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Commit | Line | Data |
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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 | /// | |
94222f64 XL |
21 | /// A generic trait bound of the form `T: Drop` is most likely misleading |
22 | /// and not what the programmer intended (they probably should have used | |
23 | /// `std::mem::needs_drop` instead). | |
29967ef6 | 24 | /// |
94222f64 XL |
25 | /// `Drop` bounds do not actually indicate whether a type can be trivially |
26 | /// dropped or not, because a composite type containing `Drop` types does | |
27 | /// not necessarily implement `Drop` itself. Naïvely, one might be tempted | |
28 | /// to write an implementation that assumes that a type can be trivially | |
29 | /// dropped while also supplying a specialization for `T: Drop` that | |
30 | /// actually calls the destructor. However, this breaks down e.g. when `T` | |
31 | /// is `String`, which does not implement `Drop` itself but contains a | |
32 | /// `Vec`, which does implement `Drop`, so assuming `T` can be trivially | |
33 | /// dropped would lead to a memory leak here. | |
34 | /// | |
35 | /// Furthermore, the `Drop` trait only contains one method, `Drop::drop`, | |
36 | /// which may not be called explicitly in user code (`E0040`), so there is | |
37 | /// really no use case for using `Drop` in trait bounds, save perhaps for | |
38 | /// some obscure corner cases, which can use `#[allow(drop_bounds)]`. | |
29967ef6 XL |
39 | pub DROP_BOUNDS, |
40 | Warn, | |
94222f64 | 41 | "bounds of the form `T: Drop` are most likely incorrect" |
29967ef6 XL |
42 | } |
43 | ||
136023e0 XL |
44 | declare_lint! { |
45 | /// The `dyn_drop` lint checks for trait objects with `std::ops::Drop`. | |
46 | /// | |
47 | /// ### Example | |
48 | /// | |
49 | /// ```rust | |
50 | /// fn foo(_x: Box<dyn Drop>) {} | |
51 | /// ``` | |
52 | /// | |
53 | /// {{produces}} | |
54 | /// | |
55 | /// ### Explanation | |
56 | /// | |
57 | /// A trait object bound of the form `dyn Drop` is most likely misleading | |
58 | /// and not what the programmer intended. | |
59 | /// | |
60 | /// `Drop` bounds do not actually indicate whether a type can be trivially | |
61 | /// dropped or not, because a composite type containing `Drop` types does | |
62 | /// not necessarily implement `Drop` itself. Naïvely, one might be tempted | |
63 | /// to write a deferred drop system, to pull cleaning up memory out of a | |
64 | /// latency-sensitive code path, using `dyn Drop` trait objects. However, | |
65 | /// this breaks down e.g. when `T` is `String`, which does not implement | |
66 | /// `Drop`, but should probably be accepted. | |
67 | /// | |
68 | /// To write a trait object bound that accepts anything, use a placeholder | |
69 | /// trait with a blanket implementation. | |
70 | /// | |
71 | /// ```rust | |
72 | /// trait Placeholder {} | |
73 | /// impl<T> Placeholder for T {} | |
74 | /// fn foo(_x: Box<dyn Placeholder>) {} | |
75 | /// ``` | |
76 | pub DYN_DROP, | |
77 | Warn, | |
78 | "trait objects of the form `dyn Drop` are useless" | |
79 | } | |
80 | ||
29967ef6 XL |
81 | declare_lint_pass!( |
82 | /// Lint for bounds of the form `T: Drop`, which usually | |
83 | /// indicate an attempt to emulate `std::mem::needs_drop`. | |
136023e0 | 84 | DropTraitConstraints => [DROP_BOUNDS, DYN_DROP] |
29967ef6 XL |
85 | ); |
86 | ||
87 | impl<'tcx> LateLintPass<'tcx> for DropTraitConstraints { | |
88 | fn check_item(&mut self, cx: &LateContext<'tcx>, item: &'tcx hir::Item<'tcx>) { | |
5869c6ff | 89 | use rustc_middle::ty::PredicateKind::*; |
29967ef6 | 90 | |
6a06907d | 91 | let predicates = cx.tcx.explicit_predicates_of(item.def_id); |
29967ef6 | 92 | for &(predicate, span) in predicates.predicates { |
a2a8927a XL |
93 | let Trait(trait_predicate) = predicate.kind().skip_binder() else { |
94 | continue | |
29967ef6 XL |
95 | }; |
96 | let def_id = trait_predicate.trait_ref.def_id; | |
97 | if cx.tcx.lang_items().drop_trait() == Some(def_id) { | |
98 | // Explicitly allow `impl Drop`, a drop-guards-as-Voldemort-type pattern. | |
99 | if trait_predicate.trait_ref.self_ty().is_impl_trait() { | |
100 | continue; | |
101 | } | |
102 | cx.struct_span_lint(DROP_BOUNDS, span, |lint| { | |
a2a8927a XL |
103 | let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else { |
104 | return | |
29967ef6 XL |
105 | }; |
106 | let msg = format!( | |
94222f64 XL |
107 | "bounds on `{}` are most likely incorrect, consider instead \ |
108 | using `{}` to detect whether a type can be trivially dropped", | |
29967ef6 XL |
109 | predicate, |
110 | cx.tcx.def_path_str(needs_drop) | |
111 | ); | |
5e7ed085 | 112 | lint.build(&msg).emit(); |
29967ef6 XL |
113 | }); |
114 | } | |
115 | } | |
116 | } | |
136023e0 XL |
117 | |
118 | fn check_ty(&mut self, cx: &LateContext<'_>, ty: &'tcx hir::Ty<'tcx>) { | |
a2a8927a XL |
119 | let hir::TyKind::TraitObject(bounds, _lifetime, _syntax) = &ty.kind else { |
120 | return | |
136023e0 XL |
121 | }; |
122 | for bound in &bounds[..] { | |
123 | let def_id = bound.trait_ref.trait_def_id(); | |
124 | if cx.tcx.lang_items().drop_trait() == def_id { | |
125 | cx.struct_span_lint(DYN_DROP, bound.span, |lint| { | |
a2a8927a XL |
126 | let Some(needs_drop) = cx.tcx.get_diagnostic_item(sym::needs_drop) else { |
127 | return | |
136023e0 XL |
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 | ); | |
5e7ed085 | 134 | lint.build(&msg).emit(); |
136023e0 XL |
135 | }); |
136 | } | |
137 | } | |
138 | } | |
29967ef6 | 139 | } |