Stack overflow regression

[KamilaBorowska]

I tried this code (this is #57156 by the way causing regression AGAIN, so it may be a good idea to add this to tests):

use std::cell::Cell;

trait CloneablePredicate<'a, T>: 'a + Fn(T) -> bool {
    fn clone_boxed(&self) -> Box<dyn CloneablePredicate<'a, T, Output = bool>>;
}

impl<'a, T, F> CloneablePredicate<'a, T> for F
where
    F: 'a + Clone + Fn(T) -> bool,
{
    fn clone_boxed(&self) -> Box<dyn CloneablePredicate<'a, T, Output = bool>> {
        Box::new(self.clone())
    }
}

impl<'a, T> Clone for Box<dyn CloneablePredicate<'a, T, Output = bool>>
where
    T: 'a,
{
    fn clone(&self) -> Self {
        self.clone_boxed()
    }
}

fn main() {
    let cell = Cell::new(4);
    let x: Box<dyn CloneablePredicate<i32, Output = bool>> = Box::new(move |x| {
        let minus_one = cell.get() - 1;
        cell.set(minus_one);
        x == minus_one
    });
    let y = x.clone();
    println!("{}", x(3));
    println!("{}", x(3));
    println!("{}", y(3));
    println!("{}", y(3));
}

When running in Rust 1.35, it causes a stack overflow.

$ rustc +1.34.0 ex.rs; ./ex
true
false
true
false
$ rustc +1.35.0 ex.rs; ./ex

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

Meta:

$ rustc +1.34.0 --version --verbose
rustc 1.34.0 (91856ed52 2019-04-10)
binary: rustc
commit-hash: 91856ed52c58aa5ba66a015354d1cc69e9779bdf
commit-date: 2019-04-10
host: x86_64-unknown-linux-gnu
release: 1.34.0
LLVM version: 8.0
$ rustc +1.35.0 --version --verbose
rustc 1.35.0 (3c235d560 2019-05-20)
binary: rustc
commit-hash: 3c235d5600393dfe6c36eeed34042efad8d4f26e
commit-date: 2019-05-20
host: x86_64-unknown-linux-gnu
release: 1.35.0
LLVM version: 8.0

[KamilaBorowska]

Nevermind, this is a side-effect of introducing Fn implementation for Box<impl Fn<A> + ?Sized>. (**self).clone_boxed() is a viable workaround. Considering how contrived the entire thing is, it's probably fine.