Lint about usage of format!("string literal")

tests/compile-fail/format.rs

@@ -0,0 +1,11 @@
+#![feature(plugin)]
+#![plugin(clippy)]
+#![deny(useless_format)]
+
+fn main() {
+    format!("foo"); //~ERROR useless use of `format!`
+    format!("foo {}", 42);
+
+    println!("foo");
+    println!("foo {}", 42);
+}