RFC: Inherent traits

text/0000-inherent-traits.md

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+- Feature Name: inherent_traits
+- Start Date: 2018-01-21
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Provides a mechanism to declare "inherent traits" for a type defined in the same crate. Methods on these traits are callable on instances
+of the specified type without needing to import the trait.
+
+# Motivation
+[motivation]: #motivation
+
+There are two similar cases where this is valuable:
+
+- Mapping object-oriented APIs.
+
+  When mapping these APIs to rust, base classes are usually mapped to traits: methods on those base classes will need to be callable on any
+  derived type. This is sub-optimal because to use a class method a user must now know which class in the hierachy defined that
+  method, so that they can import and use the corresponding trait. This knowledge is not required when using the same API from an
+  object-oriented language.
+
+- Frequently used types.
+
+  Sometimes getting the right abstractions require breaking up a type's implementation into many traits, with only a few methods per
+  trait. Every use of such a type results in a large number of imports to ensure the correct traits are in scope. If such a type is used
+  frequently, then this burden quickly becomes a pain point for users of the API.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+The feature is implemented using a new attribute which can be applied to `impl` blocks:
+
+```rust
+pub struct Foo;
+
+trait Bar {
+    fn bar(&self);
+}
+
+impl Bar for Foo {
+    fn bar(&self) { println!("foo::bar"); }
+}
+
+#[include(Bar)]
+impl Foo {
+    fn foo(&self) { println!("foo::foo"); }
+}
+```
+
+The method `bar` is now callable on any instance of `Foo`, regardless of whether the `Bar` trait is currently in scope, or even whether
+the `Bar` trait is publically visible.
+
+The `impl` block may be empty, in which case the only methods defined on the type are those from any included traits.
+
+The `include` attribute may include multiple traits.
+
+The `include` attribute is not valid on `impl <trait> for <type>` style `impl` blocks.
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The `include` attribute in the above example makes the `impl` block equivalent to:
+
+```rust
+impl Foo {
+    #[inline]
+    pub fn bar(&self) { <Self as Bar>::bar(self); }
+
+    fn foo(&self) { println!("foo::foo"); }
+}
+```
+
+Any questions regarding coherence, visibility or syntax can be resolved by comparing against this expansion, although the feature need not
+be implemented as an expansion within the compiler.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+- Increased complexity of the language.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+- Do nothing: users may choose to workaround the issue by manually performing the expansion if required.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Syntax bike-shedding