lint: only consider actual calls as unconditional recursion.

Specifically, just mentioning the function name as a value is fine, as
long as it isn't called, e.g. `fn main() { let _ = main; }`.

Closes #21705.

src/librustc_lint/builtin.rs

@@ -1973,9 +1973,14 @@ impl LintPass for UnconditionalRecursion {
                                       fn_id: ast::NodeId,
                                       _: ast::Ident,
                                       id: ast::NodeId) -> bool {
-            tcx.def_map.borrow().get(&id)
+            match tcx.map.get(id) {
+                ast_map::NodeExpr(&ast::Expr { node: ast::ExprCall(ref callee, _), .. }) => {
+                    tcx.def_map.borrow().get(&callee.id)
                         .map_or(false, |def| def.def_id() == local_def(fn_id))
                 }
+                _ => false
+            }
+        }
 
         // check if the method call `id` refers to method `method_id`
         // (with name `method_name` contained in impl `impl_id`).

src/test/compile-fail/lint-unconditional-recursion.rs

@@ -63,4 +63,8 @@ impl Baz {
     }
 }
 
+fn all_fine() {
+    let _f = all_fine;
+}
+
 fn main() {}