feat(cordyceps): add CursorMut::as_cursor (#244)

This commit adds an `as_cursor` method to `cordyceps::list::CursorMut`
that temporarily borrows the `CursorMut` as an immutable
`Cursor`. This is the same as the `as_cursor` method on
`std::collections::linked_list`'s `CursorMut` type.

cordyceps/src/list/cursor.rs

@@ -426,6 +426,22 @@ impl<'list, T: Linked<Links<T>> + ?Sized> CursorMut<'list, T> {
 
         self.core.index += splice_len;
     }
+
+    /// Returns a read-only cursor pointing to the current element.
+    ///
+    /// The lifetime of the returned [`Cursor`] is bound to that of the
+    /// `CursorMut`, which means it cannot outlive the `CursorMut` and that the
+    /// `CursorMut` is frozen for the lifetime of the [`Cursor`].
+    #[must_use]
+    pub fn as_cursor(&self) -> Cursor<'_, T> {
+        Cursor {
+            core: CursorCore {
+                list: self.core.list,
+                curr: self.core.curr,
+                index: self.core.index,
+            },
+        }
+    }
 }
 
 impl<T: Linked<Links<T>> + ?Sized> fmt::Debug for CursorMut<'_, T> {

cordyceps/src/list/tests/cursor.rs

@@ -4,7 +4,7 @@ use super::*;
 /// interface:
 /// https://github.com/rust-lang/rust/blob/ec21d7ea3ca8e96863f175fbd4a6bfee79529d6c/library/alloc/src/collections/linked_list/tests.rs#L564-L655
 #[test]
-fn move_peek() {
+fn move_peek_front() {
     let _ = super::trace_init();
 
     let entries = [entry(1), entry(2), entry(3), entry(4), entry(5), entry(6)];
@@ -26,6 +26,17 @@ fn move_peek() {
     assert_eq!(val(cursor.peek_next()), Some(3));
     assert_eq!(val(cursor.peek_prev()), Some(1));
     assert_eq!(cursor.index(), Some(1));
+}
+
+/// Based on this test from the standard library's `linked_list::Cursor`
+/// interface:
+/// https://github.com/rust-lang/rust/blob/ec21d7ea3ca8e96863f175fbd4a6bfee79529d6c/library/alloc/src/collections/linked_list/tests.rs#L564-L655
+#[test]
+fn move_peek_back() {
+    let _ = super::trace_init();
+
+    let entries = [entry(1), entry(2), entry(3), entry(4), entry(5), entry(6)];
+    let mut list = list_from_iter(&entries);
 
     let mut cursor = list.cursor_back_mut();
     assert_eq!(val(cursor.current()), Some(6));
@@ -43,60 +54,76 @@ fn move_peek() {
     assert_eq!(val(cursor.peek_next()), Some(6));
     assert_eq!(val(cursor.peek_prev()), Some(4));
     assert_eq!(cursor.index(), Some(4));
+}
+
+/// Based on this test from the standard library's `linked_list::Cursor`
+/// interface:
+/// https://github.com/rust-lang/rust/blob/ec21d7ea3ca8e96863f175fbd4a6bfee79529d6c/library/alloc/src/collections/linked_list/tests.rs#L564-L655
+#[test]
+fn as_cursor_move_peek_front() {
+    let _ = super::trace_init();
 
-    // let mut m: LinkedList<u32> = LinkedList::new();
-    // m.extend(&[1, 2, 3, 4, 5, 6]);
-    // let mut cursor = m.cursor_front_mut();
-    // assert_eq!(val(cursor.current()), Some(&mut 1));
-    // assert_eq!(val(cursor.peek_next()), Some(&mut 2));
-    // assert_eq!(val(cursor.peek_prev()), None);
-    // assert_eq!(cursor.index(), Some(0));
-    // cursor.move_prev();
-    // assert_eq!(val(cursor.current()), None);
-    // assert_eq!(val(cursor.peek_next()), Some(&mut 1));
-    // assert_eq!(val(cursor.peek_prev()), Some(&mut 6));
-    // assert_eq!(cursor.index(), None);
-    // cursor.move_next();
-    // cursor.move_next();
-    // assert_eq!(val(cursor.current()), Some(&mut 2));
-    // assert_eq!(val(cursor.peek_next()), Some(&mut 3));
-    // assert_eq!(val(cursor.peek_prev()), Some(&mut 1));
-    // assert_eq!(cursor.index(), Some(1));
-    // let mut cursor2 = cursor.as_cursor();
-    // assert_eq!(cursor2.current(), Some(&2));
-    // assert_eq!(cursor2.index(), Some(1));
-    // cursor2.move_next();
-    // assert_eq!(cursor2.current(), Some(&3));
-    // assert_eq!(cursor2.index(), Some(2));
-    // assert_eq!(val(cursor.current()), Some(&mut 2));
-    // assert_eq!(cursor.index(), Some(1));
-
-    // let mut m: LinkedList<u32> = LinkedList::new();
-    // m.extend(&[1, 2, 3, 4, 5, 6]);
-    // let mut cursor = m.cursor_back_mut();
-    // assert_eq!(val(cursor.current()), Some(&mut 6));
-    // assert_eq!(val(cursor.peek_next()), None);
-    // assert_eq!(val(cursor.peek_prev()), Some(&mut 5));
-    // assert_eq!(cursor.index(), Some(5));
-    // cursor.move_next();
-    // assert_eq!(val(cursor.current()), None);
-    // assert_eq!(val(cursor.peek_next()), Some(&mut 1));
-    // assert_eq!(val(cursor.peek_prev()), Some(&mut 6));
-    // assert_eq!(cursor.index(), None);
-    // cursor.move_prev();
-    // cursor.move_prev();
-    // assert_eq!(val(cursor.current()), Some(&mut 5));
-    // assert_eq!(val(cursor.peek_next()), Some(&mut 6));
-    // assert_eq!(val(cursor.peek_prev()), Some(&mut 4));
-    // assert_eq!(cursor.index(), Some(4));
-    // let mut cursor2 = cursor.as_cursor();
-    // assert_eq!(cursor2.current(), Some(&5));
-    // assert_eq!(cursor2.index(), Some(4));
-    // cursor2.move_prev();
-    // assert_eq!(cursor2.current(), Some(&4));
-    // assert_eq!(cursor2.index(), Some(3));
-    // assert_eq!(val(cursor.current()), Some(&mut 5));
-    // assert_eq!(cursor.index(), Some(4));
+    let entries = [entry(1), entry(2), entry(3), entry(4), entry(5), entry(6)];
+    let mut list = list_from_iter(&entries);
+    let mut cursor = list.cursor_front_mut();
+    assert_eq!(val(cursor.current()), Some(1));
+    assert_eq!(val(cursor.peek_next()), Some(2));
+    assert_eq!(val(cursor.peek_prev()), None);
+    assert_eq!(cursor.index(), Some(0));
+    cursor.move_prev();
+    assert_eq!(val(cursor.current()), None);
+    assert_eq!(val(cursor.peek_next()), Some(1));
+    assert_eq!(val(cursor.peek_prev()), Some(6));
+    assert_eq!(cursor.index(), None);
+    cursor.move_next();
+    cursor.move_next();
+    assert_eq!(val(cursor.current()), Some(2));
+    assert_eq!(val(cursor.peek_next()), Some(3));
+    assert_eq!(val(cursor.peek_prev()), Some(1));
+    assert_eq!(cursor.index(), Some(1));
+    let mut cursor2 = cursor.as_cursor();
+    assert_eq!(val(cursor2.current()), Some(2));
+    assert_eq!(cursor2.index(), Some(1));
+    cursor2.move_next();
+    assert_eq!(val(cursor2.current()), Some(3));
+    assert_eq!(cursor2.index(), Some(2));
+    assert_eq!(val(cursor.current()), Some(2));
+    assert_eq!(cursor.index(), Some(1));
+}
+
+/// Based on this test from the standard library's `linked_list::Cursor`
+/// interface:
+/// https://github.com/rust-lang/rust/blob/ec21d7ea3ca8e96863f175fbd4a6bfee79529d6c/library/alloc/src/collections/linked_list/tests.rs#L564-L655
+#[test]
+fn as_cursor_move_peek_back() {
+    let _ = super::trace_init();
+
+    let entries = [entry(1), entry(2), entry(3), entry(4), entry(5), entry(6)];
+    let mut list = list_from_iter(&entries);
+    let mut cursor = list.cursor_back_mut();
+    assert_eq!(val(cursor.current()), Some(6));
+    assert_eq!(val(cursor.peek_next()), None);
+    assert_eq!(val(cursor.peek_prev()), Some(5));
+    assert_eq!(cursor.index(), Some(5));
+    cursor.move_next();
+    assert_eq!(val(cursor.current()), None);
+    assert_eq!(val(cursor.peek_next()), Some(1));
+    assert_eq!(val(cursor.peek_prev()), Some(6));
+    assert_eq!(cursor.index(), None);
+    cursor.move_prev();
+    cursor.move_prev();
+    assert_eq!(val(cursor.current()), Some(5));
+    assert_eq!(val(cursor.peek_next()), Some(6));
+    assert_eq!(val(cursor.peek_prev()), Some(4));
+    assert_eq!(cursor.index(), Some(4));
+    let mut cursor2 = cursor.as_cursor();
+    assert_eq!(val(cursor2.current()), Some(5));
+    assert_eq!(cursor2.index(), Some(4));
+    cursor2.move_prev();
+    assert_eq!(val(cursor2.current()), Some(4));
+    assert_eq!(cursor2.index(), Some(3));
+    assert_eq!(val(cursor.current()), Some(5));
+    assert_eq!(cursor.index(), Some(4));
 }
 
 /// Based on this test from the standard library's `linked_list::CursorMut`