Add slide about Box<dyn Error> before introducing anyhow.

src/SUMMARY.md

@@ -154,6 +154,7 @@
   - [Propagating Errors with ?](error-handling/try-operator.md)
     - [Converting Error Types](error-handling/converting-error-types.md)
     - [Deriving Error Enums](error-handling/deriving-error-enums.md)
+    - [Dynamic Error Types](error-handling/dynamic-errors.md)
     - [Adding Context to Errors](error-handling/error-contexts.md)
 - [Testing](testing.md)
   - [Unit Tests](testing/unit-tests.md)

src/error-handling/dynamic-errors.md

@@ -0,0 +1,41 @@
+# Dynamic Error Types
+
+Sometimes we want to allow any type of error to be returned without writing our own enum covering
+all the different possibilities. `std::error::Error` makes this easy.
+
+```rust,editable,compile_fail
+use std::fs::{self, File};
+use std::io::Read;
+use thiserror::Error;
+use std::error::Error;
+
+#[derive(Clone, Debug, Eq, Error, PartialEq)]
+#[error("Found no username in {0}")]
+struct EmptyUsernameError(String);
+
+fn read_username(path: &str) -> Result<String, Box<dyn Error>> {
+    let mut username = String::with_capacity(100);
+    File::open(path)?.read_to_string(&mut username)?;
+    if username.is_empty() {
+        return Err(EmptyUsernameError(String::from(path)).into());
+    }
+    Ok(username)
+}
+
+fn main() {
+    //fs::write("config.dat", "").unwrap();
+    match read_username("config.dat") {
+        Ok(username) => println!("Username: {username}"),
+        Err(err)     => println!("Error: {err}"),
+    }
+}
+```
+
+<details>
+
+This saves on code, but gives up the ability to cleanly handle different error cases differently in
+the program. As such it's generally not a good idea to use `Box<dyn Error>` in the public API of a
+library, but it can be a good option in a program where you just want to display the error message
+somewhere.
+
+</details>