auto merge of #7204 : alexcrichton/rust/deriving-to-string, r=pcwalton

Closes #7180 and #7179.

Before, the `deriving(ToStr)` attribute was essentially `fmt!("%?")`. This changes it to recursively invoke `to_str()` on fields instead of relying on `fmt!`-style things. This seems more natural to me and what should actually be expected.

1 files changed, 3 insertions, 1 deletions

diff --git a/doc/rust.md b/doc/rust.md
index f533bb50f75..3dc70ed10c7 100644
--- a/doc/rust.md
+++ b/doc/rust.md
@@ -1563,7 +1563,9 @@ Supported traits for `deriving` are:
 * `Rand`, to create a random instance of a data type.
 * `Zero`, to create an zero (or empty) instance of a data type.
 * `ToStr`, to convert to a string. For a type with this instance,
-  `obj.to_str()` has the same output as `fmt!("%?", obj)`.
+  `obj.to_str()` has similar output as `fmt!("%?", obj)`, but it differs in that
+  each constituent field of the type must also implement `ToStr` and will have
+  `field.to_str()` invoked to build up the result.
 
 # Statements and expressions