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Auto merge of rust-lang#120248 - WaffleLapkin:bonk-ptr-object-casts, …
…r=<try> Make casts of pointers to trait objects stricter This is an attempt to `fix` rust-lang#120222 and rust-lang#120217. This is done by adding restrictions on casting pointers to trait objects. Currently the restriction is > When casting `*const X<dyn A>` -> `*const Y<dyn B>`, if `B` has a principal trait, `dyn A + 'erased: Unsize<dyn B + 'erased>` must hold This ensures that 1. Principal trait's generic arguments match (no `*const dyn Tr<A>` -> `*const dyn Tr<B>` casts, which are a problem for [rust-lang#120222](rust-lang#120222)) 2. Principal trait's lifetime arguments match (no `*const dyn Tr<'a>` -> `*const dyn Tr<'b>` casts, which are a problem for [rust-lang#120217](rust-lang#120217)) 3. No auto traits can be _added_ (this is a problem for arbitrary self types, see [this comment](rust-lang#120248 (comment))) Some notes: - We only care about the metadata/last field, so you can still cast `*const dyn T` to `*const WithHeader<dyn T>`, etc - The lifetime of the trait object itself (`dyn A + 'lt`) is not checked, so you can still cast `*mut FnOnce() + '_` to `*mut FnOnce() + 'static`, etc - This feels fishy, but I couldn't come up with a reason it must be checked - The new checks are only done if `B` has a principal, so you can still do any kinds of cast, if the target only has auto traits - This is because auto traits are not enough to get unsoundness issues that this PR fixes - ...and so it makes sense to minimize breakage The plan is to, ~~once the checks are properly implemented~~, run crate to determine how much damage does this do. The diagnostics are currently not great, to say the least, but as far as I can tell this correctly fixes the issues. cc `@oli-obk` `@compiler-errors` `@lcnr`
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// check-fail | ||
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trait Trait<'a> {} | ||
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fn add_auto<'a>(x: *mut dyn Trait<'a>) -> *mut (dyn Trait<'a> + Send) { | ||
x as _ //~ error: the trait bound `dyn Trait<'_>: Unsize<dyn Trait<'_> + Send>` is not satisfied | ||
} | ||
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fn main() {} |
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error[E0277]: the trait bound `dyn Trait<'_>: Unsize<dyn Trait<'_> + Send>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-add-auto.rs:6:5 | ||
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LL | x as _ | ||
| ^^^^^^ the trait `Unsize<dyn Trait<'_> + Send>` is not implemented for `dyn Trait<'_>` | ||
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= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
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error: aborting due to 1 previous error | ||
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For more information about this error, try `rustc --explain E0277`. |
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// check-fail | ||
// | ||
// issue: <https://github.com/rust-lang/rust/issues/120222> | ||
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trait A {} | ||
impl<T> A for T {} | ||
trait B {} | ||
impl<T> B for T {} | ||
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trait Trait<G> {} | ||
struct X; | ||
impl<T> Trait<X> for T {} | ||
struct Y; | ||
impl<T> Trait<Y> for T {} | ||
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fn main() { | ||
let a: *const dyn A = &(); | ||
let b: *const dyn B = a as _; //~ error: the trait bound `dyn A: Unsize<dyn B>` is not satisfied | ||
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let x: *const dyn Trait<X> = &(); | ||
let y: *const dyn Trait<Y> = x as _; //~ error: the trait bound `dyn Trait<X>: Unsize<dyn Trait<Y>>` is not satisfied | ||
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_ = (b, y); | ||
} | ||
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fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) { | ||
let _: *const dyn Trait<T> = x as _; //~ error: the trait bound `dyn Trait<X>: Unsize<dyn Trait<T>>` is not satisfied | ||
let _: *const dyn Trait<X> = t as _; //~ error: the trait bound `dyn Trait<T>: Unsize<dyn Trait<X>>` is not satisfied | ||
} | ||
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trait Assocked { | ||
type Assoc: ?Sized; | ||
} | ||
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fn change_assoc(x: *mut dyn Assocked<Assoc = u8>) -> *mut dyn Assocked<Assoc = u32> { | ||
x as _ //~ error: the trait bound `dyn Assocked<Assoc = u8>: Unsize<dyn Assocked<Assoc = u32>>` is not satisfied | ||
} |
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error[E0277]: the trait bound `dyn A: Unsize<dyn B>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-different-args.rs:19:27 | ||
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LL | let b: *const dyn B = a as _; | ||
| ^^^^^^ the trait `Unsize<dyn B>` is not implemented for `dyn A` | ||
| | ||
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
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error[E0277]: the trait bound `dyn Trait<X>: Unsize<dyn Trait<Y>>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-different-args.rs:22:34 | ||
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LL | let y: *const dyn Trait<Y> = x as _; | ||
| ^^^^^^ the trait `Unsize<dyn Trait<Y>>` is not implemented for `dyn Trait<X>` | ||
| | ||
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
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error[E0277]: the trait bound `dyn Trait<X>: Unsize<dyn Trait<T>>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-different-args.rs:28:34 | ||
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LL | let _: *const dyn Trait<T> = x as _; | ||
| ^^^^^^ the trait `Unsize<dyn Trait<T>>` is not implemented for `dyn Trait<X>` | ||
| | ||
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement | ||
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LL | fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) where dyn Trait<X>: Unsize<dyn Trait<T>> { | ||
| ++++++++++++++++++++++++++++++++++++++++ | ||
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error[E0277]: the trait bound `dyn Trait<T>: Unsize<dyn Trait<X>>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-different-args.rs:29:34 | ||
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LL | let _: *const dyn Trait<X> = t as _; | ||
| ^^^^^^ the trait `Unsize<dyn Trait<X>>` is not implemented for `dyn Trait<T>` | ||
| | ||
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
help: consider introducing a `where` clause, but there might be an alternative better way to express this requirement | ||
| | ||
LL | fn generic<T>(x: *const dyn Trait<X>, t: *const dyn Trait<T>) where dyn Trait<T>: Unsize<dyn Trait<X>> { | ||
| ++++++++++++++++++++++++++++++++++++++++ | ||
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error[E0277]: the trait bound `dyn Assocked<Assoc = u8>: Unsize<dyn Assocked<Assoc = u32>>` is not satisfied | ||
--> $DIR/ptr-to-trait-obj-different-args.rs:37:5 | ||
| | ||
LL | x as _ | ||
| ^^^^^^ the trait `Unsize<dyn Assocked<Assoc = u32>>` is not implemented for `dyn Assocked<Assoc = u8>` | ||
| | ||
= note: all implementations of `Unsize` are provided automatically by the compiler, see <https://doc.rust-lang.org/stable/std/marker/trait.Unsize.html> for more information | ||
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error: aborting due to 5 previous errors | ||
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For more information about this error, try `rustc --explain E0277`. |
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