Add From<PyReadwriteArray> for PyReadonlyArray

The ordering of drops is important for the dynamic checker, so this is a
convenience for conversion.

src/borrow/mod.rs

@@ -175,8 +175,8 @@ use crate::array::{PyArray, PyArrayMethods};
 use crate::convert::NpyIndex;
 use crate::dtype::Element;
 use crate::error::{BorrowError, NotContiguousError};
-use crate::untyped_array::PyUntypedArrayMethods;
 use crate::npyffi::flags;
+use crate::untyped_array::PyUntypedArrayMethods;
 
 use shared::{acquire, acquire_mut, release, release_mut};
 
@@ -454,6 +454,18 @@ where
         unsafe { &*(self as *const Self as *const Self::Target) }
     }
 }
+impl<'py, T, D> From<PyReadwriteArray<'py, T, D>> for PyReadonlyArray<'py, T, D>
+where
+    T: Element,
+    D: Dimension,
+{
+    fn from(value: PyReadwriteArray<'py, T, D>) -> Self {
+        let array = value.array.clone();
+        ::std::mem::drop(value);
+        Self::try_new(array)
+            .expect("releasing an exclusive reference should immediately permit a shared reference")
+    }
+}
 
 impl<'py, T: Element, D: Dimension> FromPyObject<'py> for PyReadwriteArray<'py, T, D> {
     fn extract_bound(obj: &Bound<'py, PyAny>) -> PyResult<Self> {
@@ -504,12 +516,13 @@ where
     ///
     /// [writeable]: https://numpy.org/doc/stable/reference/c-api/array.html#c.NPY_ARRAY_WRITEABLE
     /// [owndata]: https://numpy.org/doc/stable/reference/c-api/array.html#c.NPY_ARRAY_OWNDATA
-    pub fn make_nonwriteable(self) {
+    pub fn make_nonwriteable(self) -> PyReadonlyArray<'py, T, D> {
         // SAFETY: consuming the only extant mutable reference guarantees we cannot invalidate an
         // existing reference, nor allow the caller to keep hold of one.
         unsafe {
             (*self.as_array_ptr()).flags &= !flags::NPY_ARRAY_WRITEABLE;
         }
+        self.into()
     }
 }