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
|
#![feature(dropck_eyepatch)]
// The point of this test is to illustrate that the `#[may_dangle]`
// attribute specifically allows, in the context of a type
// implementing `Drop`, a generic parameter to be instantiated with a
// lifetime that does not strictly outlive the owning type itself.
//
// Here we test that only the expected errors are issued.
//
// The illustration is made concrete by comparison with two variations
// on the type with `#[may_dangle]`:
//
// 1. an analogous type that does not implement `Drop` (and thus
// should exhibit maximal flexibility with respect to dropck), and
//
// 2. an analogous type that does not use `#[may_dangle]` (and thus
// should exhibit the standard limitations imposed by dropck.
//
// The types in this file follow a pattern, {D,P,S}{t,r}, where:
//
// - D means "I implement Drop"
//
// - P means "I implement Drop but guarantee my (first) parameter is
// pure, i.e., not accessed from the destructor"; no other parameters
// are pure.
//
// - S means "I do not implement Drop"
//
// - t suffix is used when the first generic is a type
//
// - r suffix is used when the first generic is a lifetime.
use std::fmt;
struct Dt<A: fmt::Debug>(&'static str, A);
struct Dr<'a, B:'a+fmt::Debug>(&'static str, &'a B);
struct Pt<A,B: fmt::Debug>(&'static str, A, B);
struct Pr<'a, 'b, B:'a+'b+fmt::Debug>(&'static str, &'a B, &'b B);
struct St<A: fmt::Debug>(&'static str, A);
struct Sr<'a, B:'a+fmt::Debug>(&'static str, &'a B);
impl<A: fmt::Debug> Drop for Dt<A> {
fn drop(&mut self) { println!("drop {} {:?}", self.0, self.1); }
}
impl<'a, B: fmt::Debug> Drop for Dr<'a, B> {
fn drop(&mut self) { println!("drop {} {:?}", self.0, self.1); }
}
unsafe impl<#[may_dangle] A, B: fmt::Debug> Drop for Pt<A, B> {
// (unsafe to access self.1 due to #[may_dangle] on A)
fn drop(&mut self) { println!("drop {} {:?}", self.0, self.2); }
}
unsafe impl<#[may_dangle] 'a, 'b, B: fmt::Debug> Drop for Pr<'a, 'b, B> {
// (unsafe to access self.1 due to #[may_dangle] on 'a)
fn drop(&mut self) { println!("drop {} {:?}", self.0, self.2); }
}
fn main() {
use std::cell::Cell;
// We use separate blocks with separate variable to prevent the error
// messages from being deduplicated.
{
let c_long;
let (mut dt, mut dr): (Dt<_>, Dr<_>);
c_long = Cell::new(1);
// No error: sufficiently long-lived state can be referenced in dtors
dt = Dt("dt", &c_long);
dr = Dr("dr", &c_long);
}
{
let (c, mut dt, mut dr): (Cell<_>, Dt<_>, Dr<_>);
c = Cell::new(1);
// No Error: destructor order precisely modelled
dt = Dt("dt", &c);
dr = Dr("dr", &c);
}
{
let (mut dt, mut dr, c_shortest): (Dt<_>, Dr<_>, Cell<_>);
c_shortest = Cell::new(1);
// Error: `c_shortest` dies too soon for the references in dtors to be valid.
dt = Dt("dt", &c_shortest);
//~^ ERROR `c_shortest` does not live long enough
dr = Dr("dr", &c_shortest);
}
{
let c_long;
let (mut pt, mut pr, c_shortest): (Pt<_, _>, Pr<_>, Cell<_>);
c_long = Cell::new(1);
c_shortest = Cell::new(1);
// No error: Drop impl asserts .1 (A and &'a _) are not accessed
pt = Pt("pt", &c_shortest, &c_long);
pr = Pr("pr", &c_shortest, &c_long);
}
{
let c_long;
let (mut pt, mut pr, c_shortest): (Pt<_, _>, Pr<_>, Cell<_>);
c_long = Cell::new(1);
c_shortest = Cell::new(1);
// Error: Drop impl's assertion does not apply to `B` nor `&'b _`
pt = Pt("pt", &c_long, &c_shortest);
//~^ ERROR `c_shortest` does not live long enough
pr = Pr("pr", &c_long, &c_shortest);
}
{
let (st, sr, c_shortest): (St<_>, Sr<_>, Cell<_>);
c_shortest = Cell::new(1);
// No error: St and Sr have no destructor.
st = St("st", &c_shortest);
sr = Sr("sr", &c_shortest);
}
}
fn use_imm<T>(_: &T) { }
|
