adds lint to detect construction of unit struct using default

Using `default` to construct a unit struct increases code complexity and
adds a function call. This can be avoided by simply removing the call to
`default` and simply construct by name.

tests/ui/default_constructed_unit_struct.rs

@@ -0,0 +1,72 @@
+#![allow(unused)]
+#![warn(clippy::default_constructed_unit_struct)]
+use std::marker::PhantomData;
+
+#[derive(Default)]
+struct UnitStruct;
+
+#[derive(Default)]
+struct TupleStruct(usize);
+
+// no lint for derived impl
+#[derive(Default)]
+struct NormalStruct {
+    inner: PhantomData<usize>,
+}
+
+struct NonDefaultStruct;
+
+impl NonDefaultStruct {
+    fn default() -> Self {
+        Self
+    }
+}
+
+#[derive(Default)]
+enum SomeEnum {
+    #[default]
+    Unit,
+    Tuple(UnitStruct),
+    Struct {
+        inner: usize,
+    },
+}
+
+impl NormalStruct {
+    fn new() -> Self {
+        // should lint
+        Self {
+            inner: PhantomData::default(),
+        }
+    }
+}
+
+#[derive(Default)]
+struct GenericStruct<T> {
+    t: T,
+}
+
+impl<T: Default> GenericStruct<T> {
+    fn new() -> Self {
+        // should not lint
+        Self { t: T::default() }
+    }
+}
+
+#[derive(Default)]
+#[non_exhaustive]
+struct NonExhaustiveStruct;
+
+fn main() {
+    // should lint
+    let _ = PhantomData::<usize>::default();
+    let _: PhantomData<i32> = PhantomData::default();
+    let _ = UnitStruct::default();
+
+    // should not lint
+    let _ = TupleStruct::default();
+    let _ = NormalStruct::default();
+    let _ = NonExhaustiveStruct::default();
+    let _ = SomeEnum::default();
+    let _ = NonDefaultStruct::default();
+}

tests/ui/default_constructed_unit_struct.stderr

@@ -0,0 +1,28 @@
+error: Use of `default` to create a unit struct.
+  --> $DIR/default_constructed_unit_struct.rs:39:33
+   |
+LL |             inner: PhantomData::default(),
+   |                                 ^^^^^^^ help: remove this call to `default`
+   |
+   = note: `-D clippy::default-constructed-unit-struct` implied by `-D warnings`
+
+error: Use of `default` to create a unit struct.
+  --> $DIR/default_constructed_unit_struct.rs:62:35
+   |
+LL |     let _ = PhantomData::<usize>::default();
+   |                                   ^^^^^^^ help: remove this call to `default`
+
+error: Use of `default` to create a unit struct.
+  --> $DIR/default_constructed_unit_struct.rs:63:44
+   |
+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_struct.rs:64:25
+   |
+LL |     let _ = UnitStruct::default();
+   |                         ^^^^^^^ help: remove this call to `default`
+
+error: aborting due to 4 previous errors
+