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29967ef6 XL |
1 | use crate::LateContext; |
2 | use crate::LateLintPass; | |
3 | use crate::LintContext; | |
4 | use rustc_hir as hir; | |
5 | use rustc_span::symbol::sym; | |
6 | ||
7 | declare_lint! { | |
8 | /// The `drop_bounds` lint checks for generics with `std::ops::Drop` as | |
9 | /// bounds. | |
10 | /// | |
11 | /// ### Example | |
12 | /// | |
13 | /// ```rust | |
14 | /// fn foo<T: Drop>() {} | |
15 | /// ``` | |
16 | /// | |
17 | /// {{produces}} | |
18 | /// | |
19 | /// ### Explanation | |
20 | /// | |
21 | /// `Drop` bounds do not really accomplish anything. A type may have | |
22 | /// compiler-generated drop glue without implementing the `Drop` trait | |
23 | /// itself. The `Drop` trait also only has one method, `Drop::drop`, and | |
24 | /// that function is by fiat not callable in user code. So there is really | |
25 | /// no use case for using `Drop` in trait bounds. | |
26 | /// | |
27 | /// The most likely use case of a drop bound is to distinguish between | |
28 | /// types that have destructors and types that don't. Combined with | |
29 | /// specialization, a naive coder would write an implementation that | |
30 | /// assumed a type could be trivially dropped, then write a specialization | |
31 | /// for `T: Drop` that actually calls the destructor. Except that doing so | |
32 | /// is not correct; String, for example, doesn't actually implement Drop, | |
33 | /// but because String contains a Vec, assuming it can be trivially dropped | |
34 | /// will leak memory. | |
35 | pub DROP_BOUNDS, | |
36 | Warn, | |
37 | "bounds of the form `T: Drop` are useless" | |
38 | } | |
39 | ||
136023e0 XL |
40 | declare_lint! { |
41 | /// The `dyn_drop` lint checks for trait objects with `std::ops::Drop`. | |
42 | /// | |
43 | /// ### Example | |
44 | /// | |
45 | /// ```rust | |
46 | /// fn foo(_x: Box<dyn Drop>) {} | |
47 | /// ``` | |
48 | /// | |
49 | /// {{produces}} | |
50 | /// | |
51 | /// ### Explanation | |
52 | /// | |
53 | /// A trait object bound of the form `dyn Drop` is most likely misleading | |
54 | /// and not what the programmer intended. | |
55 | /// | |
56 | /// `Drop` bounds do not actually indicate whether a type can be trivially | |
57 | /// dropped or not, because a composite type containing `Drop` types does | |
58 | /// not necessarily implement `Drop` itself. Naïvely, one might be tempted | |
59 | /// to write a deferred drop system, to pull cleaning up memory out of a | |
60 | /// latency-sensitive code path, using `dyn Drop` trait objects. However, | |
61 | /// this breaks down e.g. when `T` is `String`, which does not implement | |
62 | /// `Drop`, but should probably be accepted. | |
63 | /// | |
64 | /// To write a trait object bound that accepts anything, use a placeholder | |
65 | /// trait with a blanket implementation. | |
66 | /// | |
67 | /// ```rust | |
68 | /// trait Placeholder {} | |
69 | /// impl<T> Placeholder for T {} | |
70 | /// fn foo(_x: Box<dyn Placeholder>) {} | |
71 | /// ``` | |
72 | pub DYN_DROP, | |
73 | Warn, | |
74 | "trait objects of the form `dyn Drop` are useless" | |
75 | } | |
76 | ||
29967ef6 XL |
77 | declare_lint_pass!( |
78 | /// Lint for bounds of the form `T: Drop`, which usually | |
79 | /// indicate an attempt to emulate `std::mem::needs_drop`. | |
136023e0 | 80 | DropTraitConstraints => [DROP_BOUNDS, DYN_DROP] |
29967ef6 XL |
81 | ); |
82 | ||
83 | impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints { | |
84 | fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) { | |
5869c6ff | 85 | use rustc_middle::ty::PredicateKind::*; |
29967ef6 | 86 | |
6a06907d | 87 | let predicates = cx.tcx.explicit_predicates_of(item.def_id); |
29967ef6 | 88 | for &(predicate, span) in predicates.predicates { |
5869c6ff | 89 | let trait_predicate = match predicate.kind().skip_binder() { |
29967ef6 XL |
90 | Trait(trait_predicate, _constness) => trait_predicate, |
91 | _ => continue, | |
92 | }; | |
93 | let def_id = trait_predicate.trait_ref.def_id; | |
94 | if cx.tcx.lang_items().drop_trait() == Some(def_id) { | |
95 | // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern. | |
96 | if trait_predicate.trait_ref.self_ty().is_impl_trait() { | |
97 | continue; | |
98 | } | |
99 | cx.struct_span_lint(DROP_BOUNDS, span, |lint| { | |
100 | let needs_drop = match cx.tcx.get_diagnostic_item(sym::needs_drop) { | |
101 | Some(needs_drop) => needs_drop, | |
102 | None => return, | |
103 | }; | |
104 | let msg = format!( | |
105 | "bounds on `{}` are useless, consider instead \ | |
106 | using `{}` to detect if a type has a destructor", | |
107 | predicate, | |
108 | cx.tcx.def_path_str(needs_drop) | |
109 | ); | |
110 | lint.build(&msg).emit() | |
111 | }); | |
112 | } | |
113 | } | |
114 | } | |
136023e0 XL |
115 | |
116 | fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx>) { | |
117 | let bounds = match &ty.kind { | |
118 | hir::TyKind::TraitObject(bounds, _lifetime, _syntax) => bounds, | |
119 | _ => return, | |
120 | }; | |
121 | for bound in &bounds[..] { | |
122 | let def_id = bound.trait_ref.trait_def_id(); | |
123 | if cx.tcx.lang_items().drop_trait() == def_id { | |
124 | cx.struct_span_lint(DYN_DROP, bound.span, |lint| { | |
125 | let needs_drop = match cx.tcx.get_diagnostic_item(sym::needs_drop) { | |
126 | Some(needs_drop) => needs_drop, | |
127 | None => return, | |
128 | }; | |
129 | let msg = format!( | |
130 | "types that do not implement `Drop` can still have drop glue, consider \ | |
131 | instead using `{}` to detect whether a type is trivially dropped", | |
132 | cx.tcx.def_path_str(needs_drop) | |
133 | ); | |
134 | lint.build(&msg).emit() | |
135 | }); | |
136 | } | |
137 | } | |
138 | } | |
29967ef6 | 139 | } |