tests: Add borrowck/var-matching-lifetime-but-unused-not-mentioned.rs

So that the impact of the commit that fixes the test can be seen
clearly.

tests/ui/borrowck/var-matching-lifetime-but-unused-not-mentioned.rs

@@ -0,0 +1,17 @@
+//! Regression test for <https://github.com/rust-lang/rust/issues/113121>.
+
+#![allow(unused_variables)]
+
+fn consume<T: 'static>(_: T) {}
+
+fn foo<'a>(
+    used_arg: &'a u8,
+    unused_arg: &'static u16, // Unused in closure. Must not appear in error.
+) {
+    let unused_var: &'static u32 = &42; // Unused in closure. Must not appear in error.
+
+    let c = move || used_arg;
+    consume(c); //~ ERROR: borrowed data escapes outside of function
+}
+
+fn main() {}

tests/ui/borrowck/var-matching-lifetime-but-unused-not-mentioned.stderr

@@ -0,0 +1,19 @@
+error[E0521]: borrowed data escapes outside of function
+  --> $DIR/var-matching-lifetime-but-unused-not-mentioned.rs:14:5
+   |
+LL | fn foo<'a>(
+   |        -- lifetime `'a` defined here
+LL |     used_arg: &'a u8,
+   |     -------- `used_arg` is a reference that is only valid in the function body
+LL |     unused_arg: &'static u16, // Unused in closure. Must not appear in error.
+   |     ---------- `unused_arg` declared here, outside of the function body
+...
+LL |     consume(c);
+   |     ^^^^^^^^^^
+   |     |
+   |     `used_arg` escapes the function body here
+   |     argument requires that `'a` must outlive `'static`
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0521`.