Auto merge of #9511 - rust-lang:box-default, r=Alexendoo

add `box-default` lint

This adds a `box-default` lint to suggest using `Box::default()` instead of `Box::new(Default::default())`, which offers less moving parts and potentially better performance according to [the perf book](https://nnethercote.github.io/perf-book/standard-library-types.html#box).

---

changelog: add [`box_default`] lint

tests/ui/box_collection.rs

@@ -15,7 +15,7 @@ macro_rules! boxit {
 }
 
 fn test_macro() {
-    boxit!(Vec::new(), Vec<u8>);
+    boxit!(vec![1], Vec<u8>);
 }
 
 fn test1(foo: Box<Vec<bool>>) {}
@@ -50,7 +50,7 @@ fn test_local_not_linted() {
 pub fn pub_test(foo: Box<Vec<bool>>) {}
 
 pub fn pub_test_ret() -> Box<Vec<bool>> {
-    Box::new(Vec::new())
+    Box::default()
 }
 
 fn main() {}

tests/ui/box_default.rs

@@ -0,0 +1,31 @@
+#![warn(clippy::box_default)]
+
+#[derive(Default)]
+struct ImplementsDefault;
+
+struct OwnDefault;
+
+impl OwnDefault {
+    fn default() -> Self {
+        Self
+    }
+}
+
+macro_rules! outer {
+    ($e: expr) => {
+        $e
+    };
+}
+
+fn main() {
+    let _string: Box<String> = Box::new(Default::default());
+    let _byte = Box::new(u8::default());
+    let _vec = Box::new(Vec::<u8>::new());
+    let _impl = Box::new(ImplementsDefault::default());
+    let _impl2 = Box::new(<ImplementsDefault as Default>::default());
+    let _impl3: Box<ImplementsDefault> = Box::new(Default::default());
+    let _own = Box::new(OwnDefault::default()); // should not lint
+    let _in_macro = outer!(Box::new(String::new()));
+    // false negative: default is from different expansion
+    let _vec2: Box<Vec<ImplementsDefault>> = Box::new(vec![]);
+}

tests/ui/box_default.stderr

@@ -0,0 +1,59 @@
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:21:32
+   |
+LL |     let _string: Box<String> = Box::new(Default::default());
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::box-default` implied by `-D warnings`
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:22:17
+   |
+LL |     let _byte = Box::new(u8::default());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:23:16
+   |
+LL |     let _vec = Box::new(Vec::<u8>::new());
+   |                ^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:24:17
+   |
+LL |     let _impl = Box::new(ImplementsDefault::default());
+   |                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:25:18
+   |
+LL |     let _impl2 = Box::new(<ImplementsDefault as Default>::default());
+   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:26:42
+   |
+LL |     let _impl3: Box<ImplementsDefault> = Box::new(Default::default());
+   |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: `Box::new(_)` of default value
+  --> $DIR/box_default.rs:28:28
+   |
+LL |     let _in_macro = outer!(Box::new(String::new()));
+   |                            ^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = help: use `Box::default()` instead
+
+error: aborting due to 7 previous errors
+