Hide = _ as associated constant value inside impl blocks by dtolnay · Pull Request #134321 · rust-lang/rust (original) (raw)

Yeah I gave this some thought before going with the current approach. From what I figured out (I haven't worked in this code before):

Before this PR, AssocConstItem and TyAssocConstItem are separate variants because they must store different contents. The variant's data is either Constant (which is Generics + ConstantKind + Type) or just Generics + Type.

In order to merge ProvidedAssocConstItem and RequiredAssocConstItem into a single TraitAssocConstItem variant, either:

• Constant needs to contain Option<ConstantKind>, which has a big blast radius because non-associated constants and paths (dyn Foo<K = 0>) use this same Constant type in a context where not having a value doesn't make sense.
• or we need TraitAssocConstItem to store not Constant but some other ConstantWithOptionalValue type — which ends up being not less duplication than splitting ProvidedAssocConstItem vs RequiredAssocConstItem.

There is a more promising approach, which I gave up on but is still potentially salvageable. Notice this parent argument:

When printing, associated constants are supposed to know whether they are inside of a ItemType::Trait as opposed to a ItemType::Impl. This would be perfect for deciding whether = _ needs to be shown (only when parent is trait, never when parent is impl).

But this doesn't work because parent ends up being misused under the assumption that it's only relevant for indentation (?). See this call site, which is inside a function called item_trait and is responsible for printing a trait and its contents. And yet it is passing ItemType::Impl as the value of parent, which does not make sense.