[External] [mojo-stdlib] Add List.count(value) method (#38915)

[External] [mojo-stdlib] Add `List.count(value)` method

Add `List.count(value)` method to count the number of occurrences of a
value in the `List`.

Fixes #2263

---------

Co-authored-by: Kaushal Phulgirkar <kaushalpp01@gmail.com>
Closes #2406
MODULAR_ORIG_COMMIT_REV_ID: e9e9be6db11d56f951e94daf22fd0ff8520615fa

stdlib/src/collections/list.mojo

@@ -20,7 +20,6 @@ from collections import List
 """
 
 
-from builtin.value import StringableCollectionElement
 from memory import UnsafePointer, Reference
 from memory.unsafe_pointer import move_pointee, move_from_pointee
 from .optional import Optional
@@ -658,3 +657,34 @@ struct List[T: CollectionElement](CollectionElement, Sized, Boolable):
                 result += ", "
         result += "]"
         return result
+
+    @staticmethod
+    fn count[T: ComparableCollectionElement](self: List[T], value: T) -> Int:
+        """Counts the number of occurrences of a value in the list.
+        Note that since we can't condition methods on a trait yet,
+        the way to call this method is a bit special. Here is an example below.
+
+        ```mojo
+        var my_list = List[Int](1, 2, 3)
+        print(__type_of(my_list).count(my_list, 1))
+        ```
+
+        When the compiler supports conditional methods, then a simple `my_list.count(1)` will
+        be enough.
+
+        Parameters:
+            T: The type of the elements in the list. Must implement the
+              traits `EqualityComparable` and `CollectionElement`.
+
+        Args:
+            self: The list to search.
+            value: The value to count.
+
+        Returns:
+            The number of occurrences of the value in the list.
+        """
+        var count = 0
+        for elem in self:
+            if elem[] == value:
+                count += 1
+        return count

stdlib/test/collections/test_list.mojo

@@ -679,6 +679,16 @@ def test_converting_list_to_string():
     )
 
 
+def test_list_count():
+    var list = List[Int](1, 2, 3, 2, 5, 6, 7, 8, 9, 10)
+    assert_equal(1, __type_of(list).count(list, 1))
+    assert_equal(2, __type_of(list).count(list, 2))
+    assert_equal(0, __type_of(list).count(list, 4))
+
+    var list2 = List[Int]()
+    assert_equal(0, __type_of(list2).count(list2, 1))
+
+
 def main():
     test_mojo_issue_698()
     test_list()
@@ -703,3 +713,4 @@ def main():
     test_constructor_from_pointer()
     test_constructor_from_other_list_through_pointer()
     test_converting_list_to_string()
+    test_list_count()