Update section on "existential type" to "opaque type"

Author: varkor

Diff for: src/existential-types.md

@@ -0,0 +1,62 @@
+# Opaque types (type alias `impl Trait`)
+
+Opaque types are syntax to declare an opaque type alias that only
+exposes a specific set of traits as their interface; the concrete type in the
+background is inferred from a certain set of use sites of the opaque type.
+
+This is expressed by using `impl Trait` within type aliases, for example:
+
+```rust,ignore
+type Foo = impl Bar;
+```
+
+This declares an opaque type named `Foo`, of which the only information is that
+it implements `Bar`. Therefore, any of `Bar`'s interface can be used on a `Foo`,
+but nothing else (regardless of whether it implements any other traits).
+
+Since there needs to be a concrete background type, you can currently
+express that type by using the opaque type in a "defining use site".
+
+```rust,ignore
+struct Struct;
+impl Bar for Struct { /* stuff */ }
+fn foo() -> Foo {
+    Struct
+}
+```
+
+Any other "defining use site" needs to produce the exact same type.
+
+## Defining use site(s)
+
+Currently only the return value of a function can be a defining use site
+of an opaque type (and only if the return type of that function contains
+the opaque type).
+
+The defining use of an opaque type can be any code *within* the parent
+of the opaque type definition. This includes any siblings of the
+opaque type and all children of the siblings.
+
+The initiative for *"not causing fatal brain damage to developers due to
+accidentally running infinite loops in their brain while trying to
+comprehend what the type system is doing"* has decided to disallow children
+of opaque types to be defining use sites.
+
+### Associated opaque types
+
+Associated opaque types can be defined by any other associated item
+on the same trait `impl` or a child of these associated items. For instance:
+
+```rust,ignore
+trait Baz {
+    type Foo;
+    fn foo() -> Self::Foo;
+}
+
+struct Quux;
+
+impl Baz for Quux {
+    type Foo = impl Bar;
+    fn foo() -> Self::Foo { ... }
+}
+```