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Support importing enum variants through type aliases #83248
Description
RFC 2338 made it possible to access enum variants through type aliases, like so:
enum Foo { F1, F2 }
type Bar = Foo;
let x = Bar::F1;However, the following code still fails to compile:
enum Foo { F1, F2 }
type Bar = Foo;
use Bar::F1;with:
error[E0432]: unresolved import `Bar`
--> src/main.rs:6:5
|
6 | use Bar::F1;
| ^^^ `Bar` is a type alias, not a module
Type aliases being limited in this way is a problem for libstd because it means that deprecating/renaming any enum must inevitably involve a breaking change, since re-exporting cannot be used (#30827). This was encountered in #82122 (review) .
Reacted by Gus Gutoski, José Luis Cruz and Yuri Astrakhan
Activity
eddyb commentedon Mar 18, 2021
cc @petrochenkov
The reason
EnumAlias::Varianteven works, AIUI, is that it desugars to<EnumAlias<..>>::Variant, which is type-based name resolution, and therefore cannot be supported in imports (as it would require understanding the type without a typesystem to speak of).You're probably better off trying to distinguish between imports of the original vs a reexport, for determining the deprecated status.
bstrie commentedon Mar 18, 2021
@eddyb Is this the same reason that importing associated items doesn't work? I recall a tentative proposal for that from a few years ago: https://internals.rust-lang.org/t/importing-associated-constants/6610/4 , but I'm not sure what the current thinking is. Is it just something that's probably never going to be supported?
eddyb commentedon Mar 24, 2021
@bstrie Yeah. In theory, you can use
Foo::Variant(and any limitations are likely accidental) today with:You could imagine getting CTFE involved as well, so that the type can only be known after const-evaluating an arbitrarily complex expression with miri.
This is the sort of thing that separates the type-relative name resolution, from the regular name resolution: the typesystem itself has to be present for the former, but cannot soundly exist until the latter is complete.