allow & &* (&T)

Author: lengyijun

clippy_lints/src/needless_deref.rs

@@ -19,6 +19,16 @@ declare_clippy_lint! {
     /// if you want to deref explicitly, `&** (&T)` is what you need.
     /// If you want to reborrow, `&T` is enough (`&T` is Copy).
     ///
+    /// ### Known problems
+    /// false negative on such code:
+    /// ```
+    /// let x = &12;
+    /// let addr_x = &x as *const _ as usize;
+    /// let addr_y = &&*x as *const _ as usize; // assert ok now, and lint triggerd.
+    ///                                         // But if we apply it, it will assert fail.
+    /// assert_ne!(addr_x, addr_y);
+    /// ```
+    ///
     /// ### Example
     /// ```rust
     /// let s = &String::new();
@@ -60,7 +70,7 @@ impl LateLintPass<'_> for NeedlessDeref {
                     if matches!(deref_expr.kind, ExprKind::Path(..)) {
                         let parent_node = map.find(parent_hir_id);
                         if let Some(rustc_hir::Node::Expr(parent_expr)) = parent_node {
-                            if matches!(parent_expr.kind, ExprKind::AddrOf(..)) {
+                            if matches!(parent_expr.kind, ExprKind::AddrOf(_, Mutability::Mut, _)) {
                                 return;
                             }
                             if matches!(parent_expr.kind, ExprKind::Unary(UnOp::Deref, ..)) &&
@@ -73,8 +83,8 @@ impl LateLintPass<'_> for NeedlessDeref {
 
                 let mut give_2_help = true;
 
-                // if has deref trait, give 2 help
-                // if has no deref trait, give 1 help
+                // has deref trait -> give 2 help
+                // doesn't have deref trait -> give 1 help
                 if let Some(deref_trait_id) = cx.tcx.lang_items().deref_trait(){
                     if !implements_trait(cx, inner_ty, deref_trait_id, &[]) {
                          give_2_help = false;

tests/ui/needless_deref.rs

@@ -34,7 +34,7 @@ mod should_not_lint1 {
 }
 
 // this mod explains why we should not lint `& &* (&T)`
-mod should_not_lint2 {
+mod false_negative {
     fn foo() {
         let x = &12;
         let addr_x = &x as *const _ as usize;

tests/ui/needless_deref.stderr

@@ -24,5 +24,13 @@ LL |         let b = &mut &*bar(a);
    |
    = help: consider using `bar(a)` if you would like to reborrow
 
-error: aborting due to 3 previous errors
+error: deref on an immutable reference
+  --> $DIR/needless_deref.rs:41:23
+   |
+LL |         let addr_y = &&*x as *const _ as usize; // assert ok
+   |                       ^^^
+   |
+   = help: consider using `x` if you would like to reborrow
+
+error: aborting due to 4 previous errors