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Implement outlives requirements inference for dyn and projections.
Add tests, documentation and attr for feature.
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src/doc/unstable-book/src/language-features/infer-outlives-requirements.md
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# `infer_outlives_requirements` | ||
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The tracking issue for this feature is: [#44493] | ||
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[#44493]: https://github.com/rust-lang/rust/issues/44493 | ||
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------------------------ | ||
The `infer_outlives_requirements` feature indicates that certain | ||
outlives requirements can be infered by the compiler rather than | ||
stating them explicitly. | ||
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For example, currently generic struct definitions that contain | ||
references, require where-clauses of the form T: 'a. By using | ||
this feature the outlives predicates will be infered, although | ||
they may still be written explicitly. | ||
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```rust,ignore (pseudo-Rust) | ||
struct Foo<'a, T> | ||
where T: 'a // <-- currently required | ||
{ | ||
bar: &'a T, | ||
} | ||
``` | ||
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## Examples: | ||
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```rust,ignore (pseudo-Rust) | ||
#![feature(infer_outlives_requirements)] | ||
// Implicitly infer T: 'a | ||
struct Foo<'a, T> { | ||
bar: &'a T, | ||
} | ||
``` | ||
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```rust,ignore (pseudo-Rust) | ||
#![feature(infer_outlives_requirements)] | ||
// Implicitly infer `U: 'b` | ||
struct Foo<'b, U> { | ||
bar: Bar<'b, U> | ||
} | ||
struct Bar<'a, T> where T: 'a { | ||
x: &'a (), | ||
y: T, | ||
} | ||
``` | ||
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```rust,ignore (pseudo-Rust) | ||
#![feature(infer_outlives_requirements)] | ||
// Implicitly infer `b': 'a` | ||
struct Foo<'a, 'b, T> { | ||
x: &'a &'b T | ||
} | ||
``` | ||
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```rust,ignore (pseudo-Rust) | ||
#![feature(infer_outlives_requirements)] | ||
// Implicitly infer `<T as std::iter::Iterator>::Item : 'a` | ||
struct Foo<'a, T: Iterator> { | ||
bar: &'a T::Item | ||
``` |
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