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Showing content from https://github.com/rust-lang/rust/issues/45994 below:

[impl Trait] Should we allow `impl Trait` after `->` in `fn` types or parentheses sugar? · Issue #45994 · rust-lang/rust · GitHub

RFC 1951 disallowed uses of impl Trait within Fn trait sugar or higher-ranked bounds. For example, the following is disallowed:

fn foo(f: impl Fn(impl SomeTrait) -> impl OtherTrait)
fn bar() -> (impl Fn(impl SomeTrait) -> impl OtherTrait)

This tracking issue exists to discuss -- if we were to allow them -- what semantics they ought to have. Some known concerns around the syntax are:

• Should the () switch from existential to universal quantification and back?
  • I think the general feeling here is now "no", basically because "too complex".
• If HRTB were introduced, where would we (e.g.) want impl OtherTrait to be bound?

For consistency, we are disallow fn(impl SomeTrait) -> impl OtherTrait and dyn Fn(impl SomeTrait) -> impl OtherTrait as well. When considering the questions, one should also consider what the meaning would be in those contexts.

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