Changes to `impl Trait` in Rust 2024

[NewsBot]

A new Rust blog post/announcement has been posted!

The default way impl Trait works in return position is changing in Rust 2024. These changes are meant to simplify impl Trait to better match what people want most of the time. We're also adding a flexible syntax that gives you full control when you need it.

TL;DR

Starting in Rust 2024, we are changing the rules for when a generic parameter can be used in the hidden type of a return-position impl Trait:

• a new default that the hidden types for a return-position impl Trait can use any generic parameter in scope, instead of only types (applicable only in Rust 2024);
• a syntax to declare explicitly what types may be used (usable in any edition).

The new explicit syntax is called a "use bound": impl Trait + use<'x, T>, for example, would indicate that the hidden type is allowe...

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