2 #![feature(dropck_eyepatch)]
4 // The point of this test is to illustrate that the `#[may_dangle]`
5 // attribute specifically allows, in the context of a type
6 // implementing `Drop`, a generic parameter to be instantiated with a
7 // lifetime that does not strictly outlive the owning type itself.
9 // Here we test that a model use of `#[may_dangle]` will compile and run.
11 // The illustration is made concrete by comparison with two variations
12 // on the type with `#[may_dangle]`:
14 // 1. an analogous type that does not implement `Drop` (and thus
15 // should exhibit maximal flexibility with respect to dropck), and
17 // 2. an analogous type that does not use `#[may_dangle]` (and thus
18 // should exhibit the standard limitations imposed by dropck.
20 // The types in this file follow a pattern, {D,P,S}{t,r}, where:
22 // - D means "I implement Drop"
24 // - P means "I implement Drop but guarantee my (first) parameter is
25 // pure, i.e., not accessed from the destructor"; no other parameters
28 // - S means "I do not implement Drop"
30 // - t suffix is used when the first generic is a type
32 // - r suffix is used when the first generic is a lifetime.
34 trait Foo { fn foo(&self, _: &str); }
36 struct Dt<A: Foo>(&'static str, A);
37 struct Dr<'a, B:'a+Foo>(&'static str, &'a B);
38 struct Pt<A,B: Foo>(&'static str, #[allow(unused_tuple_struct_fields)] A, B);
39 struct Pr<'a, 'b, B:'a+'b+Foo>(&'static str, #[allow(unused_tuple_struct_fields)] &'a B, &'b B);
40 struct St<A: Foo>(&'static str, #[allow(unused_tuple_struct_fields)] A);
41 struct Sr<'a, B:'a+Foo>(&'static str, #[allow(unused_tuple_struct_fields)] &'a B);
43 impl<A: Foo> Drop for Dt<A> {
44 fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
46 impl<'a, B: Foo> Drop for Dr<'a, B> {
47 fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
49 unsafe impl<#[may_dangle] A, B: Foo> Drop for Pt<A, B> {
50 // (unsafe to access self.1 due to #[may_dangle] on A)
51 fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
53 unsafe impl<#[may_dangle] 'a, 'b, B: Foo> Drop for Pr<'a, 'b, B> {
54 // (unsafe to access self.1 due to #[may_dangle] on 'a)
55 fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
59 use std::cell::RefCell;
61 impl Foo for RefCell<String> {
62 fn foo(&self, s: &str) {
63 let s2 = format!("{}|{}", *self.borrow(), s);
64 *self.borrow_mut() = s2;
68 impl<'a, T:Foo> Foo for &'a T {
69 fn foo(&self, s: &str) {
74 struct CheckOnDrop(RefCell<String>, &'static str);
75 impl Drop for CheckOnDrop {
76 fn drop(&mut self) { assert_eq!(*self.0.borrow(), self.1); }
80 let (c, dt, dr, pt, pr, st, sr)
81 : (CheckOnDrop, Dt<_>, Dr<_>, Pt<_, _>, Pr<_>, St<_>, Sr<_>);
82 c_long = CheckOnDrop(RefCell::new("c_long".to_string()),
83 "c_long|pr|pt|dr|dt");
84 c = CheckOnDrop(RefCell::new("c".to_string()),
87 // No error: sufficiently long-lived state can be referenced in dtors
88 dt = Dt("dt", &c_long.0);
89 dr = Dr("dr", &c_long.0);
91 // No error: Drop impl asserts .1 (A and &'a _) are not accessed
92 pt = Pt("pt", &c.0, &c_long.0);
93 pr = Pr("pr", &c.0, &c_long.0);
95 // No error: St and Sr have no destructor.
99 println!("{:?}", (dt.0, dr.0, pt.0, pr.0, st.0, sr.0));
100 assert_eq!(*c_long.0.borrow(), "c_long");
101 assert_eq!(*c.0.borrow(), "c");