confirm assignment goes into the return place

Author: estebank

compiler/rustc_lint/src/default_could_be_derived.rs

@@ -481,15 +481,16 @@ fn check_path<'tcx>(
     let instance = Instance::try_resolve(cx.tcx, cx.typing_env(), default_fn_def_id, args);
 
     let Ok(Some(instance)) = instance else { return false };
-    // Get the MIR Body for the `<Ty as Default>::default()` function.
+    // Get the MIR Body for the `<FieldTy as Default>::default()` function.
     // If it is a value or call (either fn or ctor), we compare its DefId against the one for the
-    // resolution of the expression we had in the path.
+    // resolution of the expression we had in the path. This lets us identify, for example, that
+    // the body of `<Vec<T> as Default>::default()` is a `Vec::new()`, and the field was being
+    // initialized to `Vec::new()` as well.
     let body = cx.tcx.instance_mir(instance.def);
     for block_data in body.basic_blocks.iter() {
-        // FIXME: make another pass on this logic. For example we should be confirming that the
-        // place we assign to is `_0`.
         if block_data.statements.len() == 1
             && let mir::StatementKind::Assign(assign) = &block_data.statements[0].kind
+            && assign.0.local == mir::RETURN_PLACE
             && let mir::Rvalue::Aggregate(kind, _places) = &assign.1
             && let mir::AggregateKind::Adt(did, variant_index, _, _, _) = &**kind
             && let def = cx.tcx.adt_def(did)