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