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The rules for casting `*mut X<dyn A>` -> `*mut Y<dyn B>` are as follows: - If `B` has a principal - `A` must have exactly the same principal (including generics) - Auto traits of `B` must be a subset of autotraits in `A` Note that `X<_>` and `Y<_>` can be identity, or arbitrary structs with last field being the dyn type. The lifetime of the trait object itself (`dyn ... + 'a`) is not checked. This prevents a few soundness issues with `#![feature(arbitrary_self_types)]` and trait upcasting. Namely, these checks make sure that vtable is always valid for the pointee.
11 lines
370 B
Plaintext
11 lines
370 B
Plaintext
error: lifetime may not live long enough
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--> $DIR/ptr-to-trait-obj-different-regions-lt-ext.rs:12:5
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LL | fn bad_cast<'a>(x: *const dyn Static<'static>) -> *const dyn Static<'a> {
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| -- lifetime `'a` defined here
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LL | x as _
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| ^^^^^^ returning this value requires that `'a` must outlive `'static`
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error: aborting due to 1 previous error
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