add test for closures

tests/ui/single_call_fn.rs

@@ -1,5 +1,5 @@
 //@aux-build:proc_macros.rs
-#![allow(unused)]
+#![allow(clippy::redundant_closure_call, unused)]
 #![warn(clippy::single_call_fn)]
 #![no_main]
 
@@ -9,6 +9,23 @@ extern crate proc_macros;
 // Do not lint since it's public
 pub fn f() {}
 
+fn i() {}
+fn j() {}
+
+fn h() {
+    // Linted
+    let a = i;
+    // Do not lint closures
+    let a = (|| {
+        // Not linted
+        a();
+        // Imo, it's reasonable to lint this as the function is still only being used once. Just in
+        // a closure.
+        j();
+    });
+    a();
+}
+
 fn g() {
     f();
 }

tests/ui/single_call_fn.stderr

@@ -1,18 +1,5 @@
 error: this function is only used once
-  --> $DIR/single_call_fn.rs:26:1
-   |
-LL | fn a() {}
-   | ^^^^^^^^^
-   |
-help: used here
-  --> $DIR/single_call_fn.rs:29:5
-   |
-LL |     a();
-   |     ^
-   = note: `-D clippy::single-call-fn` implied by `-D warnings`
-
-error: this function is only used once
-  --> $DIR/single_call_fn.rs:16:1
+  --> $DIR/single_call_fn.rs:33:1
    |
 LL | / fn c() {
 LL | |     println!("really");
@@ -22,10 +9,47 @@ LL | | }
    | |_^
    |
 help: used here
-  --> $DIR/single_call_fn.rs:23:5
+  --> $DIR/single_call_fn.rs:40:5
    |
 LL |     c();
    |     ^
+   = note: `-D clippy::single-call-fn` implied by `-D warnings`
 
-error: aborting due to 2 previous errors
+error: this function is only used once
+  --> $DIR/single_call_fn.rs:12:1
+   |
+LL | fn i() {}
+   | ^^^^^^^^^
+   |
+help: used here
+  --> $DIR/single_call_fn.rs:17:13
+   |
+LL |     let a = i;
+   |             ^
+
+error: this function is only used once
+  --> $DIR/single_call_fn.rs:43:1
+   |
+LL | fn a() {}
+   | ^^^^^^^^^
+   |
+help: used here
+  --> $DIR/single_call_fn.rs:46:5
+   |
+LL |     a();
+   |     ^
+
+error: this function is only used once
+  --> $DIR/single_call_fn.rs:13:1
+   |
+LL | fn j() {}
+   | ^^^^^^^^^
+   |
+help: used here
+  --> $DIR/single_call_fn.rs:24:9
+   |
+LL |         j();
+   |         ^
+
+error: aborting due to 4 previous errors