Be clear that the methods-and-traits exercise does not require generics (#2568)

When teaching the course, I got a little tripped up thinking students
would need to make the `VerbosityFilter` generic over `Logger`. Let's be
clearer that this is not required, and will be described later.

This also updates the generic-types slide to repeat the exercise,
completing that thought.

Author: djmitche

src/generics/generic-data.md

@@ -4,46 +4,54 @@ minutes: 10
 
 # Generic Data Types
 
-You can use generics to abstract over the concrete field type:
+You can use generics to abstract over the concrete field type. Returning to the
+exercise for the previous segment:
 
 ```rust,editable
-#[derive(Debug)]
-struct Point<T> {
-    x: T,
-    y: T,
+pub trait Logger {
+    /// Log a message at the given verbosity level.
+    fn log(&self, verbosity: u8, message: &str);
 }
 
-impl<T> Point<T> {
-    fn coords(&self) -> (&T, &T) {
-        (&self.x, &self.y)
+struct StderrLogger;
+
+impl Logger for StderrLogger {
+    fn log(&self, verbosity: u8, message: &str) {
+        eprintln!("verbosity={verbosity}: {message}");
     }
+}
+
+/// Only log messages up to the given verbosity level.
+struct VerbosityFilter<L: Logger> {
+    max_verbosity: u8,
+    inner: L,
+}
 
-    fn set_x(&mut self, x: T) {
-        self.x = x;
+impl<L: Logger> Logger for VerbosityFilter<L> {
+    fn log(&self, verbosity: u8, message: &str) {
+        if verbosity <= self.max_verbosity {
+            self.inner.log(verbosity, message);
+        }
     }
 }
 
 fn main() {
-    let integer = Point { x: 5, y: 10 };
-    let float = Point { x: 1.0, y: 4.0 };
-    println!("{integer:?} and {float:?}");
-    println!("coords: {:?}", integer.coords());
+    let logger = VerbosityFilter { max_verbosity: 3, inner: StderrLogger };
+    logger.log(5, "FYI");
+    logger.log(2, "Uhoh");
 }
 ```
 
 <details>
 
-- _Q:_ Why `T` is specified twice in `impl<T> Point<T> {}`? Isn't that
-  redundant?
+- _Q:_ Why `L` is specified twice in `impl<L: Logger> .. VerbosityFilter<L>`?
+  Isn't that redundant?
   - This is because it is a generic implementation section for generic type.
     They are independently generic.
-  - It means these methods are defined for any `T`.
-  - It is possible to write `impl Point<u32> { .. }`.
-    - `Point` is still generic and you can use `Point<f64>`, but methods in this
-      block will only be available for `Point<u32>`.
-
-- Try declaring a new variable `let p = Point { x: 5, y: 10.0 };`. Update the
-  code to allow points that have elements of different types, by using two type
-  variables, e.g., `T` and `U`.
+  - It means these methods are defined for any `L`.
+  - It is possible to write `impl VerbosityFilter<StderrLogger> { .. }`.
+    - `VerbosityFilter` is still generic and you can use `VerbosityFilter<f64>`,
+      but methods in this block will only be available for
+      `Point<StderrLogger>`.
 
 </details>

src/methods-and-traits/exercise.md

@@ -1,5 +1,5 @@
 ---
-minutes: 20
+minutes: 15
 ---
 
 # Exercise: Logger Trait
@@ -14,13 +14,14 @@ verbosity. Your task is to write a `VerbosityFilter` type that will ignore
 messages above a maximum verbosity.
 
 This is a common pattern: a struct wrapping a trait implementation and
-implementing that same trait, adding behavior in the process. What other kinds
-of wrappers might be useful in a logging utility?
+implementing that same trait, adding behavior in the process. In the "Generics"
+segment this afternoon, we will see how to make the wrapper generic over the
+wrapped type.
 
 ```rust,compile_fail
 {{#include exercise.rs:setup}}
 
-// TODO: Define and implement `VerbosityFilter`.
+// TODO: Implement the `Logger` trait for `VerbosityFilter`.
 
 {{#include exercise.rs:main}}
 ```

src/methods-and-traits/exercise.rs

@@ -26,13 +26,13 @@ impl Logger for StderrLogger {
         eprintln!("verbosity={verbosity}: {message}");
     }
 }
-// ANCHOR_END: setup
 
 /// Only log messages up to the given verbosity level.
 struct VerbosityFilter {
     max_verbosity: u8,
     inner: StderrLogger,
 }
+// ANCHOR_END: setup
 
 impl Logger for VerbosityFilter {
     fn log(&self, verbosity: u8, message: &str) {