Add explanations about what derived trait implementations do

1 files changed, 5 insertions, 1 deletions

diff --git a/src/libcore/fmt/mod.rs b/src/libcore/fmt/mod.rs
index dde4d03dad8..6579e5dab54 100644
--- a/src/libcore/fmt/mod.rs
+++ b/src/libcore/fmt/mod.rs
@@ -318,7 +318,11 @@ impl<'a> Display for Arguments<'a> {
 ///
 /// [module]: ../../std/fmt/index.html
 ///
-/// This trait can be used with `#[derive]`.
+/// This trait can be used with `#[derive]` if all fields implement `Debug`. When
+/// `derive`d for structs, it will use the name of the `struct`, then `{`, then a
+/// comma-separated list of each field's name and `Debug` value, then `}`. For
+/// `enum`s, it will use the name of the variant and, if applicable, `(`, then the
+/// `Debug` values of the fields, then `)`.
 ///
 /// # Examples
 ///