borrowed value does not live long enough in closure

[95th]

From Stackoverflow:

https://stackoverflow.com/questions/63856055/borrow-checker-complains-for-closure-inside-loop-if-type-not-provided-explicitly

I tried this code:

fn f1(v: &str) {
}

fn main() {
    let c = |v| f1(v);
    for _ in 0..1 {
        let s = String::new();
        c(&s);
    }
}

I expected to see this happen: Code compiles

Instead, this happened: Got below error:

   |
10 |         c(&s);
   |         - ^^ borrowed value does not live long enough
   |         |
   |         borrow later used here
11 |     }
   |     - `s` dropped here while still borrowed

But if I add explicit type to closure, code compiles let c = |v: &str| f1(v);

Meta

rustc --version --verbose:

cargo 1.46.0 (149022b1d 2020-07-17)
release: 1.46.0
commit-hash: 149022b1d8f382e69c1616f6a46b69ebf59e2dea
commit-date: 2020-07-17

[95th]

Minimized a bit:

fn main() {
    let c = |v| {};
    for _ in 0..1 {
        let s = ();
        c(&s);
    }
}

[95th]

Running below gives error:

fn main() {
    let c = |v| {};
    x(c);
    y(c);
    for _ in 0..1 {
        let s = ();
        c(&s);
    }
}

fn x<F>(_: F) where for<'a> F: FnMut(&'a ()) {}
fn y<'a, F>(_: F) where F: FnMut(&'a ()) {}

gives:

error[E0308]: mismatched types
 --> src/main.rs:3:5
  |
3 |     x(c);
  |     ^ one type is more general than the other
  |
  = note: expected type `std::ops::FnOnce<(&'a (),)>`
             found type `std::ops::FnOnce<(&(),)>`

but running below gives no error:

fn main() {
    let c = |v: &()| {};
    x(c);
    y(c);
    for _ in 0..1 {
        let s = ();
        c(&s);
    }
}

fn x<F>(_: F) where for<'a> F: FnMut(&'a ()) {}
fn y<'a, F>(_: F) where F: FnMut(&'a ()) {}

Seems like an inference problem.

[viluon]

I think I ran into this issue a couple of hours ago. The following does not compile:

fn main() {
    struct Ref<'a>(&'a ());
    let closure = |x| Ref(x);

    while let Some(x) = None {
        closure(&x);
    }
}

error[E0597]: `x` does not live long enough
 --> src/main.rs:6:17
  |
6 |         closure(&x);
  |         ------- ^^ borrowed value does not live long enough
  |         |
  |         borrow later used here
7 |     }
  |     - `x` dropped here while still borrowed

(playground)

However, change while let to if let and it compiles.

-    while let Some(x) = None {
+    if let Some(x) = None {
         closure(&x);

However, duplicate the if let statement and it again does not compile.

     if let Some(x) = None {
         closure(&x);
     }
+
+    if let Some(x) = None {
+        closure(&x);
+    }

error[E0597]: `x` does not live long enough
  --> src/main.rs:6:17
   |
6  |         closure(&x);
   |                 ^^ borrowed value does not live long enough
7  |     }
   |     - `x` dropped here while still borrowed
...
10 |         closure(&x);
   |         ------- borrow later used here

(playground)

You can fix either of the failing examples by adding a lifetime-polymorphic type annotation to the closure:

-    let closure = |x| Ref(x);
+    let closure: for<'a> fn(&'a ()) -> Ref<'a> = |x| Ref(x);

or by manually inlining the closure:

-        closure(&x);
+        Ref(&x);

[martin-t]

Could this be related to #86921 ? There is at least some superficial similarity in that closure lifetimes are not inferred correctly although in this case the error is at the call site while in #86921 the declaration itself is rejected.

[anuradhawick]

How could this be a bug?

I thought this is a result of closures capturing the running context. Captured reference cease before closure ceases, hence the error.

Any thoughts?