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Clarify parameter and argument compatibility #516

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Clarify parameter and argument compatibility #516

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Clarify parameter and argument compatibility
emamoah Dec 30, 2025
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Clarify parameter and argument compatibility
emamoah Dec 31, 2025
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Clarify parameter and argument compatibility
emamoah Dec 31, 2025
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6 changes: 3 additions & 3 deletions src/subtyping.md
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Expand Up @@ -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.
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.
This means that `&mut &'static str` **cannot** be a *subtype* of `&mut &'b str`,
even if `'static` is a subtype of `'b`.
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 &'world str`,
even if `'static` is a subtype of `'world`.

Variance is the concept that Rust borrows to define relationships about subtypes through their generic parameters.

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