Repository: rust
Issue: Tracking Issue for `impl Trait` as output type of `Fn` traits in function return position
Labels: T-lang,C-tracking-issue,S-tracking-impl-incomplete,F-impl_trait_in_fn_trait_return

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This is a tracking issue for `impl Trait` as output type in `Fn` traits in return position of functions, for example `fn f() -> impl Fn() -> impl Sized { ... }`.
The feature gate for the issue is `#![feature(impl_trait_in_fn_trait_return)]`.

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### Steps
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- [x] Implement the feature (https://github.com/rust-lang/rust/pull/93582)
- [ ] Adjust documentation ([see instructions on rustc-dev-guide][doc-guide])
- [ ] Stabilization PR ([see instructions on rustc-dev-guide][stabilization-guide])

[stabilization-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#stabilization-pr
[doc-guide]: https://rustc-dev-guide.rust-lang.org/stabilization_guide.html#documentation-prs

### Unresolved Questions
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Questions to be answered in stabilization report (after gaining experience).

### Should we allow `impl Fn() -> impl Trait` in argument position of functions?

```rust
fn f(into_iter_fn: impl Fn() -> impl Iterator) {}
```

...and what does that desugar to? [See discussion here](https://github.com/rust-lang/rust/pull/93082#issuecomment-1021511579).

### Behavior of `'_`

What is the behavior of `'_` in impl trait in general and in this specific case? In this particular case, `'_` should be consistent with its use in other function signatures.

### Parsing rules

Let's document the parsing behavior and assure ourselves we are happy with it. [Some notes here.](https://github.com/rust-lang/rust/pull/93082#issuecomment-1027270291)

### Implementation history

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- https://github.com/rust-lang/rust/pull/93582