Add recursion check on main function

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

fixes #333

clippy_lints/src/main_recursion.rs

@@ -0,0 +1,55 @@
+
+use syntax::ast::{Crate, Expr, ExprKind};
+use syntax::symbol::sym;
+use rustc::lint::{LintArray, LintPass, EarlyLintPass, EarlyContext};
+use rustc::{declare_tool_lint, impl_lint_pass};
+
+use if_chain::if_chain;
+use crate::utils::span_help_and_lint;
+
+declare_clippy_lint! {
+    pub MAIN_RECURSION,
+    pedantic,
+    "function named `foo`, which is not a descriptive name"
+}
+
+pub struct MainRecursion {
+    has_no_std_attr: bool
+}
+
+impl_lint_pass!(MainRecursion => [MAIN_RECURSION]);
+
+impl MainRecursion {
+    pub fn new() -> MainRecursion {
+        MainRecursion {
+            has_no_std_attr: false
+        }
+    }
+}
+
+impl EarlyLintPass for MainRecursion {
+    fn check_crate(&mut self, _: &EarlyContext<'_>, krate: &Crate) {
+        self.has_no_std_attr = krate.attrs.iter().any(|attr| attr.path == sym::no_std);
+    }
+
+    fn check_expr(&mut self, cx: &EarlyContext<'_>, expr: &Expr) {
+        if self.has_no_std_attr {
+            return;
+        }
+
+        if_chain! {
+            if let ExprKind::Call(func, _) = &expr.node;
+            if let ExprKind::Path(_, path) = &func.node;
+            if *path == sym::main;
+            then {
+                span_help_and_lint(
+                    cx,
+                    MAIN_RECURSION,
+                    expr.span,
+                    "You are recursing into main()",
+                    "Consider using another function for this recursion"
+                )
+            }
+        }
+    }
+}