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Merge pull request #516 from emamoah/patch-2

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Clarify parameter and argument compatibility
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Yuki Okushi 3 days ago committed by GitHub
parent b88b56383f b646f1a5aa
commit 69c1421f15
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src/subtyping.md
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@ -118,9 +118,9 @@ This is a classic use-after-free bug!
Our first instinct might be to blame the `assign` impl, but there's really nothing wrong here. Our first instinct might be to blame the `assign` impl, but there's really nothing wrong here.
It shouldn't be surprising that we might want to assign a `T` into a `T`. It shouldn't be surprising that we might want to assign a `T` into a `T`.
The problem is that we cannot assume that `&mut &'static str` and `&mut &'b str` are compatible. The problem is that we cannot assume `&'static str` can still be downgraded into `&'world str` to satisfy `T`, once it's behind a `&mut` reference.
This means that `&mut &'static str` **cannot** be a *subtype* of `&mut &'b str`, This means that `&mut &'static str` **cannot** be a *subtype* of `&mut &'world str`,
even if `'static` is a subtype of `'b`. even if `'static` is a subtype of `'world`.
Variance is the concept that Rust borrows to define relationships about subtypes through their generic parameters. Variance is the concept that Rust borrows to define relationships about subtypes through their generic parameters.

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