Lint usage of *& and *&mut

fixes #1316

clippy_lints/src/reference.rs

@@ -0,0 +1,48 @@
+use rustc::hir::*;
+use rustc::lint::*;
+use utils::{span_lint_and_then, snippet};
+
+/// **What it does:** Checks for usage of `*&` and `*&mut` in expressions.
+///
+/// **Why is this bad?** Immediately dereferencing a reference is no-op and
+/// makes the code less clear.
+///
+/// **Known problems:** Multiple dereference/addrof pairs are not handled so
+/// the suggested fix for `x = **&&y` is `x = *&y`, which is still incorrect.
+///
+/// **Example:**
+/// ```rust
+/// let a = f(*&mut b);
+/// let c = *&d;
+/// ```
+declare_lint! {
+    pub DEREF_ADDROF,
+    Warn,
+    "use of `*&` or `*&mut` in an expression"
+}
+
+pub struct Pass;
+
+impl LintPass for Pass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(DEREF_ADDROF)
+    }
+}
+
+impl LateLintPass for Pass {
+    fn check_expr(&mut self, cx: &LateContext, e: &Expr) {
+        if let ExprUnary(UnDeref, ref deref_target) = e.node {
+            if let ExprAddrOf(_, ref addrof_target) = deref_target.node {
+                span_lint_and_then(
+                    cx,
+                    DEREF_ADDROF,
+                    e.span,
+                    "immediately dereferencing a reference",
+                    |db| {
+                        db.span_suggestion(e.span, "try this",
+                                             format!("{}", snippet(cx, addrof_target.span, "_")));
+                    });
+            }
+        }
+    }
+}