Start writing the nameable_interfaces RFC

jseyfried · jseyfried · commit a23a5c9fc51c · 2016-02-17T01:59:01.000Z
diff --git a/text/0000-nameable-interfaces.md b/text/0000-nameable-interfaces.md
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+- Feature Name: nameable_interfaces
+- Start Date: 2016-2-10
+- RFC PR:
+- Rust Issue:
+
+# Summary
+[summary]: #summary
+
+This RFC requires that an item which is nameable from a given module have an interface
+that is nameable from the module.
+
+An item is "nameable" from a module if there is a path in the module's scope that resolves
+to the item, possibly through a reexport.
+By the "interface" of an item, I am referring to the "public-facing parts" of that item as defined
+in [RFC #0136](https://github.com/rust-lang/rfcs/pull/0136),
+i.e. the parts of the item that the RFC requires to be public whenever the item is public.
+
+# Motivation
+[motivation]: #motivation
+
+The primary motivation for nameable interfaces is refactorability.
+Nameable interfaces ensure that the type of any expression is nameable, guaranteeing that many
+common types of refactoring are always possible, including
+  - type annotating a let declaration,
+  - refactoring an expression without early returns into a function item,
+  - refactoring a closure without upvars into a function item, and
+  - refactoring a constant expression into a constant item.
+
+Another motivation is documentability. This RFC would ensure that rustdoc never has to include
+an inaccessible path in the documentation for a crate.
+
+# Detailed design
+[design]: #detailed-design
+
+TODO describe design in more detail
+
+Examples:
+```rust
+mod foo {
+    mod bar {
+        pub struct Bar;
+    }
+
+    pub fn f() -> bar::Bar {}
+    //~^ ERROR/WARN `f` is visible outside `foo`, but `bar::Bar` is not.
+    //~| NOTE consider either not declaring `f` with `pub` or reexporting `bar::Bar`.
+}
+
+mod foo {
+    mod bar {
+        pub struct Bar;
+    }
+
+    mod baz {
+        pub fn f() -> bar::Bar {}
+        //~^ ERROR/WARN `f` is visible outside `foo`, but `bar::Bar` is not.
+    }
+
+    pub use self::baz::f;
+    //~^ NOTE consider either not reexporting `f` or also reexporing `bar::Bar`.
+}
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+One drawback is the hassle for developers who have to fix unnameable interfaces or face warnings and
+eventually breakage (unless we keep it a warning indefinitely).
+One way to ameliorate this is to have clear warnings that suggest adding specific reexports
+that would bring the code up to compliance.
+
+
+The other main drawback is that adding the suggested reexports can sometimes expose previously
+private implementation details. More specifically,
+  - reexporting a struct or an enum can allow access to its inherent methods,
+  - reexporting a struct can allow access to its constructor and fields,
+  - reexporting a trait allows access to its methods, and
+  - reexporting an enum allows access to its variants and their fields.
+
+[RFC #1422](https://github.com/rust-lang/rfcs/pull/1422) will address the first two points by
+allowing developers to rehide any exposed inherent methods, struct constructors, or struct fields.
+
+More specifically, once can rehide inherent methods and struct fields by declaring them with the
+pub(restricted) syntax to restrict their visibility to where the corresponding struct or enum was
+nameable.
+For structs with fields, rehiding the fields will also rehide the constructor.
+For structs without fields, one must add a pub(restricted) dummy field to rehide the constructor.
+
+To rehide enum variants and trait methods, RFC #1422 will have to be extended to allow the
+pub(restricted) syntax on trait methods, enum variants, and the variants' fields. I think this
+extension would be a good idea anyway.
+
+# Alternatives
+[alternatives]: #alternatives
+
+We could choose not to require that interfaces be nameable.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+We could make nameable interfaces not only necessary but also sufficient to pass the
+`private_in_public` check in `librustc_privacy`. This would allow, for example,
+```rust
+mod foo {
+    struct Bar; // `Bar` is nameable inside `foo`
+    mod baz {
+        pub fn f() -> Bar {} // Since f is not nameable outside `foo`, this use of `Bar` is ok.
+    }
+}
+```
+
+This addition would make nameability synonymous with visibility, which seems to be what most people
+expect (as evidenced by the discussions in rust issues
+[#18082](https://github.com/rust-lang/rust/issues/18082),
+[#23585](https://github.com/rust-lang/rust/issues/23585), and
+[#30905](https://github.com/rust-lang/rust/issues/30905)).
