box_vec: dont use Box<&T>

Author: jpospychala

CHANGELOG.md

@@ -1074,6 +1074,7 @@ Released 2018-09-13
 [`bool_comparison`]: https://rust-lang.github.io/rust-clippy/master/index.html#bool_comparison
 [`borrow_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrow_interior_mutable_const
 [`borrowed_box`]: https://rust-lang.github.io/rust-clippy/master/index.html#borrowed_box
+[`box_borrow`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_borrow
 [`box_vec`]: https://rust-lang.github.io/rust-clippy/master/index.html#box_vec
 [`boxed_local`]: https://rust-lang.github.io/rust-clippy/master/index.html#boxed_local
 [`builtin_type_shadow`]: https://rust-lang.github.io/rust-clippy/master/index.html#builtin_type_shadow

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 361 lints included in this crate!](https://rust-lang.github.io/rust-clippy/master/index.html)
+[There are 362 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

@@ -785,6 +785,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         &try_err::TRY_ERR,
         &types::ABSURD_EXTREME_COMPARISONS,
         &types::BORROWED_BOX,
+        &types::BOX_BORROW,
         &types::BOX_VEC,
         &types::CAST_LOSSLESS,
         &types::CAST_POSSIBLE_TRUNCATION,
@@ -1355,6 +1356,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&try_err::TRY_ERR),
         LintId::of(&types::ABSURD_EXTREME_COMPARISONS),
         LintId::of(&types::BORROWED_BOX),
+        LintId::of(&types::BOX_BORROW),
         LintId::of(&types::BOX_VEC),
         LintId::of(&types::CAST_PTR_ALIGNMENT),
         LintId::of(&types::CAST_REF_TO_MUT),
@@ -1647,6 +1649,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         LintId::of(&redundant_clone::REDUNDANT_CLONE),
         LintId::of(&slow_vector_initialization::SLOW_VECTOR_INITIALIZATION),
         LintId::of(&trivially_copy_pass_by_ref::TRIVIALLY_COPY_PASS_BY_REF),
+        LintId::of(&types::BOX_BORROW),
         LintId::of(&types::BOX_VEC),
         LintId::of(&vec::USELESS_VEC),
     ]);

clippy_lints/src/types.rs

@@ -171,11 +171,38 @@ declare_clippy_lint! {
     "a borrow of a boxed type"
 }
 
+declare_clippy_lint! {
+    /// **What it does:** Checks for use of `Box<&T>` anywhere in the code.
+    ///
+    /// **Why is this bad?** Any `Box<&T>` can also be a `&T`, which is more
+    /// general.
+    ///
+    /// **Known problems:** None.
+    ///
+    /// **Example:**
+    /// ```rust,ignore
+    /// struct X {
+    ///     values: Box<&T>,
+    /// }
+    /// ```
+    ///
+    /// Better:
+    ///
+    /// ```rust,ignore
+    /// struct X {
+    ///     values: &T,
+    /// }
+    /// ```
+    pub BOX_BORROW,
+    perf,
+    "a box of borrowed type"
+}
+
 pub struct Types {
     vec_box_size_threshold: u64,
 }
 
-impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX]);
+impl_lint_pass!(Types => [BOX_VEC, VEC_BOX, OPTION_OPTION, LINKEDLIST, BORROWED_BOX, BOX_BORROW]);
 
 impl<'a, 'tcx> LateLintPass<'a, 'tcx> for Types {
     fn check_fn(
@@ -236,6 +263,24 @@ fn match_type_parameter(cx: &LateContext<'_, '_>, qpath: &QPath<'_>, path: &[&st
     false
 }
 
+/// Checks if `qpath` last segment type parameter is a reference
+fn match_type_parameter_rptr(qpath: &QPath<'_>) -> bool {
+    let last = last_path_segment(qpath);
+    if_chain! {
+        if let Some(ref params) = last.args;
+        if !params.parenthesized;
+        if let Some(ty) = params.args.iter().find_map(|arg| match arg {
+            GenericArg::Type(ty) => Some(ty),
+            _ => None,
+        });
+        if let TyKind::Rptr(ref _lifetime_opt, ref _mut) = ty.kind;
+        then {
+            return true;
+        }
+    }
+    false
+}
+
 impl Types {
     pub fn new(vec_box_size_threshold: u64) -> Self {
         Self { vec_box_size_threshold }
@@ -267,6 +312,16 @@ impl Types {
                 let res = qpath_res(cx, qpath, hir_id);
                 if let Some(def_id) = res.opt_def_id() {
                     if Some(def_id) == cx.tcx.lang_items().owned_box() {
+                        if match_type_parameter_rptr(qpath) {
+                            span_lint_and_help(
+                                cx,
+                                BOX_BORROW,
+                                hir_ty.span,
+                                "you seem to be trying to use `Box<&T>`. Consider using just `&T`",
+                                "try `&T`",
+                            );
+                            return; // don't recurse into the type
+                        }
                         if match_type_parameter(cx, qpath, &paths::VEC) {
                             span_lint_and_help(
                                 cx,

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; 361] = [
+pub const ALL_LINTS: [Lint; 362] = [
     Lint {
         name: "absurd_extreme_comparisons",
         group: "correctness",
@@ -98,6 +98,13 @@ pub const ALL_LINTS: [Lint; 361] = [
         deprecation: None,
         module: "types",
     },
+    Lint {
+        name: "box_borrow",
+        group: "perf",
+        desc: "a box of borrowed type",
+        deprecation: None,
+        module: "types",
+    },
     Lint {
         name: "box_vec",
         group: "perf",

tests/ui/box_vec.rs

@@ -24,6 +24,8 @@ pub fn test_local_not_linted() {
     let _: Box<Vec<bool>>;
 }
 
+pub fn test3<T>(foo: Box<&T>) {}
+
 fn main() {
     test(Box::new(Vec::new()));
     test2(Box::new(|v| println!("{:?}", v)));

tests/ui/box_vec.stderr

@@ -7,5 +7,14 @@ LL | pub fn test(foo: Box<Vec<bool>>) {
    = note: `-D clippy::box-vec` implied by `-D warnings`
    = help: `Vec<T>` is already on the heap, `Box<Vec<T>>` makes an extra allocation.
 
-error: aborting due to previous error
+error: you seem to be trying to use `Box<&T>`. Consider using just `&T`
+  --> $DIR/box_vec.rs:27:22
+   |
+LL | pub fn test3<T>(foo: Box<&T>) {
+   |                      ^^^^^^^
+   |
+   = note: `-D clippy::box-borrow` implied by `-D warnings`
+   = help: try `&T`
+
+error: aborting due to 2 previous errors