Add recursion check on main function

Changes:
- Add MainRecursion lint to clippy
- Check for no-std setup

fixes #333

tests/ui/crate_level_checks/no_std_main_recursion.rs

@@ -0,0 +1,31 @@
+#![feature(lang_items, link_args, start, libc)]
+#![link_args="-nostartfiles"]
+#![no_std]
+
+use core::panic::PanicInfo;
+use core::sync::atomic::{AtomicUsize, Ordering};
+
+static N: AtomicUsize = AtomicUsize::new(0);
+
+#[warn(clippy::main_recursion)]
+#[allow(unconditional_recursion)]
+#[start]
+fn main(argc: isize, argv: *const *const u8) -> isize {
+    let x = N.load(Ordering::Relaxed);
+    N.store(x + 1, Ordering::Relaxed);
+
+    if x < 3 {
+        main(argc, argv);
+    }
+
+    0
+}
+
+#[allow(clippy::empty_loop)]
+#[panic_handler]
+fn panic(_info: &PanicInfo) -> ! {
+    loop {}
+}
+
+#[lang = "eh_personality"]
+extern fn eh_personality() {}

tests/ui/crate_level_checks/std_main_recursion.rs

@@ -0,0 +1,5 @@
+#[warn(clippy::main_recursion)]
+#[allow(unconditional_recursion)]
+fn main() {
+    main();
+}

tests/ui/crate_level_checks/std_main_recursion.stderr

@@ -0,0 +1,11 @@
+error: You are recursing into main()
+  --> $DIR/std_main_recursion.rs:4:5
+   |
+LL |     main();
+   |     ^^^^^^
+   |
+   = note: `-D clippy::main-recursion` implied by `-D warnings`
+   = help: Consider using another function for this recursion
+
+error: aborting due to previous error
+