fix: Prevent ICE when comparing EII impl against generic declaration

compiler/rustc_hir_analysis/src/check/compare_eii.rs

@@ -41,6 +41,10 @@ pub(crate) fn compare_eii_function_types<'tcx>(
     eii_name: Symbol,
     eii_attr_span: Span,
 ) -> Result<(), ErrorGuaranteed> {
+    // Already errored, returning to avoid icing.
+    if tcx.generics_of(foreign_item).own_requires_monomorphization() {
+        return Err(tcx.dcx().delayed_bug("compared an EII impl against a generic declaration"));
+    }
     check_eii_target(tcx, external_impl, foreign_item, eii_name, eii_attr_span)?;
     check_is_structurally_compatible(tcx, external_impl, foreign_item, eii_name, eii_attr_span)?;
 
@@ -289,10 +293,15 @@ fn check_is_structurally_compatible<'tcx>(
 fn check_no_generics<'tcx>(
     tcx: TyCtxt<'tcx>,
     external_impl: LocalDefId,
-    _declaration: DefId,
+    declaration: DefId,
     eii_name: Symbol,
     eii_attr_span: Span,
 ) -> Result<(), ErrorGuaranteed> {
+    // Already errored, return to avoid icing
+    if tcx.generics_of(declaration).own_requires_monomorphization() {
+        return Err(tcx.dcx().delayed_bug("compared an EII impl against a generic declaration"));
+    }
+
     let generics = tcx.generics_of(external_impl);
     if generics.own_requires_monomorphization()
         // When an EII implementation is automatically generated by the `#[eii]` macro,

tests/ui/eii/type_checking/generic_declaration_and_impl.rs

@@ -0,0 +1,18 @@
+//@ check-fail
+// Check that comparing an EII implementation against a generic declaration
+// (already rejected by E0044) does not ICE in error reporting.
+// Regression test for the ICE when note_and_explain_type_err tries to look up
+// declaration parameters in external_impl's empty generics.
+#![feature(extern_item_impls)]
+
+#[eii(foo)]
+fn bar<T>(x: T) where T: FnOnce() -> u64 { x() }
+//~^ ERROR externally implementable items may not have type parameters
+//~| ERROR mismatched types
+
+#[foo]
+fn y(a: u64) -> u64 {
+    a
+}
+
+fn main() {}

tests/ui/eii/type_checking/generic_declaration_and_impl.stderr

@@ -0,0 +1,27 @@
+error[E0044]: externally implementable items may not have type parameters
+  --> $DIR/generic_declaration_and_impl.rs:9:1
+   |
+LL | fn bar<T>(x: T) where T: FnOnce() -> u64 { x() }
+   | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ can't have type parameters
+   |
+   = help: replace the type parameters with concrete types like `u32`
+
+error[E0308]: mismatched types
+  --> $DIR/generic_declaration_and_impl.rs:9:44
+   |
+LL | fn bar<T>(x: T) where T: FnOnce() -> u64 { x() }
+   |                                            ^^^ expected `()`, found `u64`
+   |
+help: consider using a semicolon here
+   |
+LL | fn bar<T>(x: T) where T: FnOnce() -> u64 { x(); }
+   |                                               +
+help: try adding a return type
+   |
+LL | fn bar<T>(x: T) -> u64 where T: FnOnce() -> u64 { x() }
+   |                 ++++++
+
+error: aborting due to 2 previous errors
+
+Some errors have detailed explanations: E0044, E0308.
+For more information about an error, try `rustc --explain E0044`.