Use and_then/filter/map chain for child_args in delegation

Refactor child_args construction to use .filter() in the method chain
instead of an early return with an empty slice inside .map(). When
child_args is None (filtered out), unwrap_or_default() returns an
empty slice.

Author: GokhanKabar

compiler/rustc_hir_analysis/src/delegation.rs

@@ -596,35 +596,35 @@ fn get_delegation_user_specified_args<'tcx>(
             .as_slice()
     });
 
-    let child_args = info.child_args_segment_id.and_then(get_segment).map(|(segment, def_id)| {
-        if !matches!(tcx.def_kind(def_id), DefKind::Fn | DefKind::AssocFn) {
-            return &[][..];
-        }
-
-        let parent_args = if let Some(parent_args) = parent_args {
-            parent_args
-        } else {
-            let parent = tcx.parent(def_id);
-            if matches!(tcx.def_kind(parent), DefKind::Trait) {
-                ty::GenericArgs::identity_for_item(tcx, parent).as_slice()
+    let child_args = info
+        .child_args_segment_id
+        .and_then(get_segment)
+        .filter(|(_, def_id)| matches!(tcx.def_kind(*def_id), DefKind::Fn | DefKind::AssocFn))
+        .map(|(segment, def_id)| {
+            let parent_args = if let Some(parent_args) = parent_args {
+                parent_args
             } else {
-                &[]
-            }
-        };
-
-        let args = lowerer
-            .lower_generic_args_of_path(
-                segment.ident.span,
-                def_id,
-                parent_args,
-                segment,
-                None,
-                GenericArgPosition::Value,
-            )
-            .0;
-
-        &args[parent_args.len()..]
-    });
+                let parent = tcx.parent(def_id);
+                if matches!(tcx.def_kind(parent), DefKind::Trait) {
+                    ty::GenericArgs::identity_for_item(tcx, parent).as_slice()
+                } else {
+                    &[]
+                }
+            };
+
+            let args = lowerer
+                .lower_generic_args_of_path(
+                    segment.ident.span,
+                    def_id,
+                    parent_args,
+                    segment,
+                    None,
+                    GenericArgPosition::Value,
+                )
+                .0;
+
+            &args[parent_args.len()..]
+        });
 
     (parent_args.unwrap_or_default(), child_args.unwrap_or_default())
 }