Amend rust-lang#1268 with a more feasible proposal post-specialization

text/0000-specialization-marker-traits.md

@@ -0,0 +1,198 @@
+- Feature Name: Specialization on impls with no items
+- Start Date: 2016-04-30
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Assume that any trait impl which provides no items specializes any other impl
+which could apply to the same type, regardless of overlap.
+
+# Motivation
+[motivation]: #motivation
+
+This RFC is intended to replace [RFC #1268][1268]
+entirely. Since its acceptance, little progress has been made on that RFC due to
+implementation concerns. With the advent of specialization, there are potential
+alternatives which are potentially easier to implement, and also addresses the
+drawbacks brought by that RFC.
+
+The motivations for the feature in general are the same as the original RFC,
+improving the ergonomics of implementing marker traits.
+
+Some examples include:
+
+- the coercible trait design presents at [RFC #91][91];
+- the `ExnSafe` trait proposed in [RFC #1236][1236].
+
+# Detailed design
+[design]: #detailed-design
+
+Much of the concern around [RFC #1268][1268] was related to difficulty of
+implementation. From the "Unresolved questions" section of the original RFC:
+
+> Today, we prefer to break down an obligation like
+> `Foo: MarkerTrait` into component obligations (e.g., `Foo: Send`). Due to
+> coherence, there is always one best way to do this. That is, there is a best
+> impl to choose. But under this proposal, there would not be.
+
+> similar concerns arise with the proposals around specialization, so it may be
+> that progress on that front will answer the questions raised here
+
+Specialization appears to have made some progress on this front. However,
+specialization is primarily focused on details of the trait impl, not on the
+trait itself. For that reason, it is proposed that we amend [RFC #1268][1268] to
+a definition which is more compatible with specialization.
+
+**Any trait impl which provides no items is assumed to specialize any other
+trait impl which could apply to the same type, regardless of overlap**. At the
+time of writing, the definition of a trait item is an associated const, type, or
+method. The exact meaning of an item may change in the future as language
+features are added, but the intention is that an impl providing no items means
+that the body of the trait impl is empty.
+
+[RFC #1210][1210] states the following constraint around allowing overlap:
+
+> There has to be a way to decide which of two overlapping impls to actually use
+> for a given set of input types.
+
+This continues to hold with the proposed change, as an impl with no body is
+interchangeable with the less specific impl. This can be demonstrated with
+specialization as it exists today:
+
+```rust
+trait SayHello {
+    fn hi();
+}
+
+trait Foo {}
+trait Bar {}
+
+impl<T: Foo> SayHello for T {
+    default fn hi() {
+        println!("Hello there");
+    }
+}
+
+impl<T: Foo + Bar> SayHello for T {}
+```
+
+This change effectively has the same meaning as the original RFC, as an empty
+trait body would be rejected for a trait that requires adding items. However,
+there is an important difference in this meaning compared to the original.
+
+Overlap would become allowed between impls on traits with items, as long as
+neither overlapping trait provides any items. Expanding on the previous example,
+the following would hold:
+
+```rust
+trait SayHello {
+    fn hi();
+}
+
+trait Foo {}
+trait Bar {}
+trait Baz {}
+
+impl<T: Foo> SayHello for T {
+    default fn hi() {
+        println!("Hello there");
+    }
+}
+
+impl<T: Foo + Bar> SayHello for T {}
+impl<T: Foo + Baz> SayHello for T {}
+```
+
+This means that the only drawback from [RFC #1268](1268) is removed. Adding a
+defaulted item to a marker trait is no longer a breaking change. Adding items
+without a default is already considered to be a breaking change today.
+
+Other than overlap, the rules for what impls are accepted remain the same,
+however. Any impl which is not a `default impl` must provide an implementation
+for all required items unless they are provided by a less specific impl. That
+means that the following code would not compile:
+
+```rust
+trait SayHello {
+    fn hi();
+}
+
+trait Foo {}
+trait Bar {}
+
+impl<T: Foo> SayHello for T {
+    default fn hi() {
+        println!("Hello there");
+    }
+}
+
+impl<T: Bar> SayHello for T {}
+```
+
+For any type `T` which implements `Bar`, but not `Foo`, the second impl is
+incomplete, and hence would be rejected. However, if the item has a default at
+the trait level, the code would compile:
+
+```rust
+trait SayHello {
+    fn hi() {
+        println!("Hi, there!");
+    }
+}
+
+trait Foo {}
+trait Bar {}
+
+impl<T: Foo> SayHello for T {
+    default fn hi() {
+        println!("Hello there");
+    }
+}
+
+impl<T: Bar> SayHello for T {}
+```
+
+Since `T: Bar` provides no items, it is assumed to specialize `T: Foo` if
+permitted for the given type when checking for coherence. Typeck would be
+satisfied for any type that fulfills `T: Foo`, `T: Bar`, or `T: Foo + Bar`.
+During monomorphisation in codegen, if a type satisfies either `Foo` or
+`Foo + Bar`, the code will print "Hello there". If a type satisfies `Bar` but
+not `Foo + Bar`, it will print "Hi, there!".
+
+The overall semantics are moderately complex, but become quite simple when you
+think of them in the context of the original intention. When a trait does not
+require any items, overlap is allowed.
+
+Due to the semantics of specialization, there is one additional wrinkle, which
+is that any `default impl` with no body would be accepted. This is quirky, but
+appears to be harmless. In practice there is no reason to write an empty
+`default impl` for anything. The effects of providing a `default impl` for a
+trait with no items is intentionally left unspecified (and in fact is unclear
+from the definition of specialization without this RFC).
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+While this proposal is likely easier to implement, and more flexible than
+[RFC #1268][1268], it is more complex and has edge cases that did not exist in
+the original.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Continuing with [RFC #1268](1268) as written.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+Is this actually easier to implement than the original RFC? This RFC was written
+as a result of attempting to implement [#1268](1268), and this proved to be much
+more straightforward. However, this needs to be verified by someone more well
+versed in the compiler's internals.
+
+[1210]: https://github.com/rust-lang/rfcs/pull/1210
+[1236]: https://github.com/rust-lang/rfcs/pull/1236
+[1268]: https://github.com/rust-lang/rfcs/pull/1268
+[91]: https://github.com/rust-lang/rfcs/pull/91