Add default_box_assignments lint

Author: bluebear94

CHANGELOG.md

@@ -5705,6 +5705,7 @@ Released 2018-09-13
 [`debug_assert_with_mut_call`]: https://rust-lang.github.io/rust-clippy/master/index.html#debug_assert_with_mut_call
 [`decimal_literal_representation`]: https://rust-lang.github.io/rust-clippy/master/index.html#decimal_literal_representation
 [`declare_interior_mutable_const`]: https://rust-lang.github.io/rust-clippy/master/index.html#declare_interior_mutable_const
+[`default_box_assignments`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_box_assignments
 [`default_constructed_unit_structs`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_constructed_unit_structs
 [`default_instead_of_iter_empty`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_instead_of_iter_empty
 [`default_numeric_fallback`]: https://rust-lang.github.io/rust-clippy/master/index.html#default_numeric_fallback

clippy_lints/src/declared_lints.rs

@@ -92,6 +92,7 @@ pub static LINTS: &[&crate::LintInfo] = &[
     crate::dbg_macro::DBG_MACRO_INFO,
     crate::default::DEFAULT_TRAIT_ACCESS_INFO,
     crate::default::FIELD_REASSIGN_WITH_DEFAULT_INFO,
+    crate::default_box_assignments::DEFAULT_BOX_ASSIGNMENTS_INFO,
     crate::default_constructed_unit_structs::DEFAULT_CONSTRUCTED_UNIT_STRUCTS_INFO,
     crate::default_instead_of_iter_empty::DEFAULT_INSTEAD_OF_ITER_EMPTY_INFO,
     crate::default_numeric_fallback::DEFAULT_NUMERIC_FALLBACK_INFO,

clippy_lints/src/default_box_assignments.rs

@@ -0,0 +1,86 @@
+use clippy_utils::diagnostics::span_lint_and_then;
+use clippy_utils::is_default_equivalent_call;
+use clippy_utils::source::snippet;
+use clippy_utils::ty::implements_trait;
+use rustc_errors::Applicability;
+use rustc_hir::{Expr, ExprKind, LangItem};
+use rustc_lint::{LateContext, LateLintPass};
+use rustc_middle::ty::{self, GenericArgKind, Ty};
+use rustc_session::declare_lint_pass;
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// Detects assignments of `Default::default()` to a place of type `Box<T>`.
+    ///
+    /// ### Why is this bad?
+    /// This incurs an extra heap allocation compared to assigning the boxed
+    /// storage.
+    ///
+    /// ### Example
+    /// ```no_run
+    /// let mut b = Box::new(1u32);
+    /// b = Default::default();
+    /// ```
+    /// Use instead:
+    /// ```no_run
+    /// *b = Default::default();
+    /// ```
+    #[clippy::version = "1.89.0"]
+    pub DEFAULT_BOX_ASSIGNMENTS,
+    perf,
+    "assigning `Default::default()` to `Box<T>` is inefficient"
+}
+declare_lint_pass!(DefaultBoxAssignments => [DEFAULT_BOX_ASSIGNMENTS]);
+
+impl LateLintPass<'_> for DefaultBoxAssignments {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &'_ Expr<'_>) {
+        if let ExprKind::Assign(lhs, rhs, _) = &expr.kind {
+            let lhs_ty = cx.typeck_results().expr_ty(lhs);
+            if is_box_of_default(lhs_ty, cx) && is_default_call(rhs, cx) {
+                span_lint_and_then(
+                    cx,
+                    DEFAULT_BOX_ASSIGNMENTS,
+                    expr.span,
+                    "assigning `Default::default()` to `Box<T>`",
+                    |diag| {
+                        let suggestion = format!("*({}) = Default::default()", snippet(cx, lhs.span, "_"));
+
+                        diag.note("this creates a needless allocation").span_suggestion(
+                            expr.span,
+                            "assign to the inner value",
+                            suggestion,
+                            Applicability::MaybeIncorrect,
+                        );
+                    },
+                );
+            }
+        }
+    }
+}
+
+fn is_box_of_default<'a>(ty: Ty<'a>, cx: &LateContext<'a>) -> bool {
+    if let ty::Adt(def, args) = ty.kind()
+        && cx.tcx.lang_items().get(LangItem::OwnedBox) == Some(def.did())
+        && let Some(inner) = args.iter().find_map(|arg| match arg.kind() {
+            GenericArgKind::Type(ty) => Some(ty),
+            _ => None,
+        })
+    {
+        cx.tcx
+            .get_diagnostic_item(sym::Default)
+            .map_or(false, |id| implements_trait(cx, inner, id, &[]))
+    } else {
+        false
+    }
+}
+
+fn is_default_call(expr: &Expr<'_>, cx: &LateContext<'_>) -> bool {
+    if let ExprKind::Call(func, _args) = expr.kind
+        && is_default_equivalent_call(cx, func, Some(expr))
+    {
+        true
+    } else {
+        false
+    }
+}

clippy_lints/src/lib.rs

@@ -105,6 +105,7 @@ mod crate_in_macro_def;
 mod create_dir;
 mod dbg_macro;
 mod default;
+mod default_box_assignments;
 mod default_constructed_unit_structs;
 mod default_instead_of_iter_empty;
 mod default_numeric_fallback;
@@ -948,5 +949,6 @@ pub fn register_lints(store: &mut rustc_lint::LintStore, conf: &'static Conf) {
     store.register_late_pass(move |_| Box::new(redundant_test_prefix::RedundantTestPrefix));
     store.register_late_pass(|_| Box::new(cloned_ref_to_slice_refs::ClonedRefToSliceRefs::new(conf)));
     store.register_late_pass(|_| Box::new(infallible_try_from::InfallibleTryFrom));
+    store.register_late_pass(|_| Box::new(default_box_assignments::DefaultBoxAssignments));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }

tests/ui/default_box_assignments.fixed

@@ -0,0 +1,17 @@
+#![warn(clippy::default_box_assignments)]
+
+fn main() {
+    let mut b = Box::new(1u32);
+    *(b) = Default::default();
+    //~^ default_box_assignments
+    *(b) = Default::default();
+    //~^ default_box_assignments
+
+    // No lint for assigning to the storage
+    *b = Default::default();
+    *b = u32::default();
+
+    // No lint for assigning to Box<T> where T is unsized
+    let mut b = Box::<str>::from("hi".to_string());
+    b = Default::default();
+}

tests/ui/default_box_assignments.rs

@@ -0,0 +1,17 @@
+#![warn(clippy::default_box_assignments)]
+
+fn main() {
+    let mut b = Box::new(1u32);
+    b = Default::default();
+    //~^ default_box_assignments
+    b = Box::default();
+    //~^ default_box_assignments
+
+    // No lint for assigning to the storage
+    *b = Default::default();
+    *b = u32::default();
+
+    // No lint for assigning to Box<T> where T is unsized
+    let mut b = Box::<str>::from("hi".to_string());
+    b = Default::default();
+}

tests/ui/default_box_assignments.stderr

@@ -0,0 +1,20 @@
+error: assigning `Default::default()` to `Box<T>`
+  --> tests/ui/default_box_assignments.rs:5:5
+   |
+LL |     b = Default::default();
+   |     ^^^^^^^^^^^^^^^^^^^^^^ help: assign to the inner value: `*(b) = Default::default()`
+   |
+   = note: this creates a needless allocation
+   = note: `-D clippy::default-box-assignments` implied by `-D warnings`
+   = help: to override `-D warnings` add `#[allow(clippy::default_box_assignments)]`
+
+error: assigning `Default::default()` to `Box<T>`
+  --> tests/ui/default_box_assignments.rs:7:5
+   |
+LL |     b = Box::default();
+   |     ^^^^^^^^^^^^^^^^^^ help: assign to the inner value: `*(b) = Default::default()`
+   |
+   = note: this creates a needless allocation
+
+error: aborting due to 2 previous errors
+