add regression test for rust-lang#54124

Fixes rust-lang#54124

Author: nikomatsakis

src/test/ui/nll/user-annotations/issue-54124.rs

@@ -0,0 +1,9 @@
+#![feature(nll)]
+
+fn test<'a>() {
+    let _:fn(&()) = |_:&'a ()| {};
+}
+
+fn main() {
+    test();
+}

src/test/ui/nll/user-annotations/issue-54124.stderr

@@ -0,0 +1,20 @@
+error: unsatisfied lifetime constraints
+  --> $DIR/issue-54124.rs:4:22
+   |
+LL | fn test<'a>() {
+   |         -- lifetime `'a` defined here
+LL |     let _:fn(&()) = |_:&'a ()| {};
+   |                      ^ - let's call the lifetime of this reference `'1`
+   |                      |
+   |                      requires that `'1` must outlive `'a`
+
+error: unsatisfied lifetime constraints
+  --> $DIR/issue-54124.rs:4:22
+   |
+LL | fn test<'a>() {
+   |         -- lifetime `'a` defined here
+LL |     let _:fn(&()) = |_:&'a ()| {};
+   |                      ^ requires that `'a` must outlive `'static`
+
+error: aborting due to 2 previous errors
+