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Document that associated constants prevent a trait from being made into an object #48026
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r? @eddyb (rust_highfive has picked a reviewer for you, use r? to override) |
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The reason is that because the trait object implements the trait too, and what would the constant be for that trait object? E.g. given a trait fn takes_trait_obj(t: &dyn Trait) -> i32 { <dyn Trait>::CONST } Or as another (I think it was @eddyb?) said on IRC, a constant is effectively a no-argument static (const) function, and static functions aren't allowed for the same reason. |
The explaination makes sense, especially when compared to static functions. It's a shame you can't add |
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@Badel2 once the GAT work is fully done, we can probably support that, actually ( |
@bors r+ rollup |
📌 Commit 498ef20 has been approved by |
Wait, why does |
… r=nikomatsakis Document that associated constants prevent a trait from being made into an object Fixes rust-lang#47952 Add a short mention of associated constants to E0038
@eddyb it's not particularly dependent on GAT per se, but right now we don't have a notion of "where clauses that must be satisfied in order to project" -- I expect to be adding that as part of GAT, in which case it would apply here. Note the sort of subtle distinction between: trait Foo {
type Bar: Baz;
} This is a constraint on the impl that it supply a type that is trait Foo {
type Bar where Self: Sized;
} This is a constraint on the one doing the projection that |
@nikomatsakis We have them on methods and we check them when referring to those methods. |
Fixes #47952
Add a short mention of associated constants to E0038