add more test cases

clippy_lints/src/default_constructed_unit_structs.rs

@@ -42,14 +42,14 @@ impl LateLintPass<'_> for DefaultConstructedUnitStructs {
         if_chain!(
             // make sure we have a call to `Default::default`
             if let hir::ExprKind::Call(fn_expr, &[]) = expr.kind;
-            if let ExprKind::Path(ref qpath) = fn_expr.kind;
+            if let ExprKind::Path(ref qpath@ hir::QPath::TypeRelative(_,_)) = fn_expr.kind;
             if let Res::Def(_, def_id) = cx.qpath_res(qpath, fn_expr.hir_id);
             if match_def_path(cx, def_id, &paths::DEFAULT_TRAIT_METHOD);
             // make sure we have a struct with no fields (unit struct)
             if let ty::Adt(def, ..) = cx.typeck_results().expr_ty(expr).kind();
-            if def.is_struct() && def.is_payloadfree()
-                && !def.non_enum_variant().is_field_list_non_exhaustive()
-                && !is_from_proc_macro(cx, expr);
+            if def.is_struct();
+            if let var @ ty::VariantDef { ctor: Some((hir::def::CtorKind::Const, _)), .. } = def.non_enum_variant();
+            if !var.is_field_list_non_exhaustive() && !is_from_proc_macro(cx, expr);
             then {
                 span_lint_and_sugg(
                     cx,

tests/ui/default_constructed_unit_structs.rs

@@ -5,9 +5,23 @@ use std::marker::PhantomData;
 #[derive(Default)]
 struct UnitStruct;
 
+impl UnitStruct {
+    fn new() -> Self {
+        //should lint
+        Self::default()
+    }
+}
+
 #[derive(Default)]
 struct TupleStruct(usize);
 
+impl TupleStruct {
+    fn new() -> Self {
+        // should not lint
+        Self(Default::default())
+    }
+}
+
 // no lint for derived impl
 #[derive(Default)]
 struct NormalStruct {
@@ -39,6 +53,13 @@ impl NormalStruct {
             inner: PhantomData::default(),
         }
     }
+
+    fn new2() -> Self {
+        // should not lint
+        Self {
+            inner: Default::default(),
+        }
+    }
 }
 
 #[derive(Default)]
@@ -51,8 +72,29 @@ impl<T: Default> GenericStruct<T> {
         // should not lint
         Self { t: T::default() }
     }
+
+    fn new2() -> Self {
+        // should not lint
+        Self { t: Default::default() }
+    }
 }
 
+struct FakeDefault;
+impl FakeDefault {
+    fn default() -> Self {
+        Self
+    }
+}
+
+impl Default for FakeDefault {
+    fn default() -> Self {
+        Self
+    }
+}
+
+#[derive(Default)]
+struct EmptyStruct {}
+
 #[derive(Default)]
 #[non_exhaustive]
 struct NonExhaustiveStruct;
@@ -69,4 +111,7 @@ fn main() {
     let _ = NonExhaustiveStruct::default();
     let _ = SomeEnum::default();
     let _ = NonDefaultStruct::default();
+    let _ = EmptyStruct::default();
+    let _ = FakeDefault::default();
+    let _ = <FakeDefault as Default>::default();
 }

tests/ui/default_constructed_unit_structs.stderr

@@ -1,28 +1,34 @@
 error: use of `default` to create a unit struct
-  --> $DIR/default_constructed_unit_structs.rs:39:31
+  --> $DIR/default_constructed_unit_structs.rs:11:13
    |
-LL |             inner: PhantomData::default(),
-   |                               ^^^^^^^^^^^ help: remove this call to `default`
+LL |         Self::default()
+   |             ^^^^^^^^^^^ help: remove this call to `default`
    |
    = note: `-D clippy::default-constructed-unit-structs` implied by `-D warnings`
 
 error: use of `default` to create a unit struct
-  --> $DIR/default_constructed_unit_structs.rs:62:33
+  --> $DIR/default_constructed_unit_structs.rs:53:31
+   |
+LL |             inner: PhantomData::default(),
+   |                               ^^^^^^^^^^^ help: remove this call to `default`
+
+error: use of `default` to create a unit struct
+  --> $DIR/default_constructed_unit_structs.rs:104:33
    |
 LL |     let _ = PhantomData::<usize>::default();
    |                                 ^^^^^^^^^^^ help: remove this call to `default`
 
 error: use of `default` to create a unit struct
-  --> $DIR/default_constructed_unit_structs.rs:63:42
+  --> $DIR/default_constructed_unit_structs.rs:105:42
    |
 LL |     let _: PhantomData<i32> = PhantomData::default();
    |                                          ^^^^^^^^^^^ help: remove this call to `default`
 
 error: use of `default` to create a unit struct
-  --> $DIR/default_constructed_unit_structs.rs:64:23
+  --> $DIR/default_constructed_unit_structs.rs:106:23
    |
 LL |     let _ = UnitStruct::default();
    |                       ^^^^^^^^^^^ help: remove this call to `default`
 
-error: aborting due to 4 previous errors
+error: aborting due to 5 previous errors