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103 lines
3.5 KiB
Rust
103 lines
3.5 KiB
Rust
// run-pass
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#![feature(dropck_eyepatch)]
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// The point of this test is to illustrate that the `#[may_dangle]`
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// attribute specifically allows, in the context of a type
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// implementing `Drop`, a generic parameter to be instantiated with a
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// lifetime that does not strictly outlive the owning type itself.
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//
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// Here we test that a model use of `#[may_dangle]` will compile and run.
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//
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// The illustration is made concrete by comparison with two variations
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// on the type with `#[may_dangle]`:
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//
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// 1. an analogous type that does not implement `Drop` (and thus
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// should exhibit maximal flexibility with respect to dropck), and
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//
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// 2. an analogous type that does not use `#[may_dangle]` (and thus
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// should exhibit the standard limitations imposed by dropck.
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//
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// The types in this file follow a pattern, {D,P,S}{t,r}, where:
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//
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// - D means "I implement Drop"
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//
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// - P means "I implement Drop but guarantee my (first) parameter is
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// pure, i.e., not accessed from the destructor"; no other parameters
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// are pure.
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//
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// - S means "I do not implement Drop"
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//
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// - t suffix is used when the first generic is a type
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//
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// - r suffix is used when the first generic is a lifetime.
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trait Foo { fn foo(&self, _: &str); }
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struct Dt<A: Foo>(&'static str, A);
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struct Dr<'a, B:'a+Foo>(&'static str, &'a B);
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struct Pt<A,B: Foo>(&'static str, #[allow(dead_code)] A, B);
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struct Pr<'a, 'b, B:'a+'b+Foo>(&'static str, #[allow(dead_code)] &'a B, &'b B);
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struct St<A: Foo>(&'static str, #[allow(dead_code)] A);
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struct Sr<'a, B:'a+Foo>(&'static str, #[allow(dead_code)] &'a B);
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impl<A: Foo> Drop for Dt<A> {
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fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
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}
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impl<'a, B: Foo> Drop for Dr<'a, B> {
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fn drop(&mut self) { println!("drop {}", self.0); self.1.foo(self.0); }
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}
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unsafe impl<#[may_dangle] A, B: Foo> Drop for Pt<A, B> {
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// (unsafe to access self.1 due to #[may_dangle] on A)
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fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
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}
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unsafe impl<#[may_dangle] 'a, 'b, B: Foo> Drop for Pr<'a, 'b, B> {
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// (unsafe to access self.1 due to #[may_dangle] on 'a)
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fn drop(&mut self) { println!("drop {}", self.0); self.2.foo(self.0); }
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}
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fn main() {
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use std::cell::RefCell;
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impl Foo for RefCell<String> {
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fn foo(&self, s: &str) {
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let s2 = format!("{}|{}", *self.borrow(), s);
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*self.borrow_mut() = s2;
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}
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}
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impl<'a, T:Foo> Foo for &'a T {
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fn foo(&self, s: &str) {
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(*self).foo(s);
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}
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}
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struct CheckOnDrop(RefCell<String>, &'static str);
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impl Drop for CheckOnDrop {
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fn drop(&mut self) { assert_eq!(*self.0.borrow(), self.1); }
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}
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let c_long;
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let (c, dt, dr, pt, pr, st, sr)
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: (CheckOnDrop, Dt<_>, Dr<_>, Pt<_, _>, Pr<_>, St<_>, Sr<_>);
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c_long = CheckOnDrop(RefCell::new("c_long".to_string()),
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"c_long|pr|pt|dr|dt");
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c = CheckOnDrop(RefCell::new("c".to_string()),
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"c");
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// No error: sufficiently long-lived state can be referenced in dtors
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dt = Dt("dt", &c_long.0);
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dr = Dr("dr", &c_long.0);
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// No error: Drop impl asserts .1 (A and &'a _) are not accessed
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pt = Pt("pt", &c.0, &c_long.0);
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pr = Pr("pr", &c.0, &c_long.0);
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// No error: St and Sr have no destructor.
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st = St("st", &c.0);
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sr = Sr("sr", &c.0);
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println!("{:?}", (dt.0, dr.0, pt.0, pr.0, st.0, sr.0));
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assert_eq!(*c_long.0.borrow(), "c_long");
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assert_eq!(*c.0.borrow(), "c");
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}
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