mirror of https://github.com/rust-lang/rust
68 lines
1.6 KiB
Rust
68 lines
1.6 KiB
Rust
//@ known-bug: #114484
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use std::marker::PhantomData;
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trait MyTrait {
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fn virtualize(&self) -> &dyn MyTrait;
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}
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struct VirtualWrapper<T, const L: u8>(T);
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impl<T, const L: u8> VirtualWrapper<T, L> {
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pub fn wrap(value: &T) -> &Self {
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unsafe { &*(value as *const T as *const Self) }
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}
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}
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impl<T: MyTrait + 'static, const L: u8> MyTrait for VirtualWrapper<T, L> {
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fn virtualize(&self) -> &dyn MyTrait {
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unsafe { virtualize_my_trait(L, self) }
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// unsafe { virtualize_my_trait(L, &self.0) } // <-- this code fixes the problem
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}
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}
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const unsafe fn virtualize_my_trait<T>(level: u8, obj: &T) -> &dyn MyTrait
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where
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T: MyTrait + 'static,
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{
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const fn gen_vtable_ptr<T, const L: u8>() -> *const ()
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where
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T: MyTrait + 'static,
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{
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let [_, vtable] = unsafe {
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std::mem::transmute::<*const dyn MyTrait, [*const (); 2]>(std::ptr::null::<
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VirtualWrapper<T, L>,
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>())
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};
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vtable
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}
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struct Vtable<T>(PhantomData<T>);
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impl<T> Vtable<T>
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where
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T: MyTrait + 'static,
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{
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const LEVELS: [*const (); 2] = [gen_vtable_ptr::<T, 1>(), gen_vtable_ptr::<T, 2>()];
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}
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let vtable = Vtable::<T>::LEVELS[(level != 0) as usize];
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let data = obj as *const T as *const ();
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let ptr: *const dyn MyTrait = std::mem::transmute([data, vtable]);
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&*ptr
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}
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struct SomeData<const N: usize>([u8; N]);
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impl<const N: usize> MyTrait for SomeData<N> {
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fn virtualize(&self) -> &dyn MyTrait {
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VirtualWrapper::<Self, 0>::wrap(self)
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}
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}
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fn main() {
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let test = SomeData([0; 256]);
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test.virtualize();
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}
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