Feature gate: #![feature(try_trait_v2_residual)]

This is a tracking issue for the ops::Residual trait. cc rust-lang/rfcs#3721

This is used by try_* APIs that need to change from one member of a family to another, such as

• https://doc.rust-lang.org/nightly/std/array/fn.try_from_fn.html Tracking issue for try_array_from_fn #89379
• https://doc.rust-lang.org/nightly/std/iter/trait.Iterator.html#method.try_find Tracking Issue for Iterator::try_find #63178
• https://doc.rust-lang.org/nightly/std/primitive.array.html#method.try_map Tracking issue for array::try_map #79711

For example, the closure passed to Iterator::try_find returns Foo<bool>, but the method wants to be able to return Foo<Option<<Self as Iterator>::Item>>.

Public API

// ops::Residual
trait Residual<O> {
    type TryTrait: Try<Output = O, Residual = Self>;
}
// with implementations for `Result`, `Option`, and `ControlFlow`.

Steps / History

• Implementation: Make array::{try_from_fn, try_map} and Iterator::try_find generic over Try #91286
• Final comment period (FCP)
• Stabilization PR

Unresolved Questions

• If Try::Residual is renamed (an open question in Tracking Issue for try_trait_v2, A new design for the ? desugaring (RFC#3058) #84277), this this probably should be too.