docs: show Display forwarding in #[from] example

README.md

@@ -25,7 +25,7 @@ use thiserror::Error;
 
 #[derive(Error, Debug)]
 pub enum DataStoreError {
-    #[error("data store disconnected")]
+    #[error("data store disconnected: {0}")]
     Disconnect(#[from] io::Error),
     #[error("the data for key `{0}` is not available")]
     Redaction(String),
@@ -93,6 +93,10 @@ pub enum DataStoreError {
   source error (and possibly a backtrace — see below). Usually `#[from]`
   fields are unnamed, but `#[from]` is allowed on a named field too.
 
+  To include the source's `Display` output in your error message, use `{0}`
+  (or `{var}` for a named field) in the `#[error("...")]` format string, as in
+  the `Disconnect` example above.
+
   ```rust
   #[derive(Error, Debug)]
   pub enum MyError {

src/lib.rs

@@ -19,7 +19,7 @@
 //!
 //! #[derive(Error, Debug)]
 //! pub enum DataStoreError {
-//!     #[error("data store disconnected")]
+//!     #[error("data store disconnected: {0}")]
 //!     Disconnect(#[from] io::Error),
 //!     #[error("the data for key `{0}` is not available")]
 //!     Redaction(String),
@@ -104,6 +104,10 @@
 //!   `#[from]` fields are unnamed, but `#[from]` is allowed on a named field
 //!   too.
 //!
+//!   To include the source's [`Display`] output in your error message, use
+//!   `{0}` (or `{var}` for a named field) in the `#[error("...")]` format
+//!   string, as in the `Disconnect` example above.
+//!
 //!   ```rust
 //!   # use core::fmt::{self, Display};
 //!   # use std::io;