Function leaks passed closure, even if lifetimes are declared to be independent

[ISibboI]

(continuing from users.rust-lang.org)

I tried this code:

// the compiler thinks that the closure could be returned by by this function
// to me, the function signature indicates that the closure and the return value are independent
fn calls_closure<'closure, 'result>(
    closure: impl 'closure + FnOnce() -> (),
) -> impl 'result + Iterator<Item = i32> {
    closure();
    [2].into_iter()
}

fn creates_closure<'closure_state, 'result>(
    closure_state: &'closure_state mut i32,
) -> impl 'result + Iterator<Item = i32> {
    calls_closure(|| {
        *closure_state += 1;
    })
}

(playground)

I expected to see this happen: it should compile and run just fine. The lifetimes 'closure and 'result of function calls_closure are not related by any bounds.

Instead, this happened:

error: lifetime may not live long enough
  --> src/lib.rs:13:5
   |
10 |   fn creates_closure<'closure_state, 'result>(
   |                      --------------  ------- lifetime `'result` defined here
   |                      |
   |                      lifetime `'closure_state` defined here
...
13 | /     calls_closure(|| {
14 | |         *closure_state += 1;
15 | |     })
   | |______^ function was supposed to return data with lifetime `'result` but it is returning data with lifetime `'closure_state`
   |
   = help: consider adding the following bound: `'closure_state: 'result`

For some reason, the compiler thinks that calls_closure can return something with a lifetime related to that of the passed closure. But the lifetimes 'closure and 'result are unrelated in its signature.

On nightly, the error message is different, but still seems wrong:

error[E0700]: hidden type for `impl Iterator<Item = i32>` captures lifetime that does not appear in bounds
  --> src/lib.rs:13:5
   |
10 |   fn creates_closure<'closure_state, 'result>(
   |                      -------------- hidden type `impl Iterator<Item = i32>` captures the lifetime `'closure_state` as defined here
...
13 | /     calls_closure(|| {
14 | |         *closure_state += 1;
15 | |     })
   | |______^
   |
help: to declare that the `impl Trait` captures `'closure_state`, you can add an explicit `'closure_state` lifetime bound
   |
12 | ) -> impl 'result + Iterator<Item = i32> + 'closure_state {
   |                                          ++++++++++++++++

For more information about this error, try `rustc --explain E0700`.

Meta

rustc --version --verbose:
stable from playground: 1.64.0
nightly from playground: 1.66.0-nightly (2022-10-08 8796e7a)

[nagisa]

This looks like a duplicate of #79415?

[ISibboI]

I think so, yes. Over there they also found an even simpler example.

[nagisa]

Duplicate of #79415