1 use clippy_utils::{
2 diagnostics::span_lint_and_sugg, get_parent_node, is_default_equivalent, macros::macro_backtrace, match_path,
3 path_def_id, paths, ty::expr_sig,
4 };
5 use rustc_errors::Applicability;
6 use rustc_hir::{
7 intravisit::{walk_ty, Visitor},
8 Block, Expr, ExprKind, Local, Node, QPath, TyKind,
9 };
10 use rustc_lint::{LateContext, LateLintPass, LintContext};
11 use rustc_middle::lint::in_external_macro;
12 use rustc_middle::ty::print::with_forced_trimmed_paths;
13 use rustc_middle::ty::IsSuggestable;
14 use rustc_session::{declare_lint_pass, declare_tool_lint};
15 use rustc_span::sym;
16
17 declare_clippy_lint! {
18 /// ### What it does
19 /// checks for `Box::new(T::default())`, which is better written as
20 /// `Box::<T>::default()`.
21 ///
22 /// ### Why is this bad?
23 /// First, it's more complex, involving two calls instead of one.
24 /// Second, `Box::default()` can be faster
25 /// [in certain cases](https://nnethercote.github.io/perf-book/standard-library-types.html#box).
26 ///
27 /// ### Example
28 /// ```rust
29 /// let x: Box<String> = Box::new(Default::default());
30 /// ```
31 /// Use instead:
32 /// ```rust
33 /// let x: Box<String> = Box::default();
34 /// ```
35 #[clippy::version = "1.66.0"]
36 pub BOX_DEFAULT,
37 perf,
38 "Using Box::new(T::default()) instead of Box::default()"
39 }
40
41 declare_lint_pass!(BoxDefault => [BOX_DEFAULT]);
42
43 impl LateLintPass<'_> for BoxDefault {
check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>)44 fn check_expr(&mut self, cx: &LateContext<'_>, expr: &Expr<'_>) {
45 if let ExprKind::Call(box_new, [arg]) = expr.kind
46 && let ExprKind::Path(QPath::TypeRelative(ty, seg)) = box_new.kind
47 && let ExprKind::Call(arg_path, ..) = arg.kind
48 && !in_external_macro(cx.sess(), expr.span)
49 && (expr.span.eq_ctxt(arg.span) || is_vec_expn(cx, arg))
50 && seg.ident.name == sym::new
51 && path_def_id(cx, ty).map_or(false, |id| Some(id) == cx.tcx.lang_items().owned_box())
52 && is_default_equivalent(cx, arg)
53 {
54 span_lint_and_sugg(
55 cx,
56 BOX_DEFAULT,
57 expr.span,
58 "`Box::new(_)` of default value",
59 "try",
60 if is_plain_default(arg_path) || given_type(cx, expr) {
61 "Box::default()".into()
62 } else if let Some(arg_ty) = cx.typeck_results().expr_ty(arg).make_suggestable(cx.tcx, true) {
63 with_forced_trimmed_paths!(format!("Box::<{arg_ty}>::default()"))
64 } else {
65 return
66 },
67 Applicability::MachineApplicable
68 );
69 }
70 }
71 }
72
is_plain_default(arg_path: &Expr<'_>) -> bool73 fn is_plain_default(arg_path: &Expr<'_>) -> bool {
74 // we need to match the actual path so we don't match e.g. "u8::default"
75 if let ExprKind::Path(QPath::Resolved(None, path)) = &arg_path.kind {
76 // avoid generic parameters
77 match_path(path, &paths::DEFAULT_TRAIT_METHOD) && path.segments.iter().all(|seg| seg.args.is_none())
78 } else {
79 false
80 }
81 }
82
is_vec_expn(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool83 fn is_vec_expn(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
84 macro_backtrace(expr.span)
85 .next()
86 .map_or(false, |call| cx.tcx.is_diagnostic_item(sym::vec_macro, call.def_id))
87 }
88
89 #[derive(Default)]
90 struct InferVisitor(bool);
91
92 impl<'tcx> Visitor<'tcx> for InferVisitor {
visit_ty(&mut self, t: &rustc_hir::Ty<'_>)93 fn visit_ty(&mut self, t: &rustc_hir::Ty<'_>) {
94 self.0 |= matches!(t.kind, TyKind::Infer | TyKind::OpaqueDef(..) | TyKind::TraitObject(..));
95 if !self.0 {
96 walk_ty(self, t);
97 }
98 }
99 }
100
given_type(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool101 fn given_type(cx: &LateContext<'_>, expr: &Expr<'_>) -> bool {
102 match get_parent_node(cx.tcx, expr.hir_id) {
103 Some(Node::Local(Local { ty: Some(ty), .. })) => {
104 let mut v = InferVisitor::default();
105 v.visit_ty(ty);
106 !v.0
107 },
108 Some(
109 Node::Expr(Expr {
110 kind: ExprKind::Call(path, args),
111 ..
112 }) | Node::Block(Block {
113 expr:
114 Some(Expr {
115 kind: ExprKind::Call(path, args),
116 ..
117 }),
118 ..
119 }),
120 ) => {
121 if let Some(index) = args.iter().position(|arg| arg.hir_id == expr.hir_id) &&
122 let Some(sig) = expr_sig(cx, path) &&
123 let Some(input) = sig.input(index) &&
124 !cx.typeck_results().expr_ty_adjusted(expr).boxed_ty().is_trait()
125 {
126 input.no_bound_vars().is_some()
127 } else {
128 false
129 }
130 },
131 _ => false,
132 }
133 }
134