add `box-default` lint

5 files changed, 80 insertions, 14 deletions

diff --git a/clippy_lints/src/box_default.rs b/clippy_lints/src/box_default.rs
new file mode 100644
index 00000000000..792183ac408
--- /dev/null
+++ b/clippy_lints/src/box_default.rs
@@ -0,0 +1,61 @@
+use clippy_utils::{diagnostics::span_lint_and_help, is_default_equivalent, path_def_id};
+use rustc_hir::{Expr, ExprKind, QPath};
+use rustc_lint::{LateContext, LateLintPass, LintContext};
+use rustc_middle::lint::in_external_macro;
+use rustc_session::{declare_lint_pass, declare_tool_lint};
+use rustc_span::sym;
+
+declare_clippy_lint! {
+    /// ### What it does
+    /// checks for `Box::new(T::default())`, which is better written as
+    /// `Box::<T>::default()`.
+    ///
+    /// ### Why is this bad?
+    /// First, it's more complex, involving two calls instead of one.
+    /// Second, `Box::default()` can be faster
+    /// [in certain cases](https://nnethercote.github.io/perf-book/standard-library-types.html#box).
+    ///
+    /// ### Known problems
+    /// The lint may miss some cases (e.g. Box::new(String::from(""))).
+    /// On the other hand, it will trigger on cases where the `default`
+    /// code comes from a macro that does something different based on
+    /// e.g. target operating system.
+    ///
+    /// ### Example
+    /// ```rust
+    /// let x: Box<String> = Box::new(Default::default());
+    /// ```
+    /// Use instead:
+    /// ```rust
+    /// let x: Box<String> = Box::default();
+    /// ```
+    #[clippy::version = "1.65.0"]
+    pub BOX_DEFAULT,
+    perf,
+    "Using Box::new(T::default()) instead of Box::default()"
+}
+
+declare_lint_pass!(BoxDefault => [BOX_DEFAULT]);
+
+impl LateLintPass<'_> for BoxDefault {
+    fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
+        if let ExprKind::Call(box_new, [arg]) = expr.kind
+            && let ExprKind::Path(QPath::TypeRelative(ty, seg)) = box_new.kind
+            && let ExprKind::Call(..) = arg.kind
+            && !in_external_macro(cx.sess(), expr.span)
+            && expr.span.eq_ctxt(arg.span)
+            && seg.ident.name == sym::new
+            && path_def_id(cx, ty) == cx.tcx.lang_items().owned_box()
+            && is_default_equivalent(cx, arg)
+        {
+            span_lint_and_help(
+                cx,
+                BOX_DEFAULT,
+                expr.span,
+                "`Box::new(_)` of default value",
+                None,
+                "use `Box::default()` instead",
+            );
+        }
+    }
+}
diff --git a/clippy_lints/src/lib.register_all.rs b/clippy_lints/src/lib.register_all.rs
index ae57a9a24a7..33490d1ccc5 100644
--- a/clippy_lints/src/lib.register_all.rs
+++ b/clippy_lints/src/lib.register_all.rs
@@ -21,6 +21,7 @@ store.register_group(true, "clippy::all", Some("clippy_all"), vec![
     LintId::of(booleans::NONMINIMAL_BOOL),
     LintId::of(booleans::OVERLY_COMPLEX_BOOL_EXPR),
     LintId::of(borrow_deref_ref::BORROW_DEREF_REF),
+    LintId::of(box_default::BOX_DEFAULT),
     LintId::of(casts::CAST_ABS_TO_UNSIGNED),
     LintId::of(casts::CAST_ENUM_CONSTRUCTOR),
     LintId::of(casts::CAST_ENUM_TRUNCATION),
diff --git a/clippy_lints/src/lib.register_lints.rs b/clippy_lints/src/lib.register_lints.rs
index 02fcc8de507..93ea32fa069 100644
--- a/clippy_lints/src/lib.register_lints.rs
+++ b/clippy_lints/src/lib.register_lints.rs
@@ -60,6 +60,7 @@ store.register_lints(&[
     booleans::NONMINIMAL_BOOL,
     booleans::OVERLY_COMPLEX_BOOL_EXPR,
     borrow_deref_ref::BORROW_DEREF_REF,
+    box_default::BOX_DEFAULT,
     cargo::CARGO_COMMON_METADATA,
     cargo::MULTIPLE_CRATE_VERSIONS,
     cargo::NEGATIVE_FEATURE_NAMES,
diff --git a/clippy_lints/src/lib.register_perf.rs b/clippy_lints/src/lib.register_perf.rs
index 195ce41e31e..8e927470e02 100644
--- a/clippy_lints/src/lib.register_perf.rs
+++ b/clippy_lints/src/lib.register_perf.rs
@@ -3,6 +3,7 @@
 // Manual edits will be overwritten.
 
 store.register_group(true, "clippy::perf", Some("clippy_perf"), vec![
+    LintId::of(box_default::BOX_DEFAULT),
     LintId::of(entry::MAP_ENTRY),
     LintId::of(escape::BOXED_LOCAL),
     LintId::of(format_args::FORMAT_IN_FORMAT_ARGS),
diff --git a/clippy_lints/src/lib.rs b/clippy_lints/src/lib.rs
index 83fdc15c9f0..7bd49b11090 100644
--- a/clippy_lints/src/lib.rs
+++ b/clippy_lints/src/lib.rs
@@ -181,6 +181,7 @@ mod bool_assert_comparison;
 mod bool_to_int_with_if;
 mod booleans;
 mod borrow_deref_ref;
+mod box_default;
 mod cargo;
 mod casts;
 mod checked_conversions;
@@ -536,8 +537,8 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
         store.register_late_pass(|| Box::new(utils::internal_lints::CompilerLintFunctions::new()));
         store.register_late_pass(|| Box::new(utils::internal_lints::IfChainStyle));
         store.register_late_pass(|| Box::new(utils::internal_lints::InvalidPaths));
-        store.register_late_pass(|| Box::new(utils::internal_lints::InterningDefinedSymbol::default()));
-        store.register_late_pass(|| Box::new(utils::internal_lints::LintWithoutLintPass::default()));
+        store.register_late_pass(|| Box::<utils::internal_lints::InterningDefinedSymbol>::default());
+        store.register_late_pass(|| Box::<utils::internal_lints::LintWithoutLintPass>::default());
         store.register_late_pass(|| Box::new(utils::internal_lints::MatchTypeOnDiagItem));
         store.register_late_pass(|| Box::new(utils::internal_lints::OuterExpnDataPass));
         store.register_late_pass(|| Box::new(utils::internal_lints::MsrvAttrImpl));
@@ -630,10 +631,10 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             msrv,
         ))
     });
-    store.register_late_pass(|| Box::new(shadow::Shadow::default()));
+    store.register_late_pass(|| Box::<shadow::Shadow>::default());
     store.register_late_pass(|| Box::new(unit_types::UnitTypes));
     store.register_late_pass(|| Box::new(loops::Loops));
-    store.register_late_pass(|| Box::new(main_recursion::MainRecursion::default()));
+    store.register_late_pass(|| Box::<main_recursion::MainRecursion>::default());
     store.register_late_pass(|| Box::new(lifetimes::Lifetimes));
     store.register_late_pass(|| Box::new(entry::HashMapPass));
     store.register_late_pass(|| Box::new(minmax::MinMaxPass));
@@ -667,7 +668,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(format::UselessFormat));
     store.register_late_pass(|| Box::new(swap::Swap));
     store.register_late_pass(|| Box::new(overflow_check_conditional::OverflowCheckConditional));
-    store.register_late_pass(|| Box::new(new_without_default::NewWithoutDefault::default()));
+    store.register_late_pass(|| Box::<new_without_default::NewWithoutDefault>::default());
     let disallowed_names = conf.disallowed_names.iter().cloned().collect::<FxHashSet<_>>();
     store.register_late_pass(move || Box::new(disallowed_names::DisallowedNames::new(disallowed_names.clone())));
     let too_many_arguments_threshold = conf.too_many_arguments_threshold;
@@ -706,7 +707,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(ref_option_ref::RefOptionRef));
     store.register_late_pass(|| Box::new(infinite_iter::InfiniteIter));
     store.register_late_pass(|| Box::new(inline_fn_without_body::InlineFnWithoutBody));
-    store.register_late_pass(|| Box::new(useless_conversion::UselessConversion::default()));
+    store.register_late_pass(|| Box::<useless_conversion::UselessConversion>::default());
     store.register_late_pass(|| Box::new(implicit_hasher::ImplicitHasher));
     store.register_late_pass(|| Box::new(fallible_impl_from::FallibleImplFrom));
     store.register_late_pass(|| Box::new(question_mark::QuestionMark));
@@ -776,7 +777,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             upper_case_acronyms_aggressive,
         ))
     });
-    store.register_late_pass(|| Box::new(default::Default::default()));
+    store.register_late_pass(|| Box::<default::Default>::default());
     store.register_late_pass(move || Box::new(unused_self::UnusedSelf::new(avoid_breaking_exported_api)));
     store.register_late_pass(|| Box::new(mutable_debug_assertion::DebugAssertWithMutCall));
     store.register_late_pass(|| Box::new(exit::Exit));
@@ -799,7 +800,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_early_pass(|| Box::new(option_env_unwrap::OptionEnvUnwrap));
     let warn_on_all_wildcard_imports = conf.warn_on_all_wildcard_imports;
     store.register_late_pass(move || Box::new(wildcard_imports::WildcardImports::new(warn_on_all_wildcard_imports)));
-    store.register_late_pass(|| Box::new(redundant_pub_crate::RedundantPubCrate::default()));
+    store.register_late_pass(|| Box::<redundant_pub_crate::RedundantPubCrate>::default());
     store.register_late_pass(|| Box::new(unnamed_address::UnnamedAddress));
     store.register_late_pass(move || Box::new(dereference::Dereferencing::new(msrv)));
     store.register_late_pass(|| Box::new(option_if_let_else::OptionIfLetElse));
@@ -817,7 +818,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     });
     let macro_matcher = conf.standard_macro_braces.iter().cloned().collect::<FxHashSet<_>>();
     store.register_early_pass(move || Box::new(nonstandard_macro_braces::MacroBraces::new(&macro_matcher)));
-    store.register_late_pass(|| Box::new(macro_use::MacroUseImports::default()));
+    store.register_late_pass(|| Box::<macro_use::MacroUseImports>::default());
     store.register_late_pass(|| Box::new(pattern_type_mismatch::PatternTypeMismatch));
     store.register_late_pass(|| Box::new(unwrap_in_result::UnwrapInResult));
     store.register_late_pass(|| Box::new(semicolon_if_nothing_returned::SemicolonIfNothingReturned));
@@ -830,7 +831,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(|| Box::new(strings::StrToString));
     store.register_late_pass(|| Box::new(strings::StringToString));
     store.register_late_pass(|| Box::new(zero_sized_map_values::ZeroSizedMapValues));
-    store.register_late_pass(|| Box::new(vec_init_then_push::VecInitThenPush::default()));
+    store.register_late_pass(|| Box::<vec_init_then_push::VecInitThenPush>::default());
     store.register_late_pass(|| Box::new(redundant_slicing::RedundantSlicing));
     store.register_late_pass(|| Box::new(from_str_radix_10::FromStrRadix10));
     store.register_late_pass(move || Box::new(if_then_some_else_none::IfThenSomeElseNone::new(msrv)));
@@ -868,7 +869,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(manual_bits::ManualBits::new(msrv)));
     store.register_late_pass(|| Box::new(default_union_representation::DefaultUnionRepresentation));
     store.register_early_pass(|| Box::new(doc_link_with_quotes::DocLinkWithQuotes));
-    store.register_late_pass(|| Box::new(only_used_in_recursion::OnlyUsedInRecursion::default()));
+    store.register_late_pass(|| Box::<only_used_in_recursion::OnlyUsedInRecursion>::default());
     let allow_dbg_in_tests = conf.allow_dbg_in_tests;
     store.register_late_pass(move || Box::new(dbg_macro::DbgMacro::new(allow_dbg_in_tests)));
     let cargo_ignore_publish = conf.cargo_ignore_publish;
@@ -877,7 +878,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
             ignore_publish: cargo_ignore_publish,
         })
     });
-    store.register_late_pass(|| Box::new(write::Write::default()));
+    store.register_late_pass(|| Box::<write::Write>::default());
     store.register_early_pass(|| Box::new(crate_in_macro_def::CrateInMacroDef));
     store.register_early_pass(|| Box::new(empty_structs_with_brackets::EmptyStructsWithBrackets));
     store.register_late_pass(|| Box::new(unnecessary_owned_empty_strings::UnnecessaryOwnedEmptyStrings));
@@ -887,7 +888,7 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     store.register_late_pass(move || Box::new(large_include_file::LargeIncludeFile::new(max_include_file_size)));
     store.register_late_pass(|| Box::new(strings::TrimSplitWhitespace));
     store.register_late_pass(|| Box::new(rc_clone_in_vec_init::RcCloneInVecInit));
-    store.register_early_pass(|| Box::new(duplicate_mod::DuplicateMod::default()));
+    store.register_early_pass(|| Box::<duplicate_mod::DuplicateMod>::default());
     store.register_early_pass(|| Box::new(unused_rounding::UnusedRounding));
     store.register_early_pass(move || Box::new(almost_complete_letter_range::AlmostCompleteLetterRange::new(msrv)));
     store.register_late_pass(|| Box::new(swap_ptr_to_ref::SwapPtrToRef));
@@ -899,13 +900,14 @@ pub fn register_plugins(store: &mut rustc_lint::LintStore, sess: &Session, conf:
     let verbose_bit_mask_threshold = conf.verbose_bit_mask_threshold;
     store.register_late_pass(move || Box::new(operators::Operators::new(verbose_bit_mask_threshold)));
     store.register_late_pass(|| Box::new(invalid_utf8_in_unchecked::InvalidUtf8InUnchecked));
-    store.register_late_pass(|| Box::new(std_instead_of_core::StdReexports::default()));
+    store.register_late_pass(|| Box::<std_instead_of_core::StdReexports>::default());
     store.register_late_pass(|| Box::new(manual_instant_elapsed::ManualInstantElapsed));
     store.register_late_pass(|| Box::new(partialeq_to_none::PartialeqToNone));
     store.register_late_pass(|| Box::new(manual_string_new::ManualStringNew));
     store.register_late_pass(|| Box::new(unused_peekable::UnusedPeekable));
     store.register_early_pass(|| Box::new(multi_assignments::MultiAssignments));
     store.register_late_pass(|| Box::new(bool_to_int_with_if::BoolToIntWithIf));
+    store.register_late_pass(|| Box::new(box_default::BoxDefault));
     // add lints here, do not remove this comment, it's used in `new_lint`
 }