Missing drop in FnOnce adapter for closures

[eefriedman]

struct Foo(i32);
impl Foo {
    pub fn dowork(&self) {
        println!("foo: doing work {}", self.0);
    }
}

impl Drop for Foo {
    fn drop(&mut self) {
        println!("foo: drop {}", self.0);
    }
}

fn f<T: FnOnce()>(t: T) {
    t()
}

fn main() {
    let x = Foo(1);
    let x = move || x.dowork();
    f(x);
}

playpen

Somehow, the Foo object leaks: my guess is that it has something to do with the way we generate code for the FnOnce() trait for closures which also implement Fn().

Originally reported at #28796 (comment) .

[Manishearth]

cc @pnkfelix

I think I've seen this before, happens with other traits as well.

[nikomatsakis]

triage: P-medium

[nikomatsakis]

triage: P-medium

[thepowersgang]

This appears to only happen if 1. the closure is called via a generic function, and 2. the captured variable is not explicitly moved into the closure body.

If "let a = x;" is used in place of calling a borrowing method, the destructor is called.

[thepowersgang]

Preliminary investigation - The cause is from the closure not being treated as FnOnce when destructor calls are added to the "body" (in rustc_trans/trans/closure.rs load_closure_environment)

This is exposed when the closure is explicitly called as FnOnce (leading to ownership transfer into the closure, and hence no destructor being called).

Routes to fixing:

1. Alter ClosureKind determining to take into account usage
2. Insert a checked destructor invocation after calling a FnOnce closure (and let the drop flags prevent double-drop for working cases)
3. (Unknown if possible) When a Fn/FnMut closure is called as a FnOnce, explicitly drop after calling (in the call_once "method").