Add a test for the fix to issue #43058

Followed the instructions laid out here #43058 (comment)

src/test/ui/nll/issue-43058.rs

@@ -0,0 +1,26 @@
+#![feature(nll)]
+
+use std::borrow::Cow;
+
+#[derive(Clone, Debug)]
+struct S<'a> {
+    name: Cow<'a, str>
+}
+
+#[derive(Clone, Debug)]
+struct T<'a> {
+    s: Cow<'a, [S<'a>]>
+}
+
+fn main() {
+    let s1 = [S { name: Cow::Borrowed("Test1") }, S { name: Cow::Borrowed("Test2") }];
+    let b1 = T { s: Cow::Borrowed(&s1) };
+    let s2 = [S { name: Cow::Borrowed("Test3") }, S { name: Cow::Borrowed("Test4") }];
+    let b2 = T { s: Cow::Borrowed(&s2) };
+
+    let mut v = Vec::new();
+    v.push(b1);
+    v.push(b2);
+
+    println!("{:?}", v);
+}