Rollup merge of #152384 - enthropy7:fix-eii-root-resolution-clean, r=jdonszelmann

Restrict EII declarations to functions at lowering time

We tighten EII declaration resolution so that `lower_path_simple_eii` only accepts function‑like items (`Fn, AssocFn, Ctor(_, Fn)`) as valid EII targets. If name resolution points at something else (like a const with the same name), we now emit a direct error (“`externally implementable items must refer to a function`”) at the declaration site, which prevents bad `DefIds` from ever reaching `compare_eii_function_types` and turning into an ICE

this is more robust and root-cause oriented fix to rust-lang/rust#152337 issue and an alternate of my more simple PR rust-lang/rust#152365

test included

compiler/rustc_resolve/src/late.rs

@@ -441,6 +441,8 @@ pub(crate) enum PathSource<'a, 'ast, 'ra> {
     TraitItem(Namespace, &'a PathSource<'a, 'ast, 'ra>),
     /// Paths in delegation item
     Delegation,
+    /// Paths in externally implementable item declarations.
+    ExternItemImpl,
     /// An arg in a `use<'a, N>` precise-capturing bound.
     PreciseCapturingArg(Namespace),
     /// Paths that end with `(..)`, for return type notation.
@@ -465,6 +467,7 @@ fn namespace(self) -> Namespace {
             | PathSource::Pat
             | PathSource::TupleStruct(..)
             | PathSource::Delegation
+            | PathSource::ExternItemImpl
             | PathSource::ReturnTypeNotation => ValueNS,
             PathSource::TraitItem(ns, _) => ns,
             PathSource::PreciseCapturingArg(ns) => ns,
@@ -484,6 +487,7 @@ fn defer_to_typeck(self) -> bool {
             | PathSource::TraitItem(..)
             | PathSource::DefineOpaques
             | PathSource::Delegation
+            | PathSource::ExternItemImpl
             | PathSource::PreciseCapturingArg(..)
             | PathSource::Macro
             | PathSource::Module => false,
@@ -526,7 +530,9 @@ fn descr_expected(self) -> &'static str {
                 },
                 _ => "value",
             },
-            PathSource::ReturnTypeNotation | PathSource::Delegation => "function",
+            PathSource::ReturnTypeNotation
+            | PathSource::Delegation
+            | PathSource::ExternItemImpl => "function",
             PathSource::PreciseCapturingArg(..) => "type or const parameter",
             PathSource::Macro => "macro",
             PathSource::Module => "module",
@@ -618,6 +624,9 @@ pub(crate) fn is_expected(self, res: Res) -> bool {
                 _ => false,
             },
             PathSource::Delegation => matches!(res, Res::Def(DefKind::Fn | DefKind::AssocFn, _)),
+            PathSource::ExternItemImpl => {
+                matches!(res, Res::Def(DefKind::Fn | DefKind::AssocFn | DefKind::Ctor(..), _))
+            }
             PathSource::PreciseCapturingArg(ValueNS) => {
                 matches!(res, Res::Def(DefKind::ConstParam, _))
             }
@@ -640,8 +649,12 @@ fn error_code(self, has_unexpected_resolution: bool) -> ErrCode {
             (PathSource::Type | PathSource::DefineOpaques, false) => E0425,
             (PathSource::Struct(_), true) => E0574,
             (PathSource::Struct(_), false) => E0422,
-            (PathSource::Expr(..), true) | (PathSource::Delegation, true) => E0423,
-            (PathSource::Expr(..), false) | (PathSource::Delegation, false) => E0425,
+            (PathSource::Expr(..), true)
+            | (PathSource::Delegation, true)
+            | (PathSource::ExternItemImpl, true) => E0423,
+            (PathSource::Expr(..), false)
+            | (PathSource::Delegation, false)
+            | (PathSource::ExternItemImpl, false) => E0425,
             (PathSource::Pat | PathSource::TupleStruct(..), true) => E0532,
             (PathSource::Pat | PathSource::TupleStruct(..), false) => E0531,
             (PathSource::TraitItem(..) | PathSource::ReturnTypeNotation, true) => E0575,
@@ -1091,7 +1104,7 @@ fn visit_fn(&mut self, fn_kind: FnKind<'ast>, _: &AttrVec, sp: Span, fn_id: Node
                         *node_id,
                         &None,
                         &target.foreign_item,
-                        PathSource::Expr(None),
+                        PathSource::ExternItemImpl,
                     );
                 } else {
                     self.smart_resolve_path(*node_id, &None, &eii_macro_path, PathSource::Macro);
@@ -2198,7 +2211,8 @@ fn resolve_elided_lifetimes_in_path(
                 | PathSource::Struct(_)
                 | PathSource::TupleStruct(..)
                 | PathSource::DefineOpaques
-                | PathSource::Delegation => true,
+                | PathSource::Delegation
+                | PathSource::ExternItemImpl => true,
             };
             if inferred {
                 // Do not create a parameter for patterns and expressions: type checking can infer
@@ -3004,7 +3018,7 @@ fn resolve_item(&mut self, item: &'ast Item) {
                         item.id,
                         &None,
                         extern_item_path,
-                        PathSource::Expr(None),
+                        PathSource::ExternItemImpl,
                     );
                 }
             }

tests/ui/eii/eii-declaration-not-fn-issue-152337.rs

@@ -0,0 +1,12 @@
+// Regression test for ICE "unexpected sort of node in fn_sig()" (issue #152337).
+// When the same name is used for a const and an #[eii] function, the declaration
+// was incorrectly resolved to the const, causing fn_sig() to be called on a non-function.
+#![feature(extern_item_impls)]
+
+const A: () = ();
+#[eii]
+fn A() {} //~ ERROR the name `A` is defined multiple times
+//~^ ERROR expected function, found constant
+//~| ERROR expected function, found constant
+
+fn main() {}

tests/ui/eii/eii-declaration-not-fn-issue-152337.stderr

@@ -0,0 +1,27 @@
+error[E0428]: the name `A` is defined multiple times
+  --> $DIR/eii-declaration-not-fn-issue-152337.rs:8:1
+   |
+LL | const A: () = ();
+   | ----------------- previous definition of the value `A` here
+LL | #[eii]
+LL | fn A() {}
+   | ^^^^^^ `A` redefined here
+   |
+   = note: `A` must be defined only once in the value namespace of this module
+
+error[E0423]: expected function, found constant `self::A`
+  --> $DIR/eii-declaration-not-fn-issue-152337.rs:8:4
+   |
+LL | fn A() {}
+   |    ^ not a function
+
+error[E0423]: expected function, found constant `A`
+  --> $DIR/eii-declaration-not-fn-issue-152337.rs:8:4
+   |
+LL | fn A() {}
+   |    ^ not a function
+
+error: aborting due to 3 previous errors
+
+Some errors have detailed explanations: E0423, E0428.
+For more information about an error, try `rustc --explain E0423`.