Un-feature-gate struct variants

Author: sfackler

text/0000-struct-variants.md

@@ -0,0 +1,121 @@
+- Start Date: 2014-10-25
+- RFC PR:
+- Rust Issue:
+
+# Summary
+
+Just like structs, variants can come in three forms - unit-like, tuple-like,
+or struct-like:
+```rust
+enum Foo {
+    Foo,
+    Bar(int, String),
+    Baz { a: int, b: String }
+}
+```
+The last form is currently feature gated. This RFC proposes to remove that gate
+before 1.0.
+
+# Motivation
+
+Tuple variants with multiple fields can become difficult to work with,
+especially when the types of the fields don't make it obvious what each one is.
+It is not an uncommon sight in the compiler to see inline comments used to help
+identify the various variants of an enum, such as this snippet from
+`rustc::middle::def`:
+```rust
+pub enum Def {
+    // ...
+    DefVariant(ast::DefId /* enum */, ast::DefId /* variant */, bool /* is_structure */),
+    DefTy(ast::DefId, bool /* is_enum */),
+    // ...
+}
+```
+If these were changed to struct variants, this ad-hoc documentation would move
+into the names of the fields themselves. These names are visible in rustdoc,
+so a developer doesn't have to go source diving to figure out what's going on.
+In addition, the fields of struct variants can have documentation attached.
+```rust
+pub enum Def {
+    // ...
+    DefVariant {
+        enum_did: ast::DefId,
+        variant_did: ast::DefId,
+        /// Identifies the variant as tuple-like or struct-like
+        is_structure: bool,
+    },
+    DefTy {
+        did: ast::DefId,
+        is_enum: bool,
+    },
+    // ...
+}
+```
+
+As the number of fields in a variant increases, it becomes increasingly crucial
+to use struct variants. For example, consider this snippet from
+`rust-postgres`:
+```rust
+enum FrontendMessage<'a> {
+    // ...
+    Bind {
+        pub portal: &'a str,
+        pub statement: &'a str,
+        pub formats: &'a [i16],
+        pub values: &'a [Option<Vec<u8>>],
+        pub result_formats: &'a [i16]
+    },
+    // ...
+}
+```
+If we convert `Bind` to a tuple variant:
+```rust
+enum FrontendMessage<'a> {
+    // ...
+    Bind(&'a str, &'a str, &'a [i16], &'a [Option<Vec<u8>>], &'a [i16]),
+    // ...
+}
+```
+we run into both the documentation issues discussed above, as well as ergonomic
+issues. If code only cares about the `values` and `formats` fields, working
+with a struct variant is nicer:
+```rust
+match msg {
+    // you can reorder too!
+    Bind { values, formats, .. } => ...
+    // ...
+}
+```
+versus
+```rust
+match msg {
+    Bind(_, _, formats, values, _) => ...
+    // ...
+}
+```
+
+This feature gate was originally put in place because there were many serious
+bugs in the compiler's support for struct variants. This is not the case today.
+The issue tracker does not appear have any open correctness issues related to
+struct variants and many libraries, including rustc itself, have been using
+them without trouble for a while.
+
+# Detailed design
+
+Change the `Status` of the `struct_variant` feature from `Active` to
+`Accepted`.
+
+# Drawbacks
+
+Adding formal support for a feature increases the maintenance burden of rustc.
+
+# Alternatives
+
+If struct variants remain feature gated at 1.0, libraries that want to ensure
+that they will continue working into the future will be forced to avoid struct
+variants since there are no guarantees about backwards compatibility of
+feature-gated parts of the language.
+
+# Unresolved questions
+
+N/A