Add tests involving generics

Author: bluebear94

tests/ui/default_box_assignments.fixed

@@ -1,5 +1,20 @@
 #![warn(clippy::default_box_assignments)]
 
+fn with_default<T: Default>(b: &mut Box<T>) {
+    *(*b) = T::default();
+    //~^ default_box_assignments
+}
+
+fn with_sized<T>(b: &mut Box<T>, t: T) {
+    *(*b) = t;
+    //~^ default_box_assignments
+}
+
+fn with_unsized<const N: usize>(b: &mut Box<[u32]>) {
+    // No lint for assigning to Box<T> where T: !Default
+    *b = Box::new([42; N]);
+}
+
 macro_rules! create_default {
     () => {
         Default::default()

tests/ui/default_box_assignments.rs

@@ -1,5 +1,20 @@
 #![warn(clippy::default_box_assignments)]
 
+fn with_default<T: Default>(b: &mut Box<T>) {
+    *b = Box::new(T::default());
+    //~^ default_box_assignments
+}
+
+fn with_sized<T>(b: &mut Box<T>, t: T) {
+    *b = Box::new(t);
+    //~^ default_box_assignments
+}
+
+fn with_unsized<const N: usize>(b: &mut Box<[u32]>) {
+    // No lint for assigning to Box<T> where T: !Default
+    *b = Box::new([42; N]);
+}
+
 macro_rules! create_default {
     () => {
         Default::default()

tests/ui/default_box_assignments.stderr

@@ -1,28 +1,44 @@
+error: creating a new box
+  --> tests/ui/default_box_assignments.rs:4:5
+   |
+LL |     *b = Box::new(T::default());
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*(*b) = T::default()`
+   |
+   = note: this creates a needless allocation
+   = note: `-D clippy::default-box-assignments` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::default_box_assignments)]`
+
+error: creating a new box
+  --> tests/ui/default_box_assignments.rs:9:5
+   |
+LL |     *b = Box::new(t);
+   |     ^^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*(*b) = t`
+   |
+   = note: this creates a needless allocation
+
 error: creating a new box with default content
-  --> tests/ui/default_box_assignments.rs:11:5
+  --> tests/ui/default_box_assignments.rs:26:5
    |
 LL |     b = Default::default();
    |     ^^^^^^^^^^^^^^^^^^^^^^ help: replace existing content with default instead: `*b = Default::default()`
    |
    = note: this creates a needless allocation
-   = note: `-D clippy::default-box-assignments` implied by `-D warnings`
-   = help: to override `-D warnings` add `#[allow(clippy::default_box_assignments)]`
 
 error: creating a new box with default content
-  --> tests/ui/default_box_assignments.rs:13:5
+  --> tests/ui/default_box_assignments.rs:28:5
    |
 LL |     b = Box::default();
    |     ^^^^^^^^^^^^^^^^^^ help: replace existing content with default instead: `*b = Default::default()`
    |
    = note: this creates a needless allocation
 
 error: creating a new box
-  --> tests/ui/default_box_assignments.rs:23:5
+  --> tests/ui/default_box_assignments.rs:38:5
    |
 LL |     b = Box::new(5);
    |     ^^^^^^^^^^^^^^^ help: replace existing content with inner value instead: `*b = 5`
    |
    = note: this creates a needless allocation
 
-error: aborting due to 3 previous errors
+error: aborting due to 5 previous errors