &[Symbol] = &[sym::rustc_eii_extern_item];
+    const ON_DUPLICATE: OnDuplicate<S> = OnDuplicate::Error;
+    const ALLOWED_TARGETS: AllowedTargets = AllowedTargets::AllowList(&[Allow(Target::ForeignFn)]);
+    const CREATE: fn(Span) -> AttributeKind = |_| AttributeKind::EiiExternItem;
+}