Added lint for unnecessary references

Author: Aaron Power

clippy_lints/src/lib.rs

@@ -391,6 +391,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
     reg.register_late_lint_pass(box if_let_redundant_pattern_matching::Pass);
     reg.register_late_lint_pass(box partialeq_ne_impl::Pass);
     reg.register_early_lint_pass(box reference::Pass);
+    reg.register_early_lint_pass(box reference::DerefPass);
     reg.register_early_lint_pass(box double_parens::DoubleParens);
     reg.register_late_lint_pass(box unused_io_amount::UnusedIoAmount);
     reg.register_late_lint_pass(box large_enum_variant::LargeEnumVariant::new(conf.enum_variant_size_threshold));
@@ -812,6 +813,7 @@ pub fn register_plugins(reg: &mut rustc_plugin::Registry) {
         precedence::PRECEDENCE,
         ranges::RANGE_ZIP_WITH_LEN,
         reference::DEREF_ADDROF,
+        reference::REF_IN_DEREF,
         swap::MANUAL_SWAP,
         temporary_assignment::TEMPORARY_ASSIGNMENT,
         transmute::CROSSPOINTER_TRANSMUTE,

clippy_lints/src/reference.rs

@@ -52,3 +52,53 @@ impl EarlyLintPass for Pass {
         }
     }
 }
+
+/// **What it does:** Checks for references in expressions that use
+/// auto dereference.
+///
+/// **Why is this bad?** The reference is a no-op and is automatically
+/// dereferenced by the compiler and makes the code less clear.
+///
+/// **Example:**
+/// ```rust
+/// struct Point(u32, u32);
+/// let point = Foo(30, 20);
+/// let x = (&point).x;
+/// ```
+declare_clippy_lint! {
+    pub REF_IN_DEREF,
+    complexity,
+    "Use of reference in auto dereference expression."
+}
+
+pub struct DerefPass;
+
+impl LintPass for DerefPass {
+    fn get_lints(&self) -> LintArray {
+        lint_array!(REF_IN_DEREF)
+    }
+}
+
+impl EarlyLintPass for DerefPass {
+    fn check_expr(&mut self, cx: &EarlyContext, e: &Expr) {
+        if_chain! {
+            if let ExprKind::Field(ref object, ref field_name) = e.node;
+            if let ExprKind::Paren(ref parened) = object.node;
+            if let ExprKind::AddrOf(_, ref inner) = parened.node;
+            then {
+                span_lint_and_sugg(
+                    cx,
+                    REF_IN_DEREF,
+                    object.span,
+                    "Creating a reference that is immediately dereferenced.",
+                    "try this",
+                    format!(
+                        "{}.{}",
+                        snippet(cx, inner.span, "_"),
+                        snippet(cx, field_name.span, "_")
+                    )
+                );
+            }
+        }
+    }
+}

tests/ui/unnecessary_ref.rs

@@ -0,0 +1,12 @@
+#![feature(tool_attributes)]
+#![feature(stmt_expr_attributes)]
+
+struct Outer {
+    inner: u32,
+}
+
+#[deny(ref_in_deref)]
+fn main() {
+    let outer = Outer { inner: 0 };
+    let inner = (&outer).inner;
+}

tests/ui/unnecessary_ref.stderr

@@ -0,0 +1,14 @@
+error: Creating a reference that is immediately dereferenced.
+  --> $DIR/unnecessary_ref.rs:11:17
+   |
+11 |     let inner = (&outer).inner;
+   |                 ^^^^^^^^ help: try this: `outer.inner`
+   |
+note: lint level defined here
+  --> $DIR/unnecessary_ref.rs:8:8
+   |
+8  | #[deny(ref_in_deref)]
+   |        ^^^^^^^^^^^^
+
+error: aborting due to previous error
+