Delegation: fix ICE on late diagnostics

Author: Bryanskiy

compiler/rustc_resolve/src/late/diagnostics.rs

@@ -2041,8 +2041,11 @@ impl<'a: 'ast, 'ast, 'tcx> LateResolutionVisitor<'a, '_, 'ast, 'tcx> {
                         ast::AssocItemKind::Fn(..) => AssocSuggestion::AssocFn { called },
                         ast::AssocItemKind::Type(..) => AssocSuggestion::AssocType,
                         ast::AssocItemKind::Delegation(..)
-                            if self.r.delegation_fn_sigs[&self.r.local_def_id(assoc_item.id)]
-                                .has_self =>
+                            if self
+                                .r
+                                .delegation_fn_sigs
+                                .get(&self.r.local_def_id(assoc_item.id))
+                                .map_or(false, |sig| sig.has_self) =>
                         {
                             AssocSuggestion::MethodWithSelf { called }
                         }

tests/ui/delegation/ice-issue-124342.rs

@@ -0,0 +1,12 @@
+#![feature(fn_delegation)]
+#![allow(incomplete_features)]
+
+mod to_reuse {}
+
+trait Trait {
+    reuse to_reuse::foo { foo }
+    //~^ ERROR cannot find function `foo` in module `to_reuse`
+    //~| ERROR cannot find value `foo` in this scope
+}
+
+fn main() {}

tests/ui/delegation/ice-issue-124342.stderr

@@ -0,0 +1,20 @@
+error[E0425]: cannot find function `foo` in module `to_reuse`
+  --> $DIR/ice-issue-124342.rs:7:21
+   |
+LL |     reuse to_reuse::foo { foo }
+   |                     ^^^ not found in `to_reuse`
+
+error[E0425]: cannot find value `foo` in this scope
+  --> $DIR/ice-issue-124342.rs:7:27
+   |
+LL |     reuse to_reuse::foo { foo }
+   |                           ^^^
+   |
+help: you might have meant to refer to the associated function
+   |
+LL |     reuse to_reuse::foo { Self::foo }
+   |                           ++++++
+
+error: aborting due to 2 previous errors
+
+For more information about this error, try `rustc --explain E0425`.