Type alias enum variants

text/2338-type-alias-enum-variants.md

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+- Feature Name: `type_alias_enum_variants`
+- Start Date: 2018-02-15
+- RFC PR: [rust-lang/rfcs#2338](https://github.com/rust-lang/rfcs/pull/2338)
+- Rust Issue: [rust-lang/rust#49683](https://github.com/rust-lang/rust/issues/49683)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes to allow access to enum variants through type aliases. This
+enables better abstraction/information hiding by encapsulating enums in aliases
+without having to create another enum type and requiring the conversion from
+and into the "alias" enum.
+
+# Motivation
+[motivation]: #motivation
+
+While type aliases provide a useful means of encapsulating a type definition in
+order to hide implementation details or provide a more ergonomic API, the
+substitution principle currently falls down in the face of enum variants. It's
+reasonable to expect that a type alias can fully replace the original type
+specification, and so the lack of working support for aliased enum variants
+represents an ergonomic gap in the language/type system. This can be useful in
+exposing an interface from a dependency to library users while "hiding" the exact
+implementation details. There's at least some evidence that people have asked
+about this capability before.
+
+Since `Self` also works as an alias, this should also enable the use of `Self`
+in more places.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+In general, the simple explanation here is that type aliases can be used in
+more places where you currently have to go through the original type definition,
+as it relates to enum variants. As much as possible, enum variants should work
+as if the original type was specified rather than the alias. This should make
+type aliases easier to learn than before, because there are fewer exceptions
+to their applicability.
+
+```rust
+enum Foo {
+    Bar(i32),
+    Baz { i: i32 },
+}
+
+type Alias = Foo;
+
+fn main() {
+    let t = Alias::Bar(0);
+    let t = Alias::Baz { i: 0 };
+    match t {
+        Alias::Bar(_i) => {}
+        Alias::Baz { i: _i } => {}
+    }
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+If a path refers into an alias, the behavior for enum variants should be as
+if the alias was substituted with the original type. Here are some examples of
+the new behavior in edge cases:
+
+```rust
+type Alias<T> = Option<T>;
+
+mod foo {
+    pub use Alias::Some;
+}
+
+Option::<u8>::None // Not allowed
+Option::None::<u8> // Ok
+Alias::<u8>::None // Not allowed
+Alias::None::<u8> // Ok
+foo::Some::<u8> // Ok
+```
+
+This is the proposed handling for how to propagate type arguments from alias
+paths:
+
+* If the previous segment is a type (alias or enum), the variant segment
+  "gifts" its arguments to that previous segment.
+* If the previous segment is not a type (for example, a module), the variant
+  segment treats the arguments as arguments for the variant's enum.
+* In paths that specify both the alias and the variant, type arguments must
+  be specified after the variant, not after the aliased type. This extends the
+  current behavior to enum aliases.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+We should not do this if the edge cases make the implemented behavior too
+complex or surprising to reason about the alias substitution.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+This design seems like a straightforward extension of what type aliases are
+supposed to be for. In that sense, the main alternative seems to be to do
+nothing. Currently, there are two ways to work around this:
+
+1. Require the user to implement wrapper `enum`s instead of using aliases.
+   This hides more information, so it may provide more API stability. On the
+   other hand, it also mandates boxing and unboxing which has a run-time
+   performance cost; and API stability is already up to the user in most other
+   cases.
+
+2. Renaming of types via `use` statements. This provides a good solution in the
+   case where there are no type variables that you want to fill in as part of
+   the alias, but filling in variables is part of the motivating use case for
+   having aliases.
+
+As such, not implementing aliased enum variants this makes it harder to
+encapsulate or hide parts of an API.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+As far as I know, there are no unresolved questions at this time.