Add last() to Vec (#6870)

## Description

`last()` is not implemented for `Vec`.

## Checklist

- [x] I have linked to any relevant issues.
- [x] I have commented my code, particularly in hard-to-understand
areas.
- [x] I have updated the documentation where relevant (API docs, the
reference, and the Sway book).
- [x] If my change requires substantial documentation changes, I have
[requested support from the DevRel
team](https://github.com/FuelLabs/devrel-requests/issues/new/choose)
- [x] I have added tests that prove my fix is effective or that my
feature works.
- [x] I have added (or requested a maintainer to add) the necessary
`Breaking*` or `New Feature` labels where relevant.
- [x] I have done my best to ensure that my PR adheres to [the Fuel Labs
Code Review
Standards](https://github.com/FuelLabs/rfcs/blob/master/text/code-standards/external-contributors.md).
- [x] I have requested a review from the relevant team or maintainers.

Co-authored-by: Joshua Batty <joshpbatty@gmail.com>

Author: bitzoic

sway-lib-std/src/vec.sw

@@ -753,6 +753,32 @@ impl<T> Vec<T> {
 
         self.len = new_len;
     }
+
+    /// Returns the last element in the `Vec`.
+    ///
+    /// # Returns
+    ///
+    /// [Option<T>] - The last element in the `Vec` or `None`.
+    ///
+    /// # Examples
+    ///
+    /// ```sway
+    /// fn foo() {
+    ///     let mut vec = Vec::new();
+    ///     assert(vec.last() == None);
+    ///     vec.push(1u64);
+    ///     assert(vec.last() == Some(1u64));
+    ///     vec.push(2u64);
+    ///     assert(vec.last() == Some(2u64));
+    /// }
+    /// ```
+    pub fn last(self) -> Option<T> {
+        if self.len == 0 {
+            return None;
+        }
+
+        Some(self.buf.ptr().add::<T>(self.len - 1).read::<T>())
+    }
 }
 
 impl<T> AsRawSlice for Vec<T> {

test/src/in_language_tests/test_programs/vec_inline_tests/src/main.sw

@@ -805,3 +805,21 @@ fn vec_resize() {
     assert(vec_2.get(1) == Some(1));
     assert(vec_2.get(2) == Some(1));
 }
+
+#[test]
+fn vec_last() {
+    let (mut vec_1, a, b, c) = setup();
+    assert(vec_1.last() == Some(9));
+
+    vec_1.push(2);
+    assert(vec_1.last() == Some(2));
+
+    vec_1.push(3);
+    assert(vec_1.last() == Some(3));
+
+    let _ = vec_1.pop();
+    assert(vec_1.last() == Some(2));
+
+    let vec_2: Vec<u64> = Vec::new();
+    assert(vec_2.last() == None)
+}