rfc, associated-type-bounds: initial text

Author: Centril

text/0000-associated-type-bounds.md

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+- Feature Name: associated_type_bounds
+- Start Date: 2018-01-13
+- RFC PR: (leave this empty)
+- Rust Issue: (leave this empty)
+
+# Summary
+[summary]: #summary
+
+Introduce the bound form `MyTrait<AssociatedType: Bounds>`, permitted anywhere
+a bound of the form `MyTrait<AssociatedType = T>` would be allowed. This form
+desugars to `MyTrait<AssociatedType = impl Bounds>`.
+
+# Motivation
+[motivation]: #motivation
+
+Currently, when specifying a bound using a trait that has an associated
+type, the developer can specify the precise type via the syntax
+`MyTrait<AssociatedType = T>`. With the introduction of the `impl Trait`
+syntax for static-dispatch existential types, this syntax also permits
+`MyTrait<AssociatedType = impl Bounds>`, as a shorthand for introducing a
+new type variable and specifying those bounds.
+
+However, this introduces an unnecessary level of indirection that does not
+match the developer's intuition and mental model as well as it could. In
+particular, given the ability to write bounds on a type variable as `T: Bounds`,
+it makes sense to permit writing bounds on an associated type directly.
+This results in the simpler syntax `MyTrait<AssociatedType: Bounds>`.
+
+# Guide-level explanation
+[guide-level-explanation]: #guide-level-explanation
+
+Instead of specifying a concrete type for an associated type, we can
+specify a bound on the associated type, to ensure that it implements
+specific traits, as seen in the example below:
+
+```rust
+fn print_all<T: Iterator<Item: Display>>(printables: T) {
+    for p in printables {
+        println!("{}", p);
+    }
+}
+```
+
+## In anonymous existential types
+
+```rust
+fn printables() -> impl Iterator<Item: Display> {
+    // ..
+}
+```
+
+## Further examples
+
+Instead of writing:
+
+```rust
+impl<I> Clone for Peekable<I>
+where
+    I: Clone + Iterator,
+    <I as Iterator>::Item: Clone,
+{
+    // ..
+}
+```
+
+you may write:
+
+```rust
+impl<I> Clone for Peekable<I>
+where
+    I: Clone + Iterator<Item: Clone>
+{
+    // ..
+}
+```
+
+or replace the `where` clause entirely:
+
+```rust
+impl<I: Clone + Iterator<Item: Clone>> Clone for Peekable<I> {
+    // ..
+}
+```
+
+# Reference-level explanation
+[reference-level-explanation]: #reference-level-explanation
+
+The surface syntax `Trait<AssociatedType: Bounds>` should desugar to
+`Trait<AssociatedType = impl Bounds>` anywhere it appears. This syntax
+does not introduce any new semantics that the availability of
+`impl Bounds` does not already introduce.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+With the introduction of the `impl Trait` syntax, Rust code can already
+express this using the desugared form. This proposal just introduces a
+simpler surface syntax that parallels other uses of bounds. As always,
+when introducing new syntactic forms, an increased burden is put on
+developers to know about and understand those forms, and this proposal
+is no different. However, we believe that the parallel to the use of bounds
+elsewhere makes this new syntax immediately recognizable and understandable.
+
+# Rationale and alternatives
+[alternatives]: #alternatives
+
+As with any new surface syntax, one alternative is simply not introducing
+the syntax at all. That would still leave developers with the
+`MyTrait<AssociatedType = impl Bounds>` form. However, allowing the more
+direct bounds syntax provides a better parallel to the use of bounds elsewhere.
+The introduced form in this RFC is comparatively both shorter and clearer.
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Does this introduce any parsing ambiguities?
+- Does allowing this for `dyn` trait objects introduce any unforseen issues?