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trait Trait: Send: upcasting from dyn Trait to dyn Send does not work #126313

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lcnr opened this issue Jun 12, 2024 · 1 comment · May be fixed by #126660
Open

trait Trait: Send: upcasting from dyn Trait to dyn Send does not work #126313

lcnr opened this issue Jun 12, 2024 · 1 comment · May be fixed by #126660
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C-bug Category: This is a bug. F-trait_upcasting `#![feature(trait_upcasting)]` T-types Relevant to the types team, which will review and decide on the PR/issue.

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@lcnr
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lcnr commented Jun 12, 2024

The following compiles:

#![feature(trait_upcasting)]
trait Super {}
trait Trait: Super {}
fn foo(x: &dyn Trait) -> &dyn Super { x }

while this does not:

#![feature(trait_upcasting)]
trait Super {}
trait Trait: Send {}
fn foo(x: &dyn Trait) -> &dyn Send { x }

cc @compiler-errors I thought we actually had a PR for that at some point 🤔 I think that's actually not the case though. It wasn't part of #119338
@rustbot rustbot added the needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. label Jun 12, 2024
@lcnr lcnr changed the title cannot upcast from dyn TraitWithSendSuper to dyn Send trait Trait: Send: upcasting from dyn Trait to dyn Send does not work Jun 12, 2024
@lcnr lcnr added F-trait_upcasting `#![feature(trait_upcasting)]` T-types Relevant to the types team, which will review and decide on the PR/issue. labels Jun 12, 2024
@lcnr lcnr mentioned this issue Jun 12, 2024
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@compiler-errors
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I had a separate PR to upcast dropping the principal but I think I never FCPd it because I was too lazy: #114679 (comment)

workingjubilee added a commit to workingjubilee/rustc that referenced this issue Jun 12, 2024
@workingjubilee
Rollup merge of rust-lang#126314 - lcnr:fix-relnotes, r=pietroalbini
0a52849 
fix RELEASES: we do not support upcasting to auto traits

rust-lang#119338 does not actually support casts from `dyn Trait` to `dyn Auto`, only from `dyn Trait` to `dyn Trait   Auto`. The following test does not compile

```rust
trait Trait: Send {}
impl<T: Send> Trait for T {}

fn foo() {
    let x: &i32 = &1;
    let y = x as &dyn Trait as &dyn Send;
}
```
it actually also doesn't compile with `feature(trait_upcasting)`, opened rust-lang#126313 for that

r? `@Mark-Simulacrum` `@cuviper`
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Jun 12, 2024
@rust-timer
Unrolled build for rust-lang#126314
bf68fc4 
Rollup merge of rust-lang#126314 - lcnr:fix-relnotes, r=pietroalbini

fix RELEASES: we do not support upcasting to auto traits

rust-lang#119338 does not actually support casts from `dyn Trait` to `dyn Auto`, only from `dyn Trait` to `dyn Trait   Auto`. The following test does not compile

```rust
trait Trait: Send {}
impl<T: Send> Trait for T {}

fn foo() {
    let x: &i32 = &1;
    let y = x as &dyn Trait as &dyn Send;
}
```
it actually also doesn't compile with `feature(trait_upcasting)`, opened rust-lang#126313 for that

r? `@Mark-Simulacrum` `@cuviper`
@saethlin saethlin added C-bug Category: This is a bug. and removed needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. labels Jun 13, 2024
@Jules-Bertholet Jules-Bertholet linked a pull request Jun 19, 2024 that will close this issue
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C-bug Category: This is a bug. F-trait_upcasting `#![feature(trait_upcasting)]` T-types Relevant to the types team, which will review and decide on the PR/issue.
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Allow dropping dyn Trait principal Jules-Bertholet/rust
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@compiler-errors @saethlin @lcnr @rustbot