feat: Add TraversableFold and Folder#22
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Jun 3, 2026
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| fn fold_if_type<T, U, B, F>(this: U, fold: &mut F) -> ControlFlow<B, U> | ||
| where | ||
| T: Any, | ||
| U: Any, | ||
| F: FnMut(T) -> ControlFlow<B, T>, | ||
| { | ||
| if TypeId::of::<T>() != TypeId::of::<U>() { | ||
| return ControlFlow::Continue(this); | ||
| } | ||
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| let this = cast_between_equal_any_types::<U, T>(this); | ||
| match fold(this) { | ||
| ControlFlow::Continue(this) => { | ||
| ControlFlow::Continue(cast_between_equal_any_types::<T, U>(this)) | ||
| } | ||
| ControlFlow::Break(break_value) => ControlFlow::Break(break_value), | ||
| } | ||
| } | ||
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| fn cast_between_equal_any_types<From, To>(from: From) -> To | ||
| where | ||
| From: Any, | ||
| To: Any, | ||
| { | ||
| debug_assert_eq!(TypeId::of::<From>(), TypeId::of::<To>()); | ||
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| let from = ManuallyDrop::new(from); | ||
| // SAFETY: `fold_if_type` only calls this function after verifying that `From` and `To` have | ||
| // the same `TypeId`. `Any` is implemented only for `'static` concrete types, so equal TypeIds | ||
| // identify the same type. `ManuallyDrop` prevents dropping the source after ownership has been | ||
| // transferred through `ptr::read`. | ||
| unsafe { ptr::read((&*from as *const From).cast::<To>()) } | ||
| } |
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Use case for rust-lang/libs-team's issue.
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tisonkun
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Refs #19