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The #[pointee] attribute is required even if there is only one generic parameter #129465

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Darksonn opened this issue Aug 23, 2024 · 2 comments · Fixed by #129467
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The #[pointee] attribute is required even if there is only one generic parameter #129465

Darksonn opened this issue Aug 23, 2024 · 2 comments · Fixed by #129467
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@dingxiangfei2009
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C-bug Category: This is a bug. F-derive_smart_pointer `#![feature(derive_smart_pointer)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@Darksonn
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Darksonn commented Aug 23, 2024 •
edited by rustbot
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Currently, the following code triggers a compilation failure.

#![feature(derive_smart_pointer)]
use core::marker::SmartPointer;

#[repr(transparent)]
#[derive(SmartPointer)]
struct MyArc<T: ?Sized> {
    inner: *mut T,
}
error: At least one generic type should be designated as `#[pointee]` in order to derive `SmartPointer` traits
 --> src/lib.rs:5:10
  |
5 | #[derive(SmartPointer)]
  |          ^^^^^^^^^^^^
  |
  = note: this error originates in the derive macro `SmartPointer` (in Nightly builds, run with -Z macro-backtrace for more info)

However, the RFC says that specifying #[pointee] when the struct has only one type parameter is allowed, but not required.

See also this patch on the LKML.

Tracking issue: #123430
cc @dingxiangfei2009
@rustbot rustbot added needs-triage This issue may need triage. Remove it if it has been sufficiently triaged. F-derive_smart_pointer `#![feature(derive_smart_pointer)]` labels Aug 23, 2024
Darksonn added a commit to Darksonn/linux that referenced this issue Aug 23, 2024
@Darksonn
rust: experiment with #[derive(SmartPointer)]
572e93f 
I am sending this RFC patch to share my experience with using the new
`#[derive(SmartPointer)]` feature [1] with our custom smart pointers.
The feature is being added so that the kernel can stop using the
unstable dispatch_from_dyn and unsize features.

In general, the feature appears to work. As can be seen in the change to
`rust_minimal.rs`, it is possible to use `Arc` together with a dynamic
trait object, and the trait object is object safe even though it uses
the custom smart pointer as a self parameter.

I did run into one nit, which is that `Arc` requires the `#[pointee]`
annotation even though there's only one generic paramter. I filed an
issue [2] about this.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: rust-lang/rust#129465 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Darksonn added a commit to Darksonn/linux that referenced this issue Aug 23, 2024
@Darksonn
rust: experiment with #[derive(SmartPointer)]
56c0c1e 
I am sending this RFC patch to share my experience with using the new
`#[derive(SmartPointer)]` feature [1] with our custom smart pointers.
The feature is being added so that the kernel can stop using the
unstable dispatch_from_dyn and unsize features.

In general, the feature appears to work. As can be seen in the change to
`rust_minimal.rs`, it is possible to use `Arc` together with a dynamic
trait object, and the trait object is object safe even though it uses
the custom smart pointer as a self parameter.

I did run into one nit, which is that `Arc` requires the `#[pointee]`
annotation even though there's only one generic paramter. I filed an
issue [2] about this.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: rust-lang/rust#129465 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Darksonn added a commit to Darksonn/linux that referenced this issue Aug 23, 2024
@Darksonn
rust: experiment with #[derive(SmartPointer)]
eeddb87 
I am sending this RFC patch to share my experience with using the new
`#[derive(SmartPointer)]` feature [1] with our custom smart pointers.
The feature is being added so that the kernel can stop using the
unstable dispatch_from_dyn and unsize features.

In general, the feature appears to work. As can be seen in the change to
`rust_minimal.rs`, it is possible to use `Arc` together with a dynamic
trait object, and the trait object is object safe even though it uses
the custom smart pointer as a self parameter.

I did run into one nit, which is that `Arc` requires the `#[pointee]`
annotation even though there's only one generic paramter. I filed an
issue [2] about this.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: rust-lang/rust#129465 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
Darksonn added a commit to Darksonn/linux that referenced this issue Aug 23, 2024
@Darksonn
rust: experiment with #[derive(SmartPointer)]
74f615c 
I am sending this RFC patch to share my experience with using the new
`#[derive(SmartPointer)]` feature [1] with our custom smart pointers.
The feature is being added so that the kernel can stop using the
unstable dispatch_from_dyn and unsize features.

In general, the feature appears to work. As can be seen in the change to
`rust_minimal.rs`, it is possible to use `Arc` together with a dynamic
trait object, and the trait object is object safe even though it uses
the custom smart pointer as a self parameter.

I did run into one nit, which is that `Arc` requires the `#[pointee]`
annotation even though there's only one generic paramter. I filed an
issue [2] about this.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: rust-lang/rust#129465 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
@ojeda ojeda mentioned this issue Aug 23, 2024
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@dingxiangfei2009
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@Darksonn
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Oh, also, the error message says "At least one generic" but it's not valid to have several pointees.

@dingxiangfei2009 dingxiangfei2009 mentioned this issue Aug 23, 2024
Merged
intel-lab-lkp pushed a commit to intel-lab-lkp/linux that referenced this issue Aug 26, 2024
@Darksonn @intel-lab-lkp
rust: experiment with #[derive(SmartPointer)]
e8b105c 
I am sending this RFC patch to share my experience with using the new
`#[derive(SmartPointer)]` feature [1] with our custom smart pointers.
The feature is being added so that the kernel can stop using the
unstable dispatch_from_dyn and unsize features.

In general, the feature appears to work. As can be seen in the change to
`rust_minimal.rs`, it is possible to use `Arc` together with a dynamic
trait object, and the trait object is object safe even though it uses
the custom smart pointer as a self parameter.

I did run into one nit, which is that `Arc` requires the `#[pointee]`
annotation even though there's only one generic paramter. I filed an
issue [2] about this.

Link: https://rust-lang.github.io/rfcs/3621-derive-smart-pointer.html [1]
Link: rust-lang/rust#129465 [2]
Signed-off-by: Alice Ryhl <aliceryhl@google.com>
matthiaskrgr added a commit to matthiaskrgr/rust that referenced this issue Aug 28, 2024
@matthiaskrgr
Rollup merge of rust-lang#129467 - dingxiangfei2009:smart-pointer-rel…
4168c39 
…ax-pointee, r=compiler-errors

derive(SmartPointer): assume pointee from the single generic and better error messages

Fix rust-lang#129465

Actually RFC says that `#[pointee]` can be inferred when there is no ambiguity, or there is only one generic type parameter so to say.

cc `@Darksonn`

r? `@compiler-errors`
workingjubilee added a commit to workingjubilee/rustc that referenced this issue Aug 29, 2024
@workingjubilee
Rollup merge of rust-lang#129467 - dingxiangfei2009:smart-pointer-rel…
c05e451 
…ax-pointee, r=compiler-errors

derive(SmartPointer): assume pointee from the single generic and better error messages

Fix rust-lang#129465

Actually RFC says that `#[pointee]` can be inferred when there is no ambiguity, or there is only one generic type parameter so to say.

cc ``@Darksonn``

r? ``@compiler-errors``
@bors bors closed this as completed in d2418cb Aug 29, 2024
rust-timer added a commit to rust-lang-ci/rust that referenced this issue Aug 29, 2024
@rust-timer
Unrolled build for rust-lang#129467
52c9ca0 
Rollup merge of rust-lang#129467 - dingxiangfei2009:smart-pointer-relax-pointee, r=compiler-errors

derive(SmartPointer): assume pointee from the single generic and better error messages

Fix rust-lang#129465

Actually RFC says that `#[pointee]` can be inferred when there is no ambiguity, or there is only one generic type parameter so to say.

cc ```@Darksonn```

r? ```@compiler-errors```
@saethlin saethlin added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. 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 Aug 29, 2024
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C-bug Category: This is a bug. F-derive_smart_pointer `#![feature(derive_smart_pointer)]` T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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