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Tweak reference for precise capturing in traits #1769

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Mar 26, 2025
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Tweak reference for precise capturing in traits #1769

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5 changes: 4 additions & 1 deletion src/types/impl-trait.md
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Expand Up @@ -132,14 +132,17 @@ fn capture<'a, 'b, T>(x: &'a (), y: T) -> impl Sized + use<'a, T> {
```

r[type.impl-trait.generic-capture.precise.constraint-single]
Currently, only one `use<..>` bound may be present in a bounds list, such bounds are not allowed in the signature of items of a trait definition, all in-scope type and const generic parameters must be included, and all lifetime parameters that appear in other bounds of the abstract type must be included.
Currently, only one `use<..>` bound may be present in a bounds list, all in-scope type and const generic parameters must be included, and all lifetime parameters that appear in other bounds of the abstract type must be included.

r[type.impl-trait.generic-capture.precise.constraint-lifetime]
Within the `use<..>` bound, any lifetime parameters present must appear before all type and const generic parameters, and the elided lifetime (`'_`) may be present if it is otherwise allowed to appear within the `impl Trait` return type.

r[type.impl-trait.generic-capture.precise.constraint-param-impl-trait]
Because all in-scope type parameters must be included by name, a `use<..>` bound may not be used in the signature of items that use argument-position `impl Trait`, as those items have anonymous type parameters in scope.

r[type.impl-trait.generic-capture.precise.constraint-in-trait]
Any `use<..>` bound that is present in an associated function in a trait definition must include all generic parameters of the trait, including the implicit `Self` generic type parameter of the trait.

## Differences between generics and `impl Trait` in return position

In argument position, `impl Trait` is very similar in semantics to a generic type parameter.
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