Clarify Fn traits slide (#2333)

Improve naming and always capture something in our closures; explain
other details/variations in speaker notes.

Fixes #2251.

Author: fw-immunant

src/std-traits/closures.md

@@ -10,39 +10,41 @@ implement special [`Fn`](https://doc.rust-lang.org/std/ops/trait.Fn.html),
 [`FnOnce`](https://doc.rust-lang.org/std/ops/trait.FnOnce.html) traits:
 
 ```rust,editable
-fn apply_with_log(func: impl FnOnce(i32) -> i32, input: i32) -> i32 {
-    println!("Calling function on {input}");
-    func(input)
+fn apply_and_log(func: impl FnOnce(i32) -> i32, func_name: &str, input: i32) {
+    println!("Calling {func_name}({input}): {}", func(input))
 }
 
 fn main() {
-    let add_3 = |x| x + 3;
-    println!("add_3: {}", apply_with_log(add_3, 10));
-    println!("add_3: {}", apply_with_log(add_3, 20));
+    let n = 3;
+    let add_3 = |x| x + n;
+    apply_and_log(&add_3, "add_3", 10);
+    apply_and_log(&add_3, "add_3", 20);
 
     let mut v = Vec::new();
     let mut accumulate = |x: i32| {
         v.push(x);
         v.iter().sum::<i32>()
     };
-    println!("accumulate: {}", apply_with_log(&mut accumulate, 4));
-    println!("accumulate: {}", apply_with_log(&mut accumulate, 5));
+    apply_and_log(&mut accumulate, "accumulate", 4);
+    apply_and_log(&mut accumulate, "accumulate", 5);
 
     let multiply_sum = |x| x * v.into_iter().sum::<i32>();
-    println!("multiply_sum: {}", apply_with_log(multiply_sum, 3));
+    apply_and_log(multiply_sum, "multiply_sum", 3);
 }
 ```
 
 <details>
 
-An `Fn` (e.g. `add_3`) neither consumes nor mutates captured values, or perhaps
-captures nothing at all. It can be called multiple times concurrently.
+An `Fn` (e.g. `add_3`) neither consumes nor mutates captured values. It can be
+called needing only a shared reference to the closure, which means the closure
+can be executed repeatedly and even concurrently.
 
-An `FnMut` (e.g. `accumulate`) might mutate captured values. You can call it
-multiple times, but not concurrently.
+An `FnMut` (e.g. `accumulate`) might mutate captured values. The closure object
+is accessed via exclusive reference, so it can be called repeatedly but not
+concurrently.
 
-If you have an `FnOnce` (e.g. `multiply_sum`), you may only call it once. It
-might consume captured values.
+If you have an `FnOnce` (e.g. `multiply_sum`), you may only call it once. Doing
+so consumes the closure and any values captured by move.
 
 `FnMut` is a subtype of `FnOnce`. `Fn` is a subtype of `FnMut` and `FnOnce`.
 I.e. you can use an `FnMut` wherever an `FnOnce` is called for, and you can use
@@ -52,14 +54,17 @@ When you define a function that takes a closure, you should take `FnOnce` if you
 can (i.e. you call it once), or `FnMut` else, and last `Fn`. This allows the
 most flexibility for the caller.
 
-In contrast, when you have a closure, the most flexible you can have is `Fn` (it
-can be passed everywhere), then `FnMut`, and lastly `FnOnce`.
+In contrast, when you have a closure, the most flexible you can have is `Fn`
+(which can be passed to a consumer of any of the 3 closure traits), then
+`FnMut`, and lastly `FnOnce`.
 
 The compiler also infers `Copy` (e.g. for `add_3`) and `Clone` (e.g.
-`multiply_sum`), depending on what the closure captures.
+`multiply_sum`), depending on what the closure captures. Function pointers
+(references to `fn` items) implement `Copy` and `Fn`.
 
-By default, closures will capture by reference if they can. The `move` keyword
-makes them capture by value.
+By default, closures will capture each variable from an outer scope by the least
+demanding form of access they can (by shared reference if possible, then
+exclusive reference, then by move). The `move` keyword forces capture by value.
 
 ```rust,editable
 fn make_greeter(prefix: String) -> impl Fn(&str) {