impl FnOnce hidden type captures more than the closure itself

[kuviman]

I tried this code:

trait FooTrait {}

struct Foo<'a> {
    value: i32,
    x: &'a mut dyn FooTrait,
}

impl<'a> Foo<'a> {
    fn works(&self) -> Box<dyn FnOnce() -> i32 + '_> {
        Box::new(|| self.value)
    }
    fn doent(&self) -> impl FnOnce() -> i32 + '_ {
        || self.value
    }
}

I expected both methods to compile, but only the dyn version does.

The error says:

error[E0700]: hidden type for `impl FnOnce() -> i32 + '_` captures lifetime that does not appear in bounds
  --> src/lib.rs:13:9
   |
8  | impl<'a> Foo<'a> {
   |      -- hidden type `{closure@src/lib.rs:13:9: 13:11}` captures the lifetime `'a` as defined here
...
12 |     fn doent(&self) -> impl FnOnce() -> i32 + '_ {
   |                        ------------------------- opaque type defined here
13 |         || self.value
   |         ^^^^^^^^^^^^^

The error says that the closure captures lifetime 'a, although the closure in the dyn version is the same and compiles successfully without capturing 'a

Tried both current stable and nightly

playground

[ryanavella]

It compiles on nightly if you add #![feature(lifetime_capture_rules_2024)], so this is probably the expected (if surprising) behavior of lifetime capture rules in return-position impl Traits. At least until the 2024 edition.

Related: #117587, rust-lang/rfcs#3498