Extend support to unit-structs and add comments

Author: estebank

compiler/rustc_lint/src/default_could_be_derived.rs

@@ -242,6 +242,20 @@ impl<'tcx> LateLintPass<'tcx> for DefaultCouldBeDerived {
                     {
                         let type_def_id = cx.tcx.parent(ctor_def_id); // From Ctor to struct
                         if args.iter().all(|expr| check_expr(cx.tcx, expr.kind)) {
+                            // We have a struct literal
+                            //
+                            // struct Foo(Type);
+                            //
+                            // impl Default for Foo {
+                            //     fn default() -> Foo {
+                            //         Foo(val)
+                            //     }
+                            // }
+                            //
+                            // We suggest #[derive(Default)] if
+                            //  - `val` is `Default::default()`
+                            //  - `val` is `0`
+                            //  - `val` is `false`
                             cx.tcx.node_span_lint(
                                 DEFAULT_COULD_BE_DERIVED,
                                 item.hir_id(),
@@ -264,6 +278,41 @@ impl<'tcx> LateLintPass<'tcx> for DefaultCouldBeDerived {
                         }
                     }
                 }
+                hir::ExprKind::Path(hir::QPath::Resolved(_, path))
+                    if let Res::Def(DefKind::Ctor(CtorOf::Struct, CtorKind::Const), _) =
+                        path.res =>
+                {
+                    // We have a struct literal
+                    //
+                    // struct Foo;
+                    //
+                    // impl Default for Foo {
+                    //     fn default() -> Foo {
+                    //         Foo
+                    //     }
+                    // }
+                    //
+                    // We always suggest `#[derive(Default)]`.
+                    cx.tcx.node_span_lint(
+                        DEFAULT_COULD_BE_DERIVED,
+                        item.hir_id(),
+                        item.span,
+                        |diag| {
+                            diag.primary_message("`impl Default` that could be derived");
+                            diag.multipart_suggestion_verbose(
+                                "you don't need to manually `impl Default`, you can derive it",
+                                vec![
+                                    (
+                                        cx.tcx.def_span(type_def_id).shrink_to_lo(),
+                                        "#[derive(Default)] ".to_string(),
+                                    ),
+                                    (item.span, String::new()),
+                                ],
+                                Applicability::MachineApplicable,
+                            );
+                        },
+                    );
+                }
                 _ => {}
             }
         }

tests/ui/structs/manual-default-impl-could-be-derived.fixed

@@ -4,13 +4,10 @@
 #![deny(default_could_be_derived)]
 #![feature(default_field_values)]
 
-// #[derive(Debug)]
-// struct A;
-// 
-// impl Default for A {
-//     fn default() -> Self { A }
-// }
-// 
+#[derive(Debug)]
+#[derive(Default)] struct A;
+
+
 #[derive(Debug)]
 #[derive(Default)] struct B(Option<i32>);
 
@@ -63,9 +60,9 @@
 
 
 fn main() {
-//    let _ = A::default();
-//    let _ = B::default();
-//    let _ = C::default();
+    let _ = A::default();
+    let _ = B::default();
+    let _ = C::default();
     let _ = D::default();
     let _ = E::default();
     let _ = F::<i32>::default();

tests/ui/structs/manual-default-impl-could-be-derived.rs

@@ -4,13 +4,13 @@
 #![deny(default_could_be_derived)]
 #![feature(default_field_values)]
 
-// #[derive(Debug)]
-// struct A;
-// 
-// impl Default for A {
-//     fn default() -> Self { A }
-// }
-// 
+#[derive(Debug)]
+struct A;
+
+impl Default for A { //~ ERROR
+    fn default() -> Self { A }
+}
+
 #[derive(Debug)]
 struct B(Option<i32>);
 
@@ -111,7 +111,7 @@ impl Default for I { //~ ERROR
 }
 
 fn main() {
-//    let _ = A::default();
+    let _ = A::default();
     let _ = B::default();
     let _ = C::default();
     let _ = D::default();

tests/ui/structs/manual-default-impl-could-be-derived.stderr

@@ -1,8 +1,8 @@
 error: `impl Default` that could be derived
-  --> $DIR/manual-default-impl-could-be-derived.rs:17:1
+  --> $DIR/manual-default-impl-could-be-derived.rs:10:1
    |
-LL | / impl Default for B {
-LL | |     fn default() -> Self { B(Default::default()) }
+LL | / impl Default for A {
+LL | |     fn default() -> Self { A }
 LL | | }
    | |_^
    |
@@ -13,6 +13,20 @@ LL | #![deny(default_could_be_derived)]
    |         ^^^^^^^^^^^^^^^^^^^^^^^^
 help: you don't need to manually `impl Default`, you can derive it
    |
+LL - struct A;
+LL + #[derive(Default)] struct A;
+   |
+
+error: `impl Default` that could be derived
+  --> $DIR/manual-default-impl-could-be-derived.rs:17:1
+   |
+LL | / impl Default for B {
+LL | |     fn default() -> Self { B(Default::default()) }
+LL | | }
+   | |_^
+   |
+help: you don't need to manually `impl Default`, you can derive it
+   |
 LL - struct B(Option<i32>);
 LL + #[derive(Default)] struct B(Option<i32>);
    |
@@ -144,5 +158,5 @@ help: to avoid divergence in behavior between `Struct { .. }` and `<Struct as De
 LL ~ #[derive(Default)] struct I {
    |
 
-error: aborting due to 8 previous errors
+error: aborting due to 9 previous errors