Rollup merge of #150822 - fix-149981, r=@Kivooeo

Fix for ICE: eii: fn / macro rules None in find_attr()

Closes #149981

This used to ICE:
```rust
macro_rules! foo_impl {}
#[eii]
fn foo_impl() {}
```

`#[eii]` generates a macro (called `foo_impl`) and a default impl. So the partial expansion used to roughly look like the following:

```rust
macro_rules! foo_impl {} // actually resolves here

extern "Rust" {
    fn foo_impl();
}

#[eii_extern_target(foo_impl)]
macro foo_impl {
    () => {};
}

const _: () = {
    #[implements_eii(foo_impl)] // assumed to name resolve to the macro v2 above
    fn foo_impl() {}
};
```

Now, shadowing rules for macrov2 and macrov1 are super weird! Take a look at this: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2024&gist=23f21421921360478b0ec0276711ad36

So instead of resolving to the macrov2, we resolve the macrov1 named the same thing.

A regression test was added to this, and some span_delayed_bugs were added to make sure we catch this in the right places. But that didn't fix the root cause.

To make sure this simply cannot happen again, I made it so that we don't even need to do a name resolution for the default. In other words, the new partial expansion looks more like:

```rust
macro_rules! foo_impl {}

extern "Rust" {
    fn foo_impl(); // resolves to here now!!!
}

#[eii_extern_target(foo_impl)]
macro foo_impl {
    () => {};
}

const _: () = {
    #[implements_eii(known_extern_target=foo_impl)] // still name resolved, but directly to the foreign function.
    fn foo_impl() {}
};
```

The reason this helps is that name resolution for non-macros is much more predictable. It's not possible to have two functions like that with the same name in scope.

We used to key externally implementable items off of the defid of the macro, but now the unique identifier is the foreign function's defid which seems much more sane.

Finally, I lied a tiny bit because the above partial expansion doesn't actually work.
```rust
extern "Rust" {
    fn foo_impl(); // not to here
}

const _: () = {
    #[implements_eii(known_extern_target=foo_impl)] // actually resolves to this function itself
    fn foo_impl() {} // <--- so to here
};
```

So the last few commits change the expansion to actually be this:

```rust
macro_rules! foo_impl {}

extern "Rust" {
    fn foo_impl(); // resolves to here now!!!
}

#[eii_extern_target(foo_impl)]
macro foo_impl {
    () => {};
}

const _: () = {
    mod dflt { // necessary, otherwise `super` doesn't work
        use super::*;
        #[implements_eii(known_extern_target=super::foo_impl)] // now resolves to outside the `dflt` module, so the foreign item.
        fn foo_impl() {}
    }
};
```

I apologize to whoever needs to review this, this is very subtle and I hope this makes it clear enough 😭.

Author: Urgau

compiler/rustc_ast/src/ast.rs

@@ -2117,10 +2117,9 @@ pub struct MacroDef {
 
 #[derive(Clone, Encodable, Decodable, Debug, HashStable_Generic, Walkable)]
 pub struct EiiExternTarget {
-    /// path to the extern item we're targetting
+    /// path to the extern item we're targeting
     pub extern_item_path: Path,
     pub impl_unsafe: bool,
-    pub span: Span,
 }
 
 #[derive(Clone, Encodable, Decodable, Debug, Copy, Hash, Eq, PartialEq)]
@@ -3813,6 +3812,19 @@ pub struct Fn {
 pub struct EiiImpl {
     pub node_id: NodeId,
     pub eii_macro_path: Path,
+    /// This field is an implementation detail that prevents a lot of bugs.
+    /// See <https://github.com/rust-lang/rust/issues/149981> for an example.
+    ///
+    /// The problem is, that if we generate a declaration *together* with its default,
+    /// we generate both a declaration and an implementation. The generated implementation
+    /// uses the same mechanism to register itself as a user-defined implementation would,
+    /// despite being invisible to users. What does happen is a name resolution step.
+    /// The invisible default implementation has to find the declaration.
+    /// Both are generated at the same time, so we can skip that name resolution step.
+    ///
+    /// This field is that shortcut: we prefill the extern target to skip a name resolution step,
+    /// making sure it never fails. It'd be awful UX if we fail name resolution in code invisible to the user.
+    pub known_eii_macro_resolution: Option<EiiExternTarget>,
     pub impl_safety: Safety,
     pub span: Span,
     pub inner_span: Span,

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, EiiDecl};
+use rustc_hir::attrs::{AttributeKind, EiiDecl, EiiImplResolution};
 use rustc_hir::def::{DefKind, PerNS, Res};
 use rustc_hir::def_id::{CRATE_DEF_ID, LocalDefId};
 use rustc_hir::{
@@ -134,6 +134,56 @@ impl<'hir> LoweringContext<'_, 'hir> {
         }
     }
 
+    fn lower_eii_extern_target(
+        &mut self,
+        id: NodeId,
+        eii_name: Ident,
+        EiiExternTarget { extern_item_path, impl_unsafe }: &EiiExternTarget,
+    ) -> Option<EiiDecl> {
+        self.lower_path_simple_eii(id, extern_item_path).map(|did| EiiDecl {
+            eii_extern_target: did,
+            impl_unsafe: *impl_unsafe,
+            name: eii_name,
+        })
+    }
+
+    fn lower_eii_impl(
+        &mut self,
+        EiiImpl {
+            node_id,
+            eii_macro_path,
+            impl_safety,
+            span,
+            inner_span,
+            is_default,
+            known_eii_macro_resolution,
+        }: &EiiImpl,
+    ) -> hir::attrs::EiiImpl {
+        let resolution = if let Some(target) = known_eii_macro_resolution
+            && let Some(decl) = self.lower_eii_extern_target(
+                *node_id,
+                // the expect is ok here since we always generate this path in the eii macro.
+                eii_macro_path.segments.last().expect("at least one segment").ident,
+                target,
+            ) {
+            EiiImplResolution::Known(decl)
+        } else if let Some(macro_did) = self.lower_path_simple_eii(*node_id, eii_macro_path) {
+            EiiImplResolution::Macro(macro_did)
+        } else {
+            EiiImplResolution::Error(
+                self.dcx().span_delayed_bug(*span, "eii never resolved without errors given"),
+            )
+        };
+
+        hir::attrs::EiiImpl {
+            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,
+            resolution,
+        }
+    }
+
     fn generate_extra_attrs_for_item_kind(
         &mut self,
         id: NodeId,
@@ -143,49 +193,14 @@ impl<'hir> LoweringContext<'_, 'hir> {
             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(),
+                    eii_impls.iter().map(|i| self.lower_eii_impl(i)).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),
-                    }))]
-                })
+            ItemKind::MacroDef(name, MacroDef { eii_extern_target: Some(target), .. }) => self
+                .lower_eii_extern_target(id, *name, target)
+                .map(|decl| vec![hir::Attribute::Parsed(AttributeKind::EiiExternTarget(decl))])
                 .unwrap_or_default(),
+
             ItemKind::ExternCrate(..)
             | ItemKind::Use(..)
             | ItemKind::Static(..)

compiler/rustc_builtin_macros/src/eii.rs

@@ -103,19 +103,19 @@ fn eii_(
 
     // span of the declaring item without attributes
     let item_span = func.sig.span;
-    // span of the eii attribute and the item below it, i.e. the full declaration
-    let decl_span = eii_attr_span.to(item_span);
     let foreign_item_name = func.ident;
 
     let mut return_items = Vec::new();
 
     if func.body.is_some() {
         return_items.push(Box::new(generate_default_impl(
+            ecx,
             &func,
             impl_unsafe,
             macro_name,
             eii_attr_span,
             item_span,
+            foreign_item_name,
         )))
     }
 
@@ -133,7 +133,6 @@ fn eii_(
         macro_name,
         foreign_item_name,
         impl_unsafe,
-        decl_span,
     )));
 
     return_items.into_iter().map(wrap_item).collect()
@@ -187,11 +186,13 @@ fn filter_attrs_for_multiple_eii_attr(
 }
 
 fn generate_default_impl(
+    ecx: &mut ExtCtxt<'_>,
     func: &ast::Fn,
     impl_unsafe: bool,
     macro_name: Ident,
     eii_attr_span: Span,
     item_span: Span,
+    foreign_item_name: Ident,
 ) -> ast::Item {
     // FIXME: re-add some original attrs
     let attrs = ThinVec::new();
@@ -208,6 +209,21 @@ fn generate_default_impl(
         },
         span: eii_attr_span,
         is_default: true,
+        known_eii_macro_resolution: Some(ast::EiiExternTarget {
+            extern_item_path: ast::Path {
+                span: foreign_item_name.span,
+                segments: thin_vec![
+                    ast::PathSegment {
+                        ident: Ident::from_str_and_span("super", foreign_item_name.span,),
+                        id: DUMMY_NODE_ID,
+                        args: None
+                    },
+                    ast::PathSegment { ident: foreign_item_name, id: DUMMY_NODE_ID, args: None },
+                ],
+                tokens: None,
+            },
+            impl_unsafe,
+        }),
     });
 
     ast::Item {
@@ -236,18 +252,66 @@ fn generate_default_impl(
                         stmts: thin_vec![ast::Stmt {
                             id: DUMMY_NODE_ID,
                             kind: ast::StmtKind::Item(Box::new(ast::Item {
-                                attrs,
+                                attrs: ThinVec::new(),
                                 id: DUMMY_NODE_ID,
                                 span: item_span,
                                 vis: ast::Visibility {
-                                    span: eii_attr_span,
+                                    span: item_span,
                                     kind: ast::VisibilityKind::Inherited,
                                     tokens: None
                                 },
-                                kind: ItemKind::Fn(Box::new(default_func)),
+                                kind: ItemKind::Mod(
+                                    ast::Safety::Default,
+                                    Ident::from_str_and_span("dflt", item_span),
+                                    ast::ModKind::Loaded(
+                                        thin_vec![
+                                            Box::new(ast::Item {
+                                                attrs: thin_vec![ecx.attr_nested_word(
+                                                    sym::allow,
+                                                    sym::unused_imports,
+                                                    item_span
+                                                ),],
+                                                id: DUMMY_NODE_ID,
+                                                span: item_span,
+                                                vis: ast::Visibility {
+                                                    span: eii_attr_span,
+                                                    kind: ast::VisibilityKind::Inherited,
+                                                    tokens: None
+                                                },
+                                                kind: ItemKind::Use(ast::UseTree {
+                                                    prefix: ast::Path::from_ident(
+                                                        Ident::from_str_and_span(
+                                                            "super", item_span,
+                                                        )
+                                                    ),
+                                                    kind: ast::UseTreeKind::Glob,
+                                                    span: item_span,
+                                                }),
+                                                tokens: None,
+                                            }),
+                                            Box::new(ast::Item {
+                                                attrs,
+                                                id: DUMMY_NODE_ID,
+                                                span: item_span,
+                                                vis: ast::Visibility {
+                                                    span: eii_attr_span,
+                                                    kind: ast::VisibilityKind::Inherited,
+                                                    tokens: None
+                                                },
+                                                kind: ItemKind::Fn(Box::new(default_func)),
+                                                tokens: None,
+                                            }),
+                                        ],
+                                        ast::Inline::Yes,
+                                        ast::ModSpans {
+                                            inner_span: item_span,
+                                            inject_use_span: item_span,
+                                        }
+                                    )
+                                ),
                                 tokens: None,
                             })),
-                            span: eii_attr_span
+                            span: eii_attr_span,
                         }],
                         id: DUMMY_NODE_ID,
                         rules: ast::BlockCheckMode::Default,
@@ -352,7 +416,6 @@ fn generate_attribute_macro_to_implement(
     macro_name: Ident,
     foreign_item_name: Ident,
     impl_unsafe: bool,
-    decl_span: Span,
 ) -> ast::Item {
     let mut macro_attrs = ThinVec::new();
 
@@ -394,7 +457,6 @@ fn generate_attribute_macro_to_implement(
                 eii_extern_target: Some(ast::EiiExternTarget {
                     extern_item_path: ast::Path::from_ident(foreign_item_name),
                     impl_unsafe,
-                    span: decl_span,
                 }),
             },
         ),
@@ -451,7 +513,7 @@ pub(crate) fn eii_extern_target(
         false
     };
 
-    d.eii_extern_target = Some(EiiExternTarget { extern_item_path, impl_unsafe, span });
+    d.eii_extern_target = Some(EiiExternTarget { extern_item_path, impl_unsafe });
 
     // Return the original item and the new methods.
     vec![item]
@@ -508,6 +570,7 @@ pub(crate) fn eii_shared_macro(
         impl_safety: meta_item.unsafety,
         span,
         is_default,
+        known_eii_macro_resolution: None,
     });
 
     vec![item]

compiler/rustc_codegen_ssa/src/codegen_attrs.rs

@@ -3,7 +3,9 @@ use std::str::FromStr;
 use rustc_abi::{Align, ExternAbi};
 use rustc_ast::expand::autodiff_attrs::{AutoDiffAttrs, DiffActivity, DiffMode};
 use rustc_ast::{LitKind, MetaItem, MetaItemInner, attr};
-use rustc_hir::attrs::{AttributeKind, InlineAttr, Linkage, RtsanSetting, UsedBy};
+use rustc_hir::attrs::{
+    AttributeKind, EiiImplResolution, InlineAttr, Linkage, RtsanSetting, UsedBy,
+};
 use rustc_hir::def::DefKind;
 use rustc_hir::def_id::{DefId, LOCAL_CRATE, LocalDefId};
 use rustc_hir::{self as hir, Attribute, LangItem, find_attr, lang_items};
@@ -285,11 +287,23 @@ fn process_builtin_attrs(
                 }
                 AttributeKind::EiiImpls(impls) => {
                     for i in impls {
-                        let extern_item = find_attr!(
-                            tcx.get_all_attrs(i.eii_macro),
-                            AttributeKind::EiiExternTarget(target) => target.eii_extern_target
-                        )
-                        .expect("eii should have declaration macro with extern target attribute");
+                        let extern_item = match i.resolution {
+                            EiiImplResolution::Macro(def_id) => {
+                                let Some(extern_item) = find_attr!(
+                                    tcx.get_all_attrs(def_id),
+                                    AttributeKind::EiiExternTarget(target) => target.eii_extern_target
+                                ) else {
+                                    tcx.dcx().span_delayed_bug(
+                                        i.span,
+                                        "resolved to something that's not an EII",
+                                    );
+                                    continue;
+                                };
+                                extern_item
+                            }
+                            EiiImplResolution::Known(decl) => decl.eii_extern_target,
+                            EiiImplResolution::Error(_eg) => continue,
+                        };
 
                         // this is to prevent a bug where a single crate defines both the default and explicit implementation
                         // for an EII. In that case, both of them may be part of the same final object file. I'm not 100% sure
@@ -302,7 +316,7 @@ fn process_builtin_attrs(
                             // iterate over all implementations *in the current crate*
                             // (this is ok since we generate codegen fn attrs in the local crate)
                             // if any of them is *not default* then don't emit the alias.
-                            && tcx.externally_implementable_items(LOCAL_CRATE).get(&i.eii_macro).expect("at least one").1.iter().any(|(_, imp)| !imp.is_default)
+                            && tcx.externally_implementable_items(LOCAL_CRATE).get(&extern_item).expect("at least one").1.iter().any(|(_, imp)| !imp.is_default)
                         {
                             continue;
                         }