'static closures/FnDefs/futures with non-'static return type are unsound #84366
Description
Argument-free closures can return non-'static types but be 'static themselves:
fn foo<'a>() {
let closure = || -> &'a str { "" };
assert_static(closure);
}
fn assert_static<T: 'static>(_: T) {}However, associated types of 'static types must be 'static, too. So the fact that the above compiles results in unsoundness, as demonstrated below:
use std::fmt;
trait Trait {
type Associated;
}
impl<R, F: Fn() -> R> Trait for F {
type Associated = R;
}
fn static_transfers_to_associated<T: Trait + 'static>(
_: &T,
x: T::Associated,
) -> Box<dyn fmt::Display /* + 'static */>
where
T::Associated: fmt::Display,
{
Box::new(x) // T::Associated: 'static follows from T: 'static
}
fn make_static_displayable<'a>(s: &'a str) -> Box<dyn fmt::Display> {
let f = || -> &'a str { "" };
// problem is: the closure type of `f` is 'static
static_transfers_to_associated(&f, s)
}
fn main() {
let d;
{
let x = "Hello World".to_string();
d = make_static_displayable(&x);
}
println!("{}", d);
}����IV�
@rustbot modify labels: T-compiler, T-lang, A-closures, A-lifetimes, A-traits, A-typesystem, A-associated-items
and someone please add “I-unsound 💥”.
Found this bug while experimenting with variations of #84305.
Without the dyn keyword, this seems to compile “successfully” since Rust 1.4.0. Godbolt link.