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lines changed Original file line number Diff line number Diff line change @@ -308,6 +308,20 @@ impl<T: ?Sized> Clone for ContravariantType<T> {
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///
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/// For more information about variance, refer to this Wikipedia
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/// article <http://en.wikipedia.org/wiki/Variance_%28computer_science%29>.
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+ ///
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+ /// # Example
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+ ///
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+ /// The Cell type is an example of an `InvariantType` which uses unsafe
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+ /// code to achieve "interior" mutability:
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+ ///
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+ /// ```
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+ /// struct Cell<T> { value: T }
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+ /// ```
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+ ///
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+ /// The type system would infer that `value` is only read here
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+ /// and never written, but in fact `Cell` uses unsafe code to achieve
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+ /// interior mutability. In order to get correct behavior, the
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+ /// `InvariantType` marker must be applied.
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#[ unstable( feature = "core" ,
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reason = "likely to change with new variance strategy" ) ]
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#[ lang="invariant_type" ]
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