rust/tests/ui/span/dropck-object-cycle.rs
2023-01-11 09:32:08 +00:00

48 lines
1.4 KiB
Rust

// This test used to be part of a run-pass test, but revised outlives
// rule means that it no longer compiles.
#![allow(unused_variables)]
trait Trait<'a> {
fn long(&'a self) -> isize;
fn short<'b>(&'b self) -> isize;
}
fn object_invoke1<'d>(x: &'d dyn Trait<'d>) -> (isize, isize) { loop { } }
trait MakerTrait {
fn mk() -> Self;
}
fn make_val<T:MakerTrait>() -> T {
MakerTrait::mk()
}
impl<'t> MakerTrait for Box<dyn Trait<'t>+'static> {
fn mk() -> Box<dyn Trait<'t>+'static> { loop { } }
}
pub fn main() {
let m : Box<dyn Trait+'static> = make_val();
assert_eq!(object_invoke1(&*m), (4,5));
//~^ ERROR `*m` does not live long enough
// the problem here is that the full type of `m` is
//
// Box<Trait<'m>+'static>
//
// Here `'m` must be exactly the lifetime of the variable `m`.
// This is because of two requirements:
// 1. First, the basic type rules require that the
// type of `m`'s value outlives the lifetime of `m`. This puts a lower
// bound `'m`.
//
// 2. Meanwhile, the signature of `object_invoke1` requires that
// we create a reference of type `&'d Trait<'d>` for some `'d`.
// `'d` cannot outlive `'m`, so that forces the lifetime to be `'m`.
//
// This then conflicts with the dropck rules, which require that
// the type of `m` *strictly outlives* `'m`. Hence we get an
// error.
}