useless Rc<Rc<T>>, Rc<Box<T>>

CHANGELOG.md

@@ -1433,6 +1433,8 @@ Released 2018-09-13
 [`range_plus_one`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_plus_one
 [`range_step_by_zero`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_step_by_zero
 [`range_zip_with_len`]: https://rust-lang.github.io/rust-clippy/master/index.html#range_zip_with_len
+[`rc_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#rc_box
+[`rc_rc`]: https://rust-lang.github.io/rust-clippy/master/index.html#rc_rc
 [`redundant_clone`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_clone
 [`redundant_closure`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure
 [`redundant_closure_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#redundant_closure_call

README.md

@@ -5,7 +5,7 @@
 
 A collection of lints to catch common mistakes and improve your [Rust](https://github.com/rust-lang/rust) code.
 
-[There are 362 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 364 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
 
 We have a bunch of lint categories to allow you to choose how much Clippy is supposed to ~~annoy~~ help you:
 

clippy_lints/src/lib.rs

@@ -807,6 +807,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &types::LET_UNIT_VALUE,
         &types::LINKEDLIST,
         &types::OPTION_OPTION,
+        &types::RC_BOX,
+        &types::RC_RC,
         &types::TYPE_COMPLEXITY,
         &types::UNIT_ARG,
         &types::UNIT_CMP,
@@ -1372,6 +1374,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&types::IMPLICIT_HASHER),
         LintId::of(&types::LET_UNIT_VALUE),
         LintId::of(&types::OPTION_OPTION),
+        LintId::of(&types::RC_BOX),
+        LintId::of(&types::RC_RC),
         LintId::of(&types::TYPE_COMPLEXITY),
         LintId::of(&types::UNIT_ARG),
         LintId::of(&types::UNIT_CMP),
@@ -1657,6 +1661,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&trivially_copy_pass_by_ref::TRIVIALLY_COPY_PASS_BY_REF),
         LintId::of(&types::BOX_BORROWS),
         LintId::of(&types::BOX_VEC),
+        LintId::of(&types::RC_BOX),
+        LintId::of(&types::RC_RC),
         LintId::of(&vec::USELESS_VEC),
     ]);
 

clippy_lints/src/types.rs

@@ -194,11 +194,63 @@ declare_clippy_lint! {
     "a box of borrowed type"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for use of `Rc<Rc<T>>` anywhere in the code.
+    ///
+    /// **Why is this bad?** Reference counting pointer of reference counting pointer
+    /// is an unnecessary level of indirection.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # use std::rc::Rc;
+    /// fn foo(bar: Rc<Rc<usize>>) {}
+    /// ```
+    ///
+    /// Better:
+    ///
+    /// ```rust
+    /// # use std::rc::Rc;
+    /// fn foo(bar: Rc<usize>) {}
+    /// ```
+    pub RC_RC,
+    perf,
+    "an Rc of Rc"
+}
+
+declare_clippy_lint! {
+    /// **What it does:** Checks for use of `Rc<Box<T>>` anywhere in the code.
+    ///
+    /// **Why is this bad?** `Rc` already keeps its contents in a separate area on
+    /// the heap. So if you `Box` its contents, you just add another level of indirection.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust
+    /// # use std::rc::Rc;
+    /// # use std::boxed::Box;
+    /// fn foo(bar: Rc<Box<usize>>) {}
+    /// ```
+    ///
+    /// Better:
+    ///
+    /// ```rust
+    /// # use std::rc::Rc;
+    /// # use std::boxed::Box;
+    /// fn foo(bar: Rc<usize>) {}
+    /// ```
+    pub RC_BOX,
+    perf,
+    "an Rc of Box"
+}
+
 pub struct Types {
     vec_box_size_threshold: u64,
 }
 
-impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, BOX_BORROWS]);
+impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, BOX_BORROWS, RC_RC, RC_BOX]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Types {
     fn check_fn(
@@ -322,6 +374,27 @@ impl Types {
                             );
                             return; // don't recurse into the type
                         }
+                    } else if Some(def_id) == cx.tcx.lang_items().rc() {
+                        if match_type_parameter(cx, qpath, &paths::RC) {
+                            span_lint_and_help(
+                                cx,
+                                RC_RC,
+                                hir_ty.span,
+                                "usage of `Rc<Rc<T>>`",
+                                "Rc<Rc<T>> can be simplified to Rc<T>",
+                            );
+                            return; // don't recurse into the type
+                        }
+                        if match_type_parameter(cx, qpath, &paths::BOX) {
+                            span_lint_and_help(
+                                cx,
+                                RC_BOX,
+                                hir_ty.span,
+                                "usage of `Rc<Box<T>>`",
+                                "Rc<Box<T>> can be simplified to Rc<T>",
+                            );
+                            return; // don't recurse into the type
+                        }
                     } else if cx.tcx.is_diagnostic_item(Symbol::intern("vec_type"), def_id) {
                         if_chain! {
                             // Get the _ part of Vec<_>

clippy_lints/src/utils/paths.rs

@@ -9,6 +9,7 @@ pub const BEGIN_PANIC: [&str; 3] = ["std", "panicking", "begin_panic"];
 pub const BEGIN_PANIC_FMT: [&str; 3] = ["std", "panicking", "begin_panic_fmt"];
 pub const BINARY_HEAP: [&str; 4] = ["alloc", "collections", "binary_heap", "BinaryHeap"];
 pub const BORROW_TRAIT: [&str; 3] = ["core", "borrow", "Borrow"];
+pub const BOX: [&str; 3] = ["alloc", "boxed", "Box"];
 pub const BTREEMAP: [&str; 5] = ["alloc", "collections", "btree", "map", "BTreeMap"];
 pub const BTREEMAP_ENTRY: [&str; 5] = ["alloc", "collections", "btree", "map", "Entry"];
 pub const BTREESET: [&str; 5] = ["alloc", "collections", "btree", "set", "BTreeSet"];

src/lintlist/mod.rs

@@ -6,7 +6,7 @@ pub use lint::Lint;
 pub use lint::LINT_LEVELS;
 
 // begin lint list, do not remove this comment, it’s used in `update_lints`
-pub const ALL_LINTS: [Lint; 362] = [
+pub const ALL_LINTS: [Lint; 364] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -1722,6 +1722,20 @@ pub const ALL_LINTS: [Lint; 362] = [
         deprecation: None,
         module: "ranges",
     },
+    Lint {
+        name: "rc_box",
+        group: "perf",
+        desc: "an Rc of Box",
+        deprecation: None,
+        module: "types",
+    },
+    Lint {
+        name: "rc_rc",
+        group: "perf",
+        desc: "an Rc of Rc",
+        deprecation: None,
+        module: "types",
+    },
     Lint {
         name: "redundant_clone",
         group: "perf",

tests/ui/rc_rc.rs

@@ -0,0 +1,8 @@
+use std::boxed::Box;
+use std::rc::Rc;
+
+pub fn test(a: Rc<Rc<bool>>) {}
+
+pub fn test2(a: Rc<Box<bool>>) {}
+
+fn main() {}

tests/ui/rc_rc.stderr

@@ -0,0 +1,20 @@
+error: usage of `Rc<Rc<T>>`
+  --> $DIR/rc_rc.rs:4:16
+   |
+LL | pub fn test(a: Rc<Rc<bool>>) {}
+   |                ^^^^^^^^^^^^
+   |
+   = note: `-D clippy::rc-rc` implied by `-D warnings`
+   = help: Rc<Rc<T>> can be simplified to Rc<T>
+
+error: usage of `Rc<Box<T>>`
+  --> $DIR/rc_rc.rs:6:17
+   |
+LL | pub fn test2(a: Rc<Box<bool>>) {}
+   |                 ^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::rc-box` implied by `-D warnings`
+   = help: Rc<Box<T>> can be simplified to Rc<T>
+
+error: aborting due to 2 previous errors
+