Split let control flow into mutliple sub-slides (#2567)

There are three kinds of syntax here, making for a very long and
hard-to-navigate slide. Splitting it up helps!

src/SUMMARY.md

@@ -81,6 +81,9 @@
   - [Destructuring Structs](pattern-matching/destructuring-structs.md)
   - [Destructuring Enums](pattern-matching/destructuring-enums.md)
   - [Let Control Flow](pattern-matching/let-control-flow.md)
+    - [`if let` Expressions](pattern-matching/let-control-flow/if-let.md)
+    - [`while let` Statements](pattern-matching/let-control-flow/while-let.md)
+    - [`let else`](pattern-matching/let-control-flow/let-else.md)
   - [Exercise: Expression Evaluation](pattern-matching/exercise.md)
     - [Solution](pattern-matching/solution.md)
 - [Methods and Traits](methods-and-traits.md)

src/pattern-matching/let-control-flow.md

@@ -8,123 +8,5 @@ Rust has a few control flow constructs which differ from other languages. They
 are used for pattern matching:
 
 - `if let` expressions
-- `let else` expressions
 - `while let` expressions
-
+- `let else` expressions
-# `if let` expressions
-
-The
-[`if let` expression](https://doc.rust-lang.org/reference/expressions/if-expr.html#if-let-expressions)
-lets you execute different code depending on whether a value matches a pattern:
-
-```rust,editable
-use std::time::Duration;
-
-fn sleep_for(secs: f32) {
-    if let Ok(duration) = Duration::try_from_secs_f32(secs) {
-        std::thread::sleep(duration);
-        println!("slept for {duration:?}");
-    }
-}
-
-fn main() {
-    sleep_for(-10.0);
-    sleep_for(0.8);
-}
-```
-
-# `let else` expressions
-
-For the common case of matching a pattern and returning from the function, use
-[`let else`](https://doc.rust-lang.org/rust-by-example/flow_control/let_else.html).
-The "else" case must diverge (`return`, `break`, or panic - anything but falling
-off the end of the block).
-
-```rust,editable
-fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
-    // TODO: The structure of this code is difficult to follow -- rewrite it with let-else!
-    if let Some(s) = maybe_string {
-        if let Some(first_byte_char) = s.chars().next() {
-            if let Some(digit) = first_byte_char.to_digit(16) {
-                Ok(digit)
-            } else {
-                return Err(String::from("not a hex digit"));
-            }
-        } else {
-            return Err(String::from("got empty string"));
-        }
-    } else {
-        return Err(String::from("got None"));
-    }
-}
-
-fn main() {
-    println!("result: {:?}", hex_or_die_trying(Some(String::from("foo"))));
-}
-```
-
-Like with `if let`, there is a
-[`while let`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#predicate-pattern-loops)
-variant which repeatedly tests a value against a pattern:
-
-<!-- mdbook-xgettext: skip -->
-
-```rust,editable
-fn main() {
-    let mut name = String::from("Comprehensive Rust 🦀");
-    while let Some(c) = name.pop() {
-        println!("character: {c}");
-    }
-    // (There are more efficient ways to reverse a string!)
-}
-```
-
-Here
-[`String::pop`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.pop)
-returns `Some(c)` until the string is empty, after which it will return `None`.
-The `while let` lets us keep iterating through all items.
-
-<details>
-
-## if-let
-
-- Unlike `match`, `if let` does not have to cover all branches. This can make it
-  more concise than `match`.
-- A common usage is handling `Some` values when working with `Option`.
-- Unlike `match`, `if let` does not support guard clauses for pattern matching.
-
-## let-else
-
-`if-let`s can pile up, as shown. The `let-else` construct supports flattening
-this nested code. Rewrite the awkward version for students, so they can see the
-transformation.
-
-The rewritten version is:
-
-```rust
-fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
-    let Some(s) = maybe_string else {
-        return Err(String::from("got None"));
-    };
-
-    let Some(first_byte_char) = s.chars().next() else {
-        return Err(String::from("got empty string"));
-    };
-
-    let Some(digit) = first_byte_char.to_digit(16) else {
-        return Err(String::from("not a hex digit"));
-    };
-
-    return Ok(digit);
-}
-```
-
-# while-let
-
-- Point out that the `while let` loop will keep going as long as the value
-  matches the pattern.
-- You could rewrite the `while let` loop as an infinite loop with an if
-  statement that breaks when there is no value to unwrap for `name.pop()`. The
-  `while let` provides syntactic sugar for the above scenario.
-
-</details>

src/pattern-matching/let-control-flow/if-let.md

@@ -0,0 +1,31 @@
+# `if let` Expressions
+
+The
+[`if let` expression](https://doc.rust-lang.org/reference/expressions/if-expr.html#if-let-expressions)
+lets you execute different code depending on whether a value matches a pattern:
+
+```rust,editable
+use std::time::Duration;
+
+fn sleep_for(secs: f32) {
+    if let Ok(duration) = Duration::try_from_secs_f32(secs) {
+        std::thread::sleep(duration);
+        println!("slept for {duration:?}");
+    }
+}
+
+fn main() {
+    sleep_for(-10.0);
+    sleep_for(0.8);
+}
+```
+
+<details>
+
+- Unlike `match`, `if let` does not have to cover all branches. This can make it
+  more concise than `match`.
+- A common usage is handling `Some` values when working with `Option`.
+- Unlike `match`, `if let` does not support guard clauses for pattern matching.
+- With an `else` clause, this can be used as an expression.
+
+</details>

src/pattern-matching/let-control-flow/let-else.md

@@ -0,0 +1,57 @@
+# `let else` Statements
+
+For the common case of matching a pattern and returning from the function, use
+[`let else`](https://doc.rust-lang.org/rust-by-example/flow_control/let_else.html).
+The "else" case must diverge (`return`, `break`, or panic - anything but falling
+off the end of the block).
+
+```rust,editable
+fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
+    // TODO: The structure of this code is difficult to follow -- rewrite it with let-else!
+    if let Some(s) = maybe_string {
+        if let Some(first_byte_char) = s.chars().next() {
+            if let Some(digit) = first_byte_char.to_digit(16) {
+                Ok(digit)
+            } else {
+                Err(String::from("not a hex digit"))
+            }
+        } else {
+            Err(String::from("got empty string"))
+        }
+    } else {
+        Err(String::from("got None"))
+    }
+}
+
+fn main() {
+    println!("result: {:?}", hex_or_die_trying(Some(String::from("foo"))));
+}
+```
+
+<details>
+
+`if-let`s can pile up, as shown. The `let-else` construct supports flattening
+this nested code. Rewrite the awkward version for students, so they can see the
+transformation.
+
+The rewritten version is:
+
+```rust
+fn hex_or_die_trying(maybe_string: Option<String>) -> Result<u32, String> {
+    let Some(s) = maybe_string else {
+        return Err(String::from("got None"));
+    };
+
+    let Some(first_byte_char) = s.chars().next() else {
+        return Err(String::from("got empty string"));
+    };
+
+    let Some(digit) = first_byte_char.to_digit(16) else {
+        return Err(String::from("not a hex digit"));
+    };
+
+    Ok(digit)
+}
+```
+
+</details>

src/pattern-matching/let-control-flow/while-let.md

@@ -0,0 +1,34 @@
+# `while let` Statements
+
+Like with `if let`, there is a
+[`while let`](https://doc.rust-lang.org/reference/expressions/loop-expr.html#predicate-pattern-loops)
+variant which repeatedly tests a value against a pattern:
+
+<!-- mdbook-xgettext: skip -->
+
+```rust,editable
+fn main() {
+    let mut name = String::from("Comprehensive Rust 🦀");
+    while let Some(c) = name.pop() {
+        println!("character: {c}");
+    }
+    // (There are more efficient ways to reverse a string!)
+}
+```
+
+Here
+[`String::pop`](https://doc.rust-lang.org/stable/std/string/struct.String.html#method.pop)
+returns `Some(c)` until the string is empty, after which it will return `None`.
+The `while let` lets us keep iterating through all items.
+
+<details>
+
+- Point out that the `while let` loop will keep going as long as the value
+  matches the pattern.
+- You could rewrite the `while let` loop as an infinite loop with an if
+  statement that breaks when there is no value to unwrap for `name.pop()`. The
+  `while let` provides syntactic sugar for the above scenario.
+- This form cannot be used as an expression, because it may have no value if the
+  condition is false.
+
+</details>