RFC for trait object guidelines

Author: aturon

errors/signaling.md

@@ -77,7 +77,7 @@ parameters beyond their static type.
 
 ### For preventable errors, prefer `Result`.
 
-Preventable errors present API designers with a choice:
+For preventable errors, API designers have to make a choice:
 * Permit erroneous input, return `Result`, and use the `Err` variant to inform
   the client of the error.
 * Treat erroneous input as a _contract violation_ (i.e., assertion failure) and `fail!`.

features/traits/README.md

@@ -3,14 +3,19 @@
 Traits are probably Rust's most complex feature, supporting a wide range of use
 cases and design tradeoffs. Patterns of trait usage are still emerging.
 
-### Know whether a trait will be used as an object. [OPEN]
+### Know whether a trait will be used as an object. [RFC]
 
-Trait objects have some [significant limitations](objects.md).
+Trait objects have some [significant limitations](objects.md): methods
+invoked through a trait object cannot use generics, and cannot use
+`Self` except in receiver position.
 
-When designing a trait, decide early on whether the trait will be used as an
-object, in part by considering these limitations.
+When designing a trait, decide early on whether the trait will be used
+as an [object](objects.md) or as a [bound on generics](generics.md);
+the tradeoffs are discussed in each of the linked sections.
+
+If a trait is meant to be used as an object, its methods should take
+and return trait objects rather than use generics.
 
-> **[OPEN]** Is there more specific advice we can give here?
 
 ### Default methods [OPEN]