Add test for Box<Vec<T>> with known layout

Author: mistaste

tests/ui/unnecessary_box_returns.rs

@@ -81,6 +81,13 @@ fn generic_element_array<T>(value: Box<[T; 10]>) -> Box<[T; 10]> {
     value
 }
 
+// lint: the size of `Vec<T>` is known regardless of the generic element type
+#[expect(clippy::box_collection)]
+fn generic_vec<T>(value: Box<Vec<T>>) -> Box<Vec<T>> {
+    //~^ unnecessary_box_returns
+    value
+}
+
 fn main() {
     // don't lint: this is a closure
     let a = || -> Box<usize> { Box::new(5) };

tests/ui/unnecessary_box_returns.stderr

@@ -32,5 +32,13 @@ LL | fn _bxed_foo() -> Box<Foo> {
    |
    = help: changing this also requires a change to the return expressions in this function
 
-error: aborting due to 4 previous errors
+error: boxed return of the sized type `std::vec::Vec<T>`
+  --> tests/ui/unnecessary_box_returns.rs:86:42
+   |
+LL | fn generic_vec<T>(value: Box<Vec<T>>) -> Box<Vec<T>> {
+   |                                          ^^^^^^^^^^^ help: try: `std::vec::Vec<T>`
+   |
+   = help: changing this also requires a change to the return expressions in this function
+
+error: aborting due to 5 previous errors