RFC: Associated type bounds of form MyTrait<AssociatedType: Bounds>

text/2289-associated-type-bounds.md

@@ -0,0 +1,243 @@
+- Feature Name: `associated_type_bounds`
+- Start Date: 2018-01-13
+- RFC PR: [rust-lang/rfcs#2289](https://github.com/rust-lang/rfcs/pull/2289)
+- Rust Issue: [rust-lang/rust#52662](https://github.com/rust-lang/rust/issues/52662)
+
+# Summary
+[summary]: #summary
+
+Introduce the bound form `MyTrait<AssociatedType: Bounds>`, permitted anywhere
+a bound of the form `MyTrait<AssociatedType = T>` would be allowed. The bound
+`T: Trait<AssociatedType: Bounds>` desugars to the bounds `T: Trait` and
+`<T as Trait>::AssociatedType: Bounds`.
+See the [reference][reference-level-explanation] and [rationale][alternatives]
+for exact details.
+
+# 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 `T: Trait<AssociatedType: Bounds>` should desugar to a pair
+of bounds: `T: Trait` and `<T as Trait>::AssociatedType: Bounds`.
+Rust currently allows both of those bounds anywhere a bound can currently appear;
+the new syntax does not introduce any new semantics.
+
+Additionally, the surface syntax `impl Trait<AssociatedType: Bounds>` turns
+into a named type variable `T`, universal or existential depending on context,
+with the usual bound `T: Trait` along with the added bound
+`<T as Trait>::AssociatedType: Bounds`.
+
+Meanwhile, the surface syntax `dyn Trait<AssociatedType: Bounds>` desugars into
+`dyn Trait<AssociatedType = T>` where `T` is a named type variable `T` with the
+bound `T: Bounds`.
+
+## The desugaring for associated types
+
+In the case of an associated type having a bound of the form:
+
+```rust
+trait TraitA {
+    type AssocA: TraitB<AssocB: TraitC>;
+}
+```
+
+we desugar to an anonymous associated type for `AssocB`, which corresponds to:
+
+```rust
+trait TraitA {
+    type AssocA: TraitB<AssocB = Self::AssocA_0>;
+    type AssocA_0: TraitC; // Associated type is Unnamed!
+}
+```
+
+## Notes on the meaning of `impl Trait<Assoc: Bound>`
+
+Note that in the context `-> impl Trait<Assoc: Bound>`, since the Trait is
+existentially quantified, the `Assoc` is as well. Semantically speaking,
+`fn printables..` is equivalent to:
+
+```rust
+fn printables() -> impl Iterator<Item = impl Display> { .. }
+```
+
+For `arg: impl Trait<Assoc: Bound>`, it is semantically equivalent to:
+`arg: impl Trait<Assoc = impl Bound>`.
+
+## Meaning of `existential type Foo: Trait<Assoc: Bound>`
+
+Given:
+
+```
+existential type Foo: Trait<Assoc: Bound>;
+```
+
+it can be seen as the same as:
+
+```rust
+existential type Foo: Trait<Assoc = _0>;
+existential type _0: Bound;
+```
+
+[RFC 2071]: https://github.com/rust-lang/rfcs/blob/master/text/2071-impl-trait-type-alias.md
+
+This syntax is specified in [RFC 2071]. As in that RFC, this documentation
+uses the non-final syntax for existential type aliases.
+
+# Drawbacks
+[drawbacks]: #drawbacks
+
+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]: #rationale-and-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.
+
+### An alternative desugaring of bounds on associated types
+
+[RFC 2089]: https://github.com/rust-lang/rfcs/blob/master/text/2089-implied-bounds.md
+
+An alternative desugaring of the following definition:
+
+```rust
+trait TraitA {
+    type AssocA: TraitB<AssocB: TraitC>;
+}
+```
+
+is to add the `where` clause, as specified above, to the trait, desugaring to:
+
+```rust
+trait TraitA
+where
+    <Self::AssocA as TraitB>::AssocB: TraitC,
+{
+    type AssocA: TraitB;
+}
+```
+
+However, at the time of this writing, a Rust compiler will treat this
+differently than the desugaring proposed in the reference.
+The following snippet illustrates the difference:
+
+```rust
+trait Foo where <Self::Bar as Iterator>::Item: Copy {
+    type Bar: Iterator;
+}
+
+trait Foo2 {
+    type Bar: Iterator<Item = Self::BarItem>;
+    type BarItem: Copy;
+}
+
+fn use_foo<X: Foo>(arg: X)
+where <X::Bar as Iterator>::Item: Copy
+// ^-- Remove this line and it will error with:
+// error[E0277]: `<<X as Foo>::Bar as std::iter::Iterator>::Item` doesn't implement `Copy`
+{
+    let item: <X::Bar as Iterator>::Item;
+}
+
+fn use_foo2<X: Foo2>(arg: X) {
+    let item: <X::Bar as Iterator>::Item;
+}
+```
+
+The desugaring with a `where` therefore becomes problematic from a perspective
+of usability.
+
+However, [RFC 2089, Implied Bounds][RFC 2089] specifies that desugaring to the
+`where` clause in the trait will permit the `use_foo` function to omit its
+`where` clause. This entails that both desugarings become equivalent from the
+point of view of a user. The desugaring with `where` therefore becomes viable
+in the presence of [RFC 2089].
+
+# Unresolved questions
+[unresolved]: #unresolved-questions
+
+- Does allowing this for `dyn` trait objects introduce any unforeseen issues?
+  This can be resolved during stabilization.
+
+- The exact desugaring in the context of putting bounds on an associated type
+  of a trait is left unresolved. The semantics should however be preserved.
+  This is also the case with other desugarings in this RFC.