Added guideline on consuming structs and tuples

Author: aturon

features/let.md

@@ -93,3 +93,11 @@ let mut v = Vec::new();
 let v = v;
 // use v immutably henceforth
 ```
+
+### Prefer to bind all `struct` or tuple fields. [RFC]
+
+When consuming a `struct` or tuple via a `let`, bind all of the fields rather
+than using `..` to elide the ones you don't need. The benefit is that when
+fields are added, the compiler will pinpoint all of the places where that type
+of value was consumed, which will often need to be adjusted to take the new
+field properly into account.