prevent specialized negative impls #74648
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rust-highfive
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lcnr
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Does this completely forbid "ad-hoc" specialization that was possible before? #![feature(negative_impls)]
#![feature(optin_builtin_traits)]
pub trait Trait {}
pub struct Special;
impl<T> Trait for T where private::Is<T>: private::NotSpecial {}
impl Trait for Special {}
mod private {
pub auto trait NotSpecial {}
pub struct Is<T>(T);
impl !NotSpecial for Is<super::Special> {}
}(playground) |
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It does, see the discussion in #74525 (comment) for more details. There are subtle problems rn where negative impls don't work as expected. If you really need this kind of specialization, I would recommend you to use the approach taken in #74254 which shouldn't have the problems caused by partial negative impls. That one does depend on |
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@lcnr sadly, my trait actually has an associated type so current implementation of specialization won't work for me :( |
| @@ -1,4 +1,4 @@ | |||
| error: auto traits must not contain where bounds | |||
| error: negative impls are not allowed for tuples | |||
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do we disallow negative impls for all tuples? (I think that'd be ok, actually, though I think we could also permit them for tuples if they are only restricted based on arity, but I don't see the point.)
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I think allowing them for tuples will prevent us from ever adding variadic tuples, so it should be easier to just disallow them for now
| @@ -6,6 +6,7 @@ trait MyTrait { | |||
| type Foo; | |||
| } | |||
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| default impl !MyTrait for u32 {} //~ ERROR auto traits must not contain where bounds | |||
| default impl !MyTrait for u32 {} | |||
| //~^ ERROR negative impls on primitive types must not contain where bounds | |||
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what is the "where bound" here?
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it seems like it's more about the default keyword, perhaps?
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| trait MyTrait {} | ||
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| impl<T> !MyTrait for T {} //~ ERROR auto traits must not contain where bounds | ||
| impl<T> !MyTrait for T {} | ||
| //~^ ERROR negative impls on type parameters must not contain where bounds |
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is there any way to highlight the T: Sized bound? e.g., for drop we print something like
error[E0367]:
Dropimpl requiresT: std::marker::Sizedbut the struct it is implemented for does not
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This is still wip and I think I should reuse the drop code even if the self type is not an adt. Will ping you on zulip once it makes sense to review this again 😅 |
Muirrum
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☔ The latest upstream changes (presumably #75549) made this pull request unmergeable. Please resolve the merge conflicts. |
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let's do this at another time before stabilizing negative impls. This is not really important rn |
closes #74629
r? @nikomatsakis