Allow Self to appear in the where clause of trait impls (rust-lang#1647)

Merges rust-lang#1647

Author: nrc

text/1647-allow-self-in-where-clauses.md

@@ -0,0 +1,87 @@
+- Feature Name: `allow_self_in_where_clauses`
+- Start Date: 2016-06-13
+- RFC PR: [#1647](https://github.com/rust-lang/rfcs/pull/1647)
+- Rust Issue: [#38864](https://github.com/rust-lang/rust/issues/38864)
+
+# Summary
+[summary]: #summary
+
+This RFC proposes allowing the `Self` type to be used in every position in trait
+implementations, including where clauses and other parameters to the trait being
+implemented.
+
+# Motivation
+[motivation]: #motivation
+
+`Self` is a useful tool to have to reduce churn when the type changes for
+various reasons. One would expect to be able to write
+
+```rust
+impl SomeTrait for MySuperLongType<T, U, V, W, X> where
+  Self: SomeOtherTrait,
+```
+
+but this will fail to compile today, forcing you to repeat the type, and adding
+one more place that has to change if the type ever changes.
+
+By this same logic, we would also like to be able to reference associated types
+from the traits being implemented. When dealing with generic code, patterns like
+this often emerge:
+
+```rust
+trait MyTrait {
+    type MyType: SomeBound;
+}
+
+impl<T, U, V> MyTrait for SomeStruct<T, U, V> where
+    SomeOtherStruct<T, U, V>: SomeBound,
+{
+    type MyType = SomeOtherStruct<T, U, V>;
+}
+```
+
+the only reason the associated type is repeated at all is to restate the bound
+on the associated type. It would be nice to reduce some of that duplication.
+
+# Detailed design
+[design]: #detailed-design
+
+Instead of blocking `Self` from being used in the "header" of a trait impl,
+it will be understood to be a reference to the implementation type. For example,
+all of these would be valid:
+
+```rust
+impl SomeTrait for SomeType where Self: SomeOtherTrait { }
+
+impl SomeTrait<Self> for SomeType { }
+
+impl SomeTrait for SomeType where SomeOtherType<Self>: SomeTrait { }
+
+impl SomeTrait for SomeType where Self::AssocType: SomeOtherTrait {
+    AssocType = SomeOtherType;
+}
+```
+
+If the `Self` type is parameterized by `Self`, an error that the type definition
+is recursive is thrown, rather than not recognizing self.
+
+```rust
+// The error here is because this would be Vec<Vec<Self>>, Vec<Vec<Vec<Self>>>, ...
+impl SomeTrait for Vec<Self> { }
+```
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+`Self` is always less explicit than the alternative.
+
+# Alternatives
+[alternatives]: #alternatives
+
+Not implementing this is an alternative, as is accepting Self only in where clauses
+and not other positions in the impl header.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+None