handle clippy `new_without_default` warnings (#3971)

* handle clippy `new_without_default` warnings

* add newsfragment

newsfragments/3971.added.md

@@ -0,0 +1 @@
+Implement `Default` for `GILOnceCell`.

pyo3-ffi/src/cpython/object.rs

@@ -46,6 +46,7 @@ mod bufferinfo {
     }
 
     impl Py_buffer {
+        #[allow(clippy::new_without_default)]
         pub const fn new() -> Self {
             Py_buffer {
                 buf: ptr::null_mut(),

pyo3-ffi/src/pybuffer.rs

@@ -27,6 +27,7 @@ pub struct Py_buffer {
 }
 
 impl Py_buffer {
+    #[allow(clippy::new_without_default)]
     pub const fn new() -> Self {
         Py_buffer {
             buf: ptr::null_mut(),

src/impl_/pyclass/lazy_type_object.rs

@@ -32,6 +32,7 @@ struct LazyTypeObjectInner {
 
 impl<T> LazyTypeObject<T> {
     /// Creates an uninitialized `LazyTypeObject`.
+    #[allow(clippy::new_without_default)]
     pub const fn new() -> Self {
         LazyTypeObject(
             LazyTypeObjectInner {

src/sync.rs

@@ -90,6 +90,7 @@ unsafe impl<T> Sync for GILProtected<T> where T: Send {}
 /// }
 /// # Python::with_gil(|py| assert_eq!(get_shared_list(py).len(), 0));
 /// ```
+#[derive(Default)]
 pub struct GILOnceCell<T>(UnsafeCell<Option<T>>);
 
 // T: Send is needed for Sync because the thread which drops the GILOnceCell can be different