Tracking Issue for FCW on WF checks of closure argument and return types

What is the warning for?

Ill-formed closure argument and return types during borrow check.

Why was this change made?

#151510 introduces WF checks on closure argument and return types to fix https://github.com/rust-lang/rust/issues/104478. However, a crater run caught some crates depending on this behaviour. So we now make any errors caused by this check to be FCW, which will be turned into a hard error in a future release.

Example

trait Bound<'a> {}
impl<'a> Bound<'a> for &'a str {}

struct Container<'a, K>
where
    K: Bound<'a>,
{
    _d: &'a [u8],
    _k: std::marker::PhantomData<K>,
}

fn main() {
    let _closure = |_: Container<&str>| {};
}

Which outputs:

warning: lifetime may not live long enough
  --> repro.rs:13:21
   |
13 |     let _closure = |_: Container<&str>| {};
   |                     ^            - let's call the lifetime of this reference `'2`
   |                     |
   |                     has type `Container<'1, &str>`
   |                     requires that `'1` must outlive `'2`
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #163155 <https://github.com/rust-lang/rust/issues/163155>

warning: lifetime may not live long enough
  --> repro.rs:13:21
   |
13 |     let _closure = |_: Container<&str>| {};
   |                     ^            - let's call the lifetime of this reference `'2`
   |                     |
   |                     has type `Container<'1, &str>`
   |                     requires that `'2` must outlive `'1`
   |
   = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!
   = note: for more information, see issue #163155 <https://github.com/rust-lang/rust/issues/163155>

Recommendations

It's not really easy to circumvent this when you really want to use closures.

One way: Originally posted by @lcnr in https://github.com/rust-lang/rust/issues/151510#issuecomment-5114720867

you can work around this with deduce_closure_signature but I don't want to force users to do so.

trait Bound<'a> {}
impl<'a> Bound<'a> for &'a str {}
struct Container<'a, K: Bound<'a>>(std::marker::PhantomData<*mut (&'a (), K)>);
fn id<F: for<'a> FnOnce(Container<'a, &'a str>)>(f: F) -> F { f }
fn main() {
    let _ = id(|x: Container<'_, &str>| drop(x)); // compiles
}

Using a nightly feature #![feature(closure_lifetime_binder)] : https://github.com/rust-lang/rust/issues/97362

TODO: others?

When will this warning become a hard error?

TODO

Steps

• Implementation
• Switch to a hard error

Implementation history

See #151510