prevent specialized negative impls#74648
Conversation
rust-highfive
added
the
S-waiting-on-review
lcnr
changed the title
|
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) |
|
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 |
|
@lcnr sadly, my trait actually has an associated type so current implementation of specialization won't work for me :( |
nikomatsakis
left a comment
nikomatsakis
left a comment
There was a problem hiding this comment.
Looks about right, I didn't read it 100% yet, but some of the errors seemed a bit surprising..?
| @@ -1,4 +1,4 @@ | |||
| error: auto traits must not contain where bounds | |||
| error: negative impls are not allowed for tuples | |||
There was a problem hiding this comment.
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.)
There was a problem hiding this comment.
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
|
|
||
| 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 |
There was a problem hiding this comment.
what is the "where bound" here?
There was a problem hiding this comment.
it seems like it's more about the default keyword, perhaps?
|
|
||
| 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 |
There was a problem hiding this comment.
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
|
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
added
S-waiting-on-author
|
☔ The latest upstream changes (presumably #75549) made this pull request unmergeable. Please resolve the merge conflicts. |
|
let's do this at another time before stabilizing negative impls. This is not really important rn |
6 participants
closes #74629
r? @nikomatsakis