Feature request: Blanket Implementation for the trait #19

stevefan1999-personal · 2024-09-10T13:37:50Z

This code:

use type_uuid::{TypeUuid};
use cglue::*;


#[cglue_trait]
pub trait UUIDIdentifiable {
    fn uuid(&self) -> UUID;
}

impl<T: TypeUuid> UUIDIdentifiable for T {
    fn uuid(&self) -> UUID {
        UUID { value: <Self as TypeUuid>::UUID }
    }
}


#[repr(C)]
#[derive(Debug, Clone, Copy)]
pub struct UUID {
    value: [u8; 16],
}


#[repr(C)]
#[derive(TypeUuid)]
#[uuid = "d4adfc76-f5f4-40b0-8e28-8a51a12f5e46"]
pub struct MyType;

Is currently not able to compile:

   Compiling vtable_uuid v0.1.0 (F:\rust\vtable_uuid)
error[E0119]: conflicting implementations of trait `UUIDIdentifiable`
  --> src/main.rs:5:1
   |
5  | #[cglue_trait]
   | ^^^^^^^^^^^^^^ conflicting implementation
...
10 | impl<T: TypeUuid> UUIDIdentifiable for T {
   | ---------------------------------------- first implementation here
   |
   = note: this error originates in the attribute macro `cglue_trait` (in Nightly builds, run with -Z macro-backtrace for more info)

For more information about this error, try `rustc --explain E0119`.
error: could not compile `vtable_uuid` (bin "vtable_uuid") due to 1 previous error

Indeed there is another blanket implementation that makes it not okay:

        impl<
            'cglue_a,
            CGlueO: 'cglue_a + UUIDIdentifiableVtblGet<'cglue_a>
                + UUIDIdentifiableOpaqueObj<'cglue_a>
                + UUIDIdentifiableAssocBind<Assocs = ()>
                + ::cglue::trait_group::GetVtblBase<
                    UUIDIdentifiableVtbl<
                        'cglue_a,
                        <Self as ::cglue::trait_group::GetContainer>::ContType,
                    >,
                >,
        > UUIDIdentifiable for CGlueO {
            #[inline(always)]
            fn uuid(&self) -> UUID {
                let __cglue_vfunc = self.get_vtbl().uuid;
                let cont = self.ccont_ref();
                let mut ret = __cglue_vfunc(cont);
                ret
            }
        }

This feature would be very useful, so we can now attach UUID to a struct and then put them into a universal container for applications to search in runtime. This is adapted by Microsoft in their COM IPC mechanism with a vtable hack, and also UEFI implementation

h33p · 2025-04-28T11:35:08Z

I had a crack at this, and I think I figured out a way to work around this on CGlue's side:

#[repr(C)]
struct CGlue<T>(pub T);

impl<T: GetVtbl<MagicVtbl<()>>> Magic for CGlue<T> {
    fn magic(&self) {
        let v = self.0.get_vtbl();
        let p = self.0.get_obj();
        (v.magic)(p)
    }
}

impl<T: Magic> Magic for &T {
    fn magic(&self) {
        T::magic(*self)
    }
}

Background:

Currently, cglue codegen spits out implementation for CGlueO, which conflicts with any blanket impl. I did this for a generic, instead of CGlueObj<...>, because I needed to support arbitrary trait_group! objects. However, in both cases I can wrap around a shared object, like CGlue<T>, which can then limit the generic impl to just cglue objects. Should be in for 0.4.

h33p self-assigned this Apr 28, 2025

h33p added bug Something isn't working enhancement New feature or request labels Apr 28, 2025

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Feature request: Blanket Implementation for the trait #19

Feature request: Blanket Implementation for the trait #19

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Feature request: Blanket Implementation for the trait #19

Feature request: Blanket Implementation for the trait #19

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